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28f540f4 | 1 | @node I/O on Streams, Low-Level I/O, I/O Overview, Top |
34992338 | 2 | @c %MENU% High-level, portable I/O facilities |
28f540f4 | 3 | @chapter Input/Output on Streams |
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4 | @c fix an overfull: |
5 | @tex | |
6 | \hyphenation{which-ever} | |
7 | @end tex | |
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8 | |
9 | This chapter describes the functions for creating streams and performing | |
10 | input and output operations on them. As discussed in @ref{I/O | |
11 | Overview}, a stream is a fairly abstract, high-level concept | |
12 | representing a communications channel to a file, device, or process. | |
13 | ||
14 | @menu | |
15 | * Streams:: About the data type representing a stream. | |
19c3f208 | 16 | * Standard Streams:: Streams to the standard input and output |
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17 | devices are created for you. |
18 | * Opening Streams:: How to create a stream to talk to a file. | |
19 | * Closing Streams:: Close a stream when you are finished with it. | |
7b4161bb | 20 | * Streams and Threads:: Issues with streams in threaded programs. |
b5e73f56 | 21 | * Streams and I18N:: Streams in internationalized applications. |
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22 | * Simple Output:: Unformatted output by characters and lines. |
23 | * Character Input:: Unformatted input by characters and words. | |
24 | * Line Input:: Reading a line or a record from a stream. | |
25 | * Unreading:: Peeking ahead/pushing back input just read. | |
26 | * Block Input/Output:: Input and output operations on blocks of data. | |
27 | * Formatted Output:: @code{printf} and related functions. | |
28 | * Customizing Printf:: You can define new conversion specifiers for | |
29 | @code{printf} and friends. | |
30 | * Formatted Input:: @code{scanf} and related functions. | |
31 | * EOF and Errors:: How you can tell if an I/O error happens. | |
460e040a | 32 | * Error Recovery:: What you can do about errors. |
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33 | * Binary Streams:: Some systems distinguish between text files |
34 | and binary files. | |
35 | * File Positioning:: About random-access streams. | |
f65fd747 | 36 | * Portable Positioning:: Random access on peculiar ISO C systems. |
28f540f4 RM |
37 | * Stream Buffering:: How to control buffering of streams. |
38 | * Other Kinds of Streams:: Streams that do not necessarily correspond | |
19c3f208 | 39 | to an open file. |
0501d603 | 40 | * Formatted Messages:: Print strictly formatted messages. |
28f540f4 RM |
41 | @end menu |
42 | ||
43 | @node Streams | |
44 | @section Streams | |
45 | ||
46 | For historical reasons, the type of the C data structure that represents | |
47 | a stream is called @code{FILE} rather than ``stream''. Since most of | |
48 | the library functions deal with objects of type @code{FILE *}, sometimes | |
49 | the term @dfn{file pointer} is also used to mean ``stream''. This leads | |
50 | to unfortunate confusion over terminology in many books on C. This | |
51 | manual, however, is careful to use the terms ``file'' and ``stream'' | |
52 | only in the technical sense. | |
53 | @cindex file pointer | |
54 | ||
55 | @pindex stdio.h | |
56 | The @code{FILE} type is declared in the header file @file{stdio.h}. | |
57 | ||
58 | @comment stdio.h | |
f65fd747 | 59 | @comment ISO |
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60 | @deftp {Data Type} FILE |
61 | This is the data type used to represent stream objects. A @code{FILE} | |
62 | object holds all of the internal state information about the connection | |
63 | to the associated file, including such things as the file position | |
64 | indicator and buffering information. Each stream also has error and | |
65 | end-of-file status indicators that can be tested with the @code{ferror} | |
66 | and @code{feof} functions; see @ref{EOF and Errors}. | |
67 | @end deftp | |
68 | ||
69 | @code{FILE} objects are allocated and managed internally by the | |
70 | input/output library functions. Don't try to create your own objects of | |
71 | type @code{FILE}; let the library do it. Your programs should | |
72 | deal only with pointers to these objects (that is, @code{FILE *} values) | |
73 | rather than the objects themselves. | |
6d52618b | 74 | @c !!! should say that FILE's have "No user-serviceable parts inside." |
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75 | |
76 | @node Standard Streams | |
77 | @section Standard Streams | |
78 | @cindex standard streams | |
79 | @cindex streams, standard | |
80 | ||
81 | When the @code{main} function of your program is invoked, it already has | |
82 | three predefined streams open and available for use. These represent | |
83 | the ``standard'' input and output channels that have been established | |
84 | for the process. | |
85 | ||
86 | These streams are declared in the header file @file{stdio.h}. | |
87 | @pindex stdio.h | |
88 | ||
89 | @comment stdio.h | |
f65fd747 | 90 | @comment ISO |
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91 | @deftypevar {FILE *} stdin |
92 | The @dfn{standard input} stream, which is the normal source of input for the | |
93 | program. | |
94 | @end deftypevar | |
95 | @cindex standard input stream | |
96 | ||
97 | @comment stdio.h | |
f65fd747 | 98 | @comment ISO |
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99 | @deftypevar {FILE *} stdout |
100 | The @dfn{standard output} stream, which is used for normal output from | |
101 | the program. | |
102 | @end deftypevar | |
103 | @cindex standard output stream | |
104 | ||
105 | @comment stdio.h | |
f65fd747 | 106 | @comment ISO |
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107 | @deftypevar {FILE *} stderr |
108 | The @dfn{standard error} stream, which is used for error messages and | |
109 | diagnostics issued by the program. | |
110 | @end deftypevar | |
111 | @cindex standard error stream | |
112 | ||
113 | In the GNU system, you can specify what files or processes correspond to | |
114 | these streams using the pipe and redirection facilities provided by the | |
115 | shell. (The primitives shells use to implement these facilities are | |
116 | described in @ref{File System Interface}.) Most other operating systems | |
117 | provide similar mechanisms, but the details of how to use them can vary. | |
118 | ||
119 | In the GNU C library, @code{stdin}, @code{stdout}, and @code{stderr} are | |
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120 | normal variables which you can set just like any others. For example, |
121 | to redirect the standard output to a file, you could do: | |
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122 | |
123 | @smallexample | |
124 | fclose (stdout); | |
125 | stdout = fopen ("standard-output-file", "w"); | |
126 | @end smallexample | |
127 | ||
128 | Note however, that in other systems @code{stdin}, @code{stdout}, and | |
129 | @code{stderr} are macros that you cannot assign to in the normal way. | |
130 | But you can use @code{freopen} to get the effect of closing one and | |
131 | reopening it. @xref{Opening Streams}. | |
132 | ||
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133 | The three streams @code{stdin}, @code{stdout}, and @code{stderr} are not |
134 | unoriented at program start (@pxref{Streams and I18N}). | |
135 | ||
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136 | @node Opening Streams |
137 | @section Opening Streams | |
138 | ||
139 | @cindex opening a stream | |
140 | Opening a file with the @code{fopen} function creates a new stream and | |
141 | establishes a connection between the stream and a file. This may | |
19c3f208 | 142 | involve creating a new file. |
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143 | |
144 | @pindex stdio.h | |
145 | Everything described in this section is declared in the header file | |
146 | @file{stdio.h}. | |
147 | ||
148 | @comment stdio.h | |
f65fd747 | 149 | @comment ISO |
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150 | @deftypefun {FILE *} fopen (const char *@var{filename}, const char *@var{opentype}) |
151 | The @code{fopen} function opens a stream for I/O to the file | |
152 | @var{filename}, and returns a pointer to the stream. | |
153 | ||
154 | The @var{opentype} argument is a string that controls how the file is | |
155 | opened and specifies attributes of the resulting stream. It must begin | |
156 | with one of the following sequences of characters: | |
157 | ||
158 | @table @samp | |
159 | @item r | |
160 | Open an existing file for reading only. | |
161 | ||
162 | @item w | |
163 | Open the file for writing only. If the file already exists, it is | |
164 | truncated to zero length. Otherwise a new file is created. | |
165 | ||
166 | @item a | |
167 | Open a file for append access; that is, writing at the end of file only. | |
168 | If the file already exists, its initial contents are unchanged and | |
169 | output to the stream is appended to the end of the file. | |
170 | Otherwise, a new, empty file is created. | |
171 | ||
172 | @item r+ | |
173 | Open an existing file for both reading and writing. The initial contents | |
174 | of the file are unchanged and the initial file position is at the | |
175 | beginning of the file. | |
176 | ||
177 | @item w+ | |
178 | Open a file for both reading and writing. If the file already exists, it | |
179 | is truncated to zero length. Otherwise, a new file is created. | |
180 | ||
181 | @item a+ | |
182 | Open or create file for both reading and appending. If the file exists, | |
183 | its initial contents are unchanged. Otherwise, a new file is created. | |
184 | The initial file position for reading is at the beginning of the file, | |
185 | but output is always appended to the end of the file. | |
186 | @end table | |
187 | ||
188 | As you can see, @samp{+} requests a stream that can do both input and | |
f65fd747 | 189 | output. The ISO standard says that when using such a stream, you must |
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190 | call @code{fflush} (@pxref{Stream Buffering}) or a file positioning |
191 | function such as @code{fseek} (@pxref{File Positioning}) when switching | |
192 | from reading to writing or vice versa. Otherwise, internal buffers | |
193 | might not be emptied properly. The GNU C library does not have this | |
194 | limitation; you can do arbitrary reading and writing operations on a | |
195 | stream in whatever order. | |
196 | ||
197 | Additional characters may appear after these to specify flags for the | |
198 | call. Always put the mode (@samp{r}, @samp{w+}, etc.) first; that is | |
199 | the only part you are guaranteed will be understood by all systems. | |
200 | ||
201 | The GNU C library defines one additional character for use in | |
202 | @var{opentype}: the character @samp{x} insists on creating a new | |
203 | file---if a file @var{filename} already exists, @code{fopen} fails | |
0be8752b | 204 | rather than opening it. If you use @samp{x} you are guaranteed that |
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205 | you will not clobber an existing file. This is equivalent to the |
206 | @code{O_EXCL} option to the @code{open} function (@pxref{Opening and | |
207 | Closing Files}). | |
208 | ||
209 | The character @samp{b} in @var{opentype} has a standard meaning; it | |
210 | requests a binary stream rather than a text stream. But this makes no | |
211 | difference in POSIX systems (including the GNU system). If both | |
212 | @samp{+} and @samp{b} are specified, they can appear in either order. | |
213 | @xref{Binary Streams}. | |
214 | ||
4e2e9999 UD |
215 | @cindex stream orientation |
216 | @cindex orientation, stream | |
217 | If the @var{opentype} string contains the sequence | |
218 | @code{,ccs=@var{STRING}} then @var{STRING} is taken as the name of a | |
219 | coded character set and @code{fopen} will mark the stream as | |
220 | wide-oriented which appropriate conversion functions in place to convert | |
221 | from and to the character set @var{STRING} is place. Any other stream | |
222 | is opened initially unoriented and the orientation is decided with the | |
223 | first file operation. If the first operation is a wide character | |
224 | operation, the stream is not only marked as wide-oriented, also the | |
225 | conversion functions to convert to the coded character set used for the | |
226 | current locale are loaded. This will not change anymore from this point | |
227 | on even if the locale selected for the @code{LC_CTYPE} category is | |
228 | changed. | |
229 | ||
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230 | Any other characters in @var{opentype} are simply ignored. They may be |
231 | meaningful in other systems. | |
232 | ||
233 | If the open fails, @code{fopen} returns a null pointer. | |
a3a4a74e UD |
234 | |
235 | When the sources are compiling with @code{_FILE_OFFSET_BITS == 64} on a | |
0be8752b | 236 | 32 bit machine this function is in fact @code{fopen64} since the LFS |
a3a4a74e | 237 | interface replaces transparently the old interface. |
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238 | @end deftypefun |
239 | ||
240 | You can have multiple streams (or file descriptors) pointing to the same | |
241 | file open at the same time. If you do only input, this works | |
242 | straightforwardly, but you must be careful if any output streams are | |
243 | included. @xref{Stream/Descriptor Precautions}. This is equally true | |
244 | whether the streams are in one program (not usual) or in several | |
245 | programs (which can easily happen). It may be advantageous to use the | |
246 | file locking facilities to avoid simultaneous access. @xref{File | |
247 | Locks}. | |
248 | ||
a3a4a74e UD |
249 | @comment stdio.h |
250 | @comment Unix98 | |
251 | @deftypefun {FILE *} fopen64 (const char *@var{filename}, const char *@var{opentype}) | |
252 | This function is similar to @code{fopen} but the stream it returns a | |
253 | pointer for is opened using @code{open64}. Therefore this stream can be | |
0be8752b | 254 | used even on files larger then @math{2^31} bytes on 32 bit machines. |
a3a4a74e UD |
255 | |
256 | Please note that the return type is still @code{FILE *}. There is no | |
257 | special @code{FILE} type for the LFS interface. | |
258 | ||
259 | If the sources are compiled with @code{_FILE_OFFSET_BITS == 64} on a 32 | |
260 | bits machine this function is available under the name @code{fopen} | |
261 | and so transparently replaces the old interface. | |
262 | @end deftypefun | |
263 | ||
28f540f4 | 264 | @comment stdio.h |
f65fd747 | 265 | @comment ISO |
28f540f4 RM |
266 | @deftypevr Macro int FOPEN_MAX |
267 | The value of this macro is an integer constant expression that | |
268 | represents the minimum number of streams that the implementation | |
269 | guarantees can be open simultaneously. You might be able to open more | |
270 | than this many streams, but that is not guaranteed. The value of this | |
271 | constant is at least eight, which includes the three standard streams | |
272 | @code{stdin}, @code{stdout}, and @code{stderr}. In POSIX.1 systems this | |
273 | value is determined by the @code{OPEN_MAX} parameter; @pxref{General | |
274 | Limits}. In BSD and GNU, it is controlled by the @code{RLIMIT_NOFILE} | |
275 | resource limit; @pxref{Limits on Resources}. | |
276 | @end deftypevr | |
277 | ||
278 | @comment stdio.h | |
f65fd747 | 279 | @comment ISO |
28f540f4 RM |
280 | @deftypefun {FILE *} freopen (const char *@var{filename}, const char *@var{opentype}, FILE *@var{stream}) |
281 | This function is like a combination of @code{fclose} and @code{fopen}. | |
282 | It first closes the stream referred to by @var{stream}, ignoring any | |
283 | errors that are detected in the process. (Because errors are ignored, | |
284 | you should not use @code{freopen} on an output stream if you have | |
285 | actually done any output using the stream.) Then the file named by | |
286 | @var{filename} is opened with mode @var{opentype} as for @code{fopen}, | |
287 | and associated with the same stream object @var{stream}. | |
288 | ||
289 | If the operation fails, a null pointer is returned; otherwise, | |
290 | @code{freopen} returns @var{stream}. | |
291 | ||
292 | @code{freopen} has traditionally been used to connect a standard stream | |
293 | such as @code{stdin} with a file of your own choice. This is useful in | |
294 | programs in which use of a standard stream for certain purposes is | |
295 | hard-coded. In the GNU C library, you can simply close the standard | |
296 | streams and open new ones with @code{fopen}. But other systems lack | |
297 | this ability, so using @code{freopen} is more portable. | |
a3a4a74e UD |
298 | |
299 | When the sources are compiling with @code{_FILE_OFFSET_BITS == 64} on a | |
0be8752b | 300 | 32 bit machine this function is in fact @code{freopen64} since the LFS |
a3a4a74e UD |
301 | interface replaces transparently the old interface. |
302 | @end deftypefun | |
303 | ||
304 | @comment stdio.h | |
305 | @comment Unix98 | |
306 | @deftypefun {FILE *} freopen64 (const char *@var{filename}, const char *@var{opentype}, FILE *@var{stream}) | |
307 | This function is similar to @code{freopen}. The only difference is that | |
0be8752b | 308 | on 32 bit machine the stream returned is able to read beyond the |
a3a4a74e UD |
309 | @math{2^31} bytes limits imposed by the normal interface. It should be |
310 | noted that the stream pointed to by @var{stream} need not be opened | |
311 | using @code{fopen64} or @code{freopen64} since its mode is not important | |
312 | for this function. | |
313 | ||
314 | If the sources are compiled with @code{_FILE_OFFSET_BITS == 64} on a 32 | |
315 | bits machine this function is available under the name @code{freopen} | |
316 | and so transparently replaces the old interface. | |
28f540f4 RM |
317 | @end deftypefun |
318 | ||
f8b23b32 UD |
319 | In some situations it is useful to know whether a given stream is |
320 | available for reading or writing. This information is normally not | |
321 | available and would have to be remembered separately. Solaris | |
322 | introduced a few functions to get this information from the stream | |
323 | descriptor and these functions are also available in the GNU C library. | |
324 | ||
325 | @comment stdio_ext.h | |
326 | @comment GNU | |
327 | @deftypefun int __freadable (FILE *@var{stream}) | |
328 | The @code{__freadable} function determines whether the stream | |
329 | @var{stream} was opened to allow reading. In this case the return value | |
330 | is nonzero. For write-only streams the function returns zero. | |
331 | ||
332 | This function is declared in @file{stdio_ext.h}. | |
333 | @end deftypefun | |
334 | ||
335 | @comment stdio_ext.h | |
336 | @comment GNU | |
337 | @deftypefun int __fwritable (FILE *@var{stream}) | |
338 | The @code{__fwritable} function determines whether the stream | |
339 | @var{stream} was opened to allow writing. In this case the return value | |
340 | is nonzero. For read-only streams the function returns zero. | |
341 | ||
342 | This function is declared in @file{stdio_ext.h}. | |
343 | @end deftypefun | |
344 | ||
345 | For slightly different kind of problems there are two more functions. | |
346 | They provide even finer-grained information. | |
347 | ||
348 | @comment stdio_ext.h | |
349 | @comment GNU | |
350 | @deftypefun int __freading (FILE *@var{stream}) | |
351 | The @code{__freading} function determines whether the stream | |
352 | @var{stream} was last read from or whether it is opened read-only. In | |
353 | this case the return value is nonzero, otherwise it is zero. | |
354 | Determining whether a stream opened for reading and writing was last | |
355 | used for writing allows to draw conclusions about the content about the | |
356 | buffer, among other things. | |
357 | ||
358 | This function is declared in @file{stdio_ext.h}. | |
359 | @end deftypefun | |
360 | ||
361 | @comment stdio_ext.h | |
362 | @comment GNU | |
363 | @deftypefun int __fwriting (FILE *@var{stream}) | |
364 | The @code{__fwriting} function determines whether the stream | |
365 | @var{stream} was last written to or whether it is opened write-only. In | |
366 | this case the return value is nonzero, otherwise it is zero. | |
367 | ||
368 | This function is declared in @file{stdio_ext.h}. | |
369 | @end deftypefun | |
370 | ||
28f540f4 RM |
371 | |
372 | @node Closing Streams | |
373 | @section Closing Streams | |
374 | ||
375 | @cindex closing a stream | |
376 | When a stream is closed with @code{fclose}, the connection between the | |
0bc93a2f | 377 | stream and the file is canceled. After you have closed a stream, you |
28f540f4 RM |
378 | cannot perform any additional operations on it. |
379 | ||
380 | @comment stdio.h | |
f65fd747 | 381 | @comment ISO |
28f540f4 RM |
382 | @deftypefun int fclose (FILE *@var{stream}) |
383 | This function causes @var{stream} to be closed and the connection to | |
384 | the corresponding file to be broken. Any buffered output is written | |
385 | and any buffered input is discarded. The @code{fclose} function returns | |
386 | a value of @code{0} if the file was closed successfully, and @code{EOF} | |
19c3f208 | 387 | if an error was detected. |
28f540f4 RM |
388 | |
389 | It is important to check for errors when you call @code{fclose} to close | |
390 | an output stream, because real, everyday errors can be detected at this | |
391 | time. For example, when @code{fclose} writes the remaining buffered | |
392 | output, it might get an error because the disk is full. Even if you | |
393 | know the buffer is empty, errors can still occur when closing a file if | |
394 | you are using NFS. | |
395 | ||
396 | The function @code{fclose} is declared in @file{stdio.h}. | |
397 | @end deftypefun | |
398 | ||
6bc31da0 UD |
399 | To close all streams currently available the GNU C Library provides |
400 | another function. | |
401 | ||
402 | @comment stdio.h | |
403 | @comment GNU | |
404 | @deftypefun int fcloseall (void) | |
405 | This function causes all open streams of the process to be closed and | |
406 | the connection to corresponding files to be broken. All buffered data | |
f2ea0f5b | 407 | is written and any buffered input is discarded. The @code{fcloseall} |
6bc31da0 UD |
408 | function returns a value of @code{0} if all the files were closed |
409 | successfully, and @code{EOF} if an error was detected. | |
410 | ||
0be8752b | 411 | This function should be used only in special situations, e.g., when an |
6bc31da0 | 412 | error occurred and the program must be aborted. Normally each single |
0be8752b UD |
413 | stream should be closed separately so that problems with individual |
414 | streams can be identified. It is also problematic since the standard | |
415 | streams (@pxref{Standard Streams}) will also be closed. | |
6bc31da0 UD |
416 | |
417 | The function @code{fcloseall} is declared in @file{stdio.h}. | |
418 | @end deftypefun | |
419 | ||
28f540f4 RM |
420 | If the @code{main} function to your program returns, or if you call the |
421 | @code{exit} function (@pxref{Normal Termination}), all open streams are | |
422 | automatically closed properly. If your program terminates in any other | |
423 | manner, such as by calling the @code{abort} function (@pxref{Aborting a | |
424 | Program}) or from a fatal signal (@pxref{Signal Handling}), open streams | |
425 | might not be closed properly. Buffered output might not be flushed and | |
426 | files may be incomplete. For more information on buffering of streams, | |
427 | see @ref{Stream Buffering}. | |
428 | ||
7b4161bb UD |
429 | @node Streams and Threads |
430 | @section Streams and Threads | |
431 | ||
432 | @cindex threads | |
433 | @cindex multi-threaded application | |
434 | Streams can be used in multi-threaded applications in the same way they | |
435 | are used in single-threaded applications. But the programmer must be | |
11bf311e | 436 | aware of the possible complications. It is important to know about |
7b4161bb UD |
437 | these also if the program one writes never use threads since the design |
438 | and implementation of many stream functions is heavily influenced by the | |
f6af7428 | 439 | requirements added by multi-threaded programming. |
7b4161bb UD |
440 | |
441 | The POSIX standard requires that by default the stream operations are | |
0bc93a2f | 442 | atomic. I.e., issuing two stream operations for the same stream in two |
7b4161bb UD |
443 | threads at the same time will cause the operations to be executed as if |
444 | they were issued sequentially. The buffer operations performed while | |
445 | reading or writing are protected from other uses of the same stream. To | |
446 | do this each stream has an internal lock object which has to be | |
447 | (implicitly) acquired before any work can be done. | |
448 | ||
449 | But there are situations where this is not enough and there are also | |
450 | situations where this is not wanted. The implicit locking is not enough | |
451 | if the program requires more than one stream function call to happen | |
452 | atomically. One example would be if an output line a program wants to | |
453 | generate is created by several function calls. The functions by | |
454 | themselves would ensure only atomicity of their own operation, but not | |
455 | atomicity over all the function calls. For this it is necessary to | |
456 | perform the stream locking in the application code. | |
457 | ||
458 | @comment stdio.h | |
459 | @comment POSIX | |
460 | @deftypefun void flockfile (FILE *@var{stream}) | |
461 | The @code{flockfile} function acquires the internal locking object | |
f6af7428 | 462 | associated with the stream @var{stream}. This ensures that no other |
7b4161bb UD |
463 | thread can explicitly through @code{flockfile}/@code{ftrylockfile} or |
464 | implicit through a call of a stream function lock the stream. The | |
465 | thread will block until the lock is acquired. An explicit call to | |
466 | @code{funlockfile} has to be used to release the lock. | |
467 | @end deftypefun | |
468 | ||
469 | @comment stdio.h | |
470 | @comment POSIX | |
471 | @deftypefun int ftrylockfile (FILE *@var{stream}) | |
f6af7428 | 472 | The @code{ftrylockfile} function tries to acquire the internal locking |
7b4161bb UD |
473 | object associated with the stream @var{stream} just like |
474 | @code{flockfile}. But unlike @code{flockfile} this function does not | |
475 | block if the lock is not available. @code{ftrylockfile} returns zero if | |
476 | the lock was successfully acquired. Otherwise the stream is locked by | |
477 | another thread. | |
478 | @end deftypefun | |
479 | ||
480 | @comment stdio.h | |
481 | @comment POSIX | |
482 | @deftypefun void funlockfile (FILE *@var{stream}) | |
483 | The @code{funlockfile} function releases the internal locking object of | |
484 | the stream @var{stream}. The stream must have been locked before by a | |
485 | call to @code{flockfile} or a successful call of @code{ftrylockfile}. | |
486 | The implicit locking performed by the stream operations do not count. | |
487 | The @code{funlockfile} function does not return an error status and the | |
488 | behavior of a call for a stream which is not locked by the current | |
489 | thread is undefined. | |
490 | @end deftypefun | |
491 | ||
492 | The following example shows how the functions above can be used to | |
493 | generate an output line atomically even in multi-threaded applications | |
494 | (yes, the same job could be done with one @code{fprintf} call but it is | |
495 | sometimes not possible): | |
496 | ||
497 | @smallexample | |
498 | FILE *fp; | |
499 | @{ | |
95fdc6a0 | 500 | @dots{} |
7b4161bb UD |
501 | flockfile (fp); |
502 | fputs ("This is test number ", fp); | |
503 | fprintf (fp, "%d\n", test); | |
504 | funlockfile (fp) | |
505 | @} | |
506 | @end smallexample | |
507 | ||
508 | Without the explicit locking it would be possible for another thread to | |
509 | use the stream @var{fp} after the @code{fputs} call return and before | |
510 | @code{fprintf} was called with the result that the number does not | |
511 | follow the word @samp{number}. | |
512 | ||
513 | From this description it might already be clear that the locking objects | |
514 | in streams are no simple mutexes. Since locking the same stream twice | |
515 | in the same thread is allowed the locking objects must be equivalent to | |
516 | recursive mutexes. These mutexes keep track of the owner and the number | |
517 | of times the lock is acquired. The same number of @code{funlockfile} | |
518 | calls by the same threads is necessary to unlock the stream completely. | |
519 | For instance: | |
520 | ||
521 | @smallexample | |
522 | void | |
523 | foo (FILE *fp) | |
524 | @{ | |
525 | ftrylockfile (fp); | |
526 | fputs ("in foo\n", fp); | |
527 | /* @r{This is very wrong!!!} */ | |
528 | funlockfile (fp); | |
529 | @} | |
530 | @end smallexample | |
531 | ||
532 | It is important here that the @code{funlockfile} function is only called | |
533 | if the @code{ftrylockfile} function succeeded in locking the stream. It | |
534 | is therefore always wrong to ignore the result of @code{ftrylockfile}. | |
535 | And it makes no sense since otherwise one would use @code{flockfile}. | |
536 | The result of code like that above is that either @code{funlockfile} | |
f6af7428 | 537 | tries to free a stream that hasn't been locked by the current thread or it |
7b4161bb UD |
538 | frees the stream prematurely. The code should look like this: |
539 | ||
540 | @smallexample | |
541 | void | |
542 | foo (FILE *fp) | |
543 | @{ | |
544 | if (ftrylockfile (fp) == 0) | |
545 | @{ | |
546 | fputs ("in foo\n", fp); | |
547 | funlockfile (fp); | |
548 | @} | |
549 | @} | |
550 | @end smallexample | |
551 | ||
552 | Now that we covered why it is necessary to have these locking it is | |
f6af7428 | 553 | necessary to talk about situations when locking is unwanted and what can |
7b4161bb UD |
554 | be done. The locking operations (explicit or implicit) don't come for |
555 | free. Even if a lock is not taken the cost is not zero. The operations | |
3ca9e670 | 556 | which have to be performed require memory operations that are safe in |
7b4161bb UD |
557 | multi-processor environments. With the many local caches involved in |
558 | such systems this is quite costly. So it is best to avoid the locking | |
3ca9e670 UD |
559 | completely if it is not needed -- because the code in question is never |
560 | used in a context where two or more threads may use a stream at a time. | |
7b4161bb UD |
561 | This can be determined most of the time for application code; for |
562 | library code which can be used in many contexts one should default to be | |
563 | conservative and use locking. | |
564 | ||
565 | There are two basic mechanisms to avoid locking. The first is to use | |
566 | the @code{_unlocked} variants of the stream operations. The POSIX | |
567 | standard defines quite a few of those and the GNU library adds a few | |
568 | more. These variants of the functions behave just like the functions | |
3ca9e670 | 569 | with the name without the suffix except that they do not lock the |
7b4161bb | 570 | stream. Using these functions is very desirable since they are |
f6af7428 | 571 | potentially much faster. This is not only because the locking |
7b4161bb | 572 | operation itself is avoided. More importantly, functions like |
0bc93a2f | 573 | @code{putc} and @code{getc} are very simple and traditionally (before the |
7b4161bb | 574 | introduction of threads) were implemented as macros which are very fast |
3ca9e670 UD |
575 | if the buffer is not empty. With the addition of locking requirements |
576 | these functions are no longer implemented as macros since they would | |
577 | would expand to too much code. | |
578 | But these macros are still available with the same functionality under the new | |
7b4161bb UD |
579 | names @code{putc_unlocked} and @code{getc_unlocked}. This possibly huge |
580 | difference of speed also suggests the use of the @code{_unlocked} | |
581 | functions even if locking is required. The difference is that the | |
582 | locking then has to be performed in the program: | |
583 | ||
584 | @smallexample | |
585 | void | |
586 | foo (FILE *fp, char *buf) | |
587 | @{ | |
588 | flockfile (fp); | |
589 | while (*buf != '/') | |
590 | putc_unlocked (*buf++, fp); | |
591 | funlockfile (fp); | |
592 | @} | |
593 | @end smallexample | |
594 | ||
595 | If in this example the @code{putc} function would be used and the | |
596 | explicit locking would be missing the @code{putc} function would have to | |
597 | acquire the lock in every call, potentially many times depending on when | |
598 | the loop terminates. Writing it the way illustrated above allows the | |
599 | @code{putc_unlocked} macro to be used which means no locking and direct | |
600 | manipulation of the buffer of the stream. | |
601 | ||
602 | A second way to avoid locking is by using a non-standard function which | |
603 | was introduced in Solaris and is available in the GNU C library as well. | |
604 | ||
605 | @comment stdio_ext.h | |
606 | @comment GNU | |
607 | @deftypefun int __fsetlocking (FILE *@var{stream}, int @var{type}) | |
608 | ||
609 | The @code{__fsetlocking} function can be used to select whether the | |
610 | stream operations will implicitly acquire the locking object of the | |
611 | stream @var{stream}. By default this is done but it can be disabled and | |
612 | reinstated using this function. There are three values defined for the | |
613 | @var{type} parameter. | |
614 | ||
615 | @vtable @code | |
616 | @item FSETLOCKING_INTERNAL | |
617 | The stream @code{stream} will from now on use the default internal | |
618 | locking. Every stream operation with exception of the @code{_unlocked} | |
619 | variants will implicitly lock the stream. | |
620 | ||
621 | @item FSETLOCKING_BYCALLER | |
622 | After the @code{__fsetlocking} function returns the user is responsible | |
623 | for locking the stream. None of the stream operations will implicitly | |
624 | do this anymore until the state is set back to | |
625 | @code{FSETLOCKING_INTERNAL}. | |
626 | ||
627 | @item FSETLOCKING_QUERY | |
628 | @code{__fsetlocking} only queries the current locking state of the | |
629 | stream. The return value will be @code{FSETLOCKING_INTERNAL} or | |
630 | @code{FSETLOCKING_BYCALLER} depending on the state. | |
631 | @end vtable | |
632 | ||
633 | The return value of @code{__fsetlocking} is either | |
634 | @code{FSETLOCKING_INTERNAL} or @code{FSETLOCKING_BYCALLER} depending on | |
635 | the state of the stream before the call. | |
636 | ||
637 | This function and the values for the @var{type} parameter are declared | |
638 | in @file{stdio_ext.h}. | |
639 | @end deftypefun | |
640 | ||
641 | This function is especially useful when program code has to be used | |
642 | which is written without knowledge about the @code{_unlocked} functions | |
3ca9e670 | 643 | (or if the programmer was too lazy to use them). |
7b4161bb | 644 | |
b5e73f56 UD |
645 | @node Streams and I18N |
646 | @section Streams in Internationalized Applications | |
647 | ||
648 | @w{ISO C90} introduced the new type @code{wchar_t} to allow handling | |
649 | larger character sets. What was missing was a possibility to output | |
650 | strings of @code{wchar_t} directly. One had to convert them into | |
651 | multibyte strings using @code{mbstowcs} (there was no @code{mbsrtowcs} | |
652 | yet) and then use the normal stream functions. While this is doable it | |
653 | is very cumbersome since performing the conversions is not trivial and | |
654 | greatly increases program complexity and size. | |
655 | ||
656 | The Unix standard early on (I think in XPG4.2) introduced two additional | |
657 | format specifiers for the @code{printf} and @code{scanf} families of | |
658 | functions. Printing and reading of single wide characters was made | |
659 | possible using the @code{%C} specifier and wide character strings can be | |
660 | handled with @code{%S}. These modifiers behave just like @code{%c} and | |
661 | @code{%s} only that they expect the corresponding argument to have the | |
662 | wide character type and that the wide character and string are | |
663 | transformed into/from multibyte strings before being used. | |
664 | ||
665 | This was a beginning but it is still not good enough. Not always is it | |
666 | desirable to use @code{printf} and @code{scanf}. The other, smaller and | |
667 | faster functions cannot handle wide characters. Second, it is not | |
668 | possible to have a format string for @code{printf} and @code{scanf} | |
669 | consisting of wide characters. The result is that format strings would | |
670 | have to be generated if they have to contain non-basic characters. | |
671 | ||
672 | @cindex C++ streams | |
673 | @cindex streams, C++ | |
674 | In the @w{Amendment 1} to @w{ISO C90} a whole new set of functions was | |
675 | added to solve the problem. Most of the stream functions got a | |
676 | counterpart which take a wide character or wide character string instead | |
677 | of a character or string respectively. The new functions operate on the | |
678 | same streams (like @code{stdout}). This is different from the model of | |
679 | the C++ runtime library where separate streams for wide and normal I/O | |
680 | are used. | |
681 | ||
682 | @cindex orientation, stream | |
683 | @cindex stream orientation | |
684 | Being able to use the same stream for wide and normal operations comes | |
685 | with a restriction: a stream can be used either for wide operations or | |
686 | for normal operations. Once it is decided there is no way back. Only a | |
687 | call to @code{freopen} or @code{freopen64} can reset the | |
688 | @dfn{orientation}. The orientation can be decided in three ways: | |
689 | ||
690 | @itemize @bullet | |
691 | @item | |
692 | If any of the normal character functions is used (this includes the | |
22298d18 | 693 | @code{fread} and @code{fwrite} functions) the stream is marked as not |
b5e73f56 UD |
694 | wide oriented. |
695 | ||
696 | @item | |
697 | If any of the wide character functions is used the stream is marked as | |
22298d18 | 698 | wide oriented. |
b5e73f56 UD |
699 | |
700 | @item | |
701 | The @code{fwide} function can be used to set the orientation either way. | |
702 | @end itemize | |
703 | ||
704 | It is important to never mix the use of wide and not wide operations on | |
705 | a stream. There are no diagnostics issued. The application behavior | |
706 | will simply be strange or the application will simply crash. The | |
707 | @code{fwide} function can help avoiding this. | |
708 | ||
709 | @comment wchar.h | |
710 | @comment ISO | |
711 | @deftypefun int fwide (FILE *@var{stream}, int @var{mode}) | |
712 | ||
22298d18 | 713 | The @code{fwide} function can be used to set and query the state of the |
b5e73f56 UD |
714 | orientation of the stream @var{stream}. If the @var{mode} parameter has |
715 | a positive value the streams get wide oriented, for negative values | |
716 | narrow oriented. It is not possible to overwrite previous orientations | |
717 | with @code{fwide}. I.e., if the stream @var{stream} was already | |
718 | oriented before the call nothing is done. | |
719 | ||
720 | If @var{mode} is zero the current orientation state is queried and | |
721 | nothing is changed. | |
722 | ||
723 | The @code{fwide} function returns a negative value, zero, or a positive | |
724 | value if the stream is narrow, not at all, or wide oriented | |
725 | respectively. | |
726 | ||
727 | This function was introduced in @w{Amendment 1} to @w{ISO C90} and is | |
728 | declared in @file{wchar.h}. | |
729 | @end deftypefun | |
730 | ||
731 | It is generally a good idea to orient a stream as early as possible. | |
732 | This can prevent surprise especially for the standard streams | |
733 | @code{stdin}, @code{stdout}, and @code{stderr}. If some library | |
734 | function in some situations uses one of these streams and this use | |
735 | orients the stream in a different way the rest of the application | |
736 | expects it one might end up with hard to reproduce errors. Remember | |
737 | that no errors are signal if the streams are used incorrectly. Leaving | |
738 | a stream unoriented after creation is normally only necessary for | |
739 | library functions which create streams which can be used in different | |
740 | contexts. | |
741 | ||
742 | When writing code which uses streams and which can be used in different | |
743 | contexts it is important to query the orientation of the stream before | |
744 | using it (unless the rules of the library interface demand a specific | |
745 | orientation). The following little, silly function illustrates this. | |
746 | ||
747 | @smallexample | |
748 | void | |
749 | print_f (FILE *fp) | |
750 | @{ | |
751 | if (fwide (fp, 0) > 0) | |
752 | /* @r{Positive return value means wide orientation.} */ | |
753 | fputwc (L'f', fp); | |
754 | else | |
755 | fputc ('f', fp); | |
756 | @} | |
757 | @end smallexample | |
758 | ||
759 | Note that in this case the function @code{print_f} decides about the | |
760 | orientation of the stream if it was unoriented before (will not happen | |
761 | if the advise above is followed). | |
762 | ||
763 | The encoding used for the @code{wchar_t} values is unspecified and the | |
764 | user must not make any assumptions about it. For I/O of @code{wchar_t} | |
765 | values this means that it is impossible to write these values directly | |
766 | to the stream. This is not what follows from the @w{ISO C} locale model | |
767 | either. What happens instead is that the bytes read from or written to | |
768 | the underlying media are first converted into the internal encoding | |
769 | chosen by the implementation for @code{wchar_t}. The external encoding | |
770 | is determined by the @code{LC_CTYPE} category of the current locale or | |
771 | by the @samp{ccs} part of the mode specification given to @code{fopen}, | |
772 | @code{fopen64}, @code{freopen}, or @code{freopen64}. How and when the | |
773 | conversion happens is unspecified and it happens invisible to the user. | |
774 | ||
775 | Since a stream is created in the unoriented state it has at that point | |
776 | no conversion associated with it. The conversion which will be used is | |
777 | determined by the @code{LC_CTYPE} category selected at the time the | |
778 | stream is oriented. If the locales are changed at the runtime this | |
779 | might produce surprising results unless one pays attention. This is | |
780 | just another good reason to orient the stream explicitly as soon as | |
781 | possible, perhaps with a call to @code{fwide}. | |
782 | ||
28f540f4 RM |
783 | @node Simple Output |
784 | @section Simple Output by Characters or Lines | |
785 | ||
786 | @cindex writing to a stream, by characters | |
787 | This section describes functions for performing character- and | |
788 | line-oriented output. | |
789 | ||
b5e73f56 UD |
790 | These narrow streams functions are declared in the header file |
791 | @file{stdio.h} and the wide stream functions in @file{wchar.h}. | |
28f540f4 | 792 | @pindex stdio.h |
b5e73f56 | 793 | @pindex wchar.h |
28f540f4 RM |
794 | |
795 | @comment stdio.h | |
f65fd747 | 796 | @comment ISO |
28f540f4 RM |
797 | @deftypefun int fputc (int @var{c}, FILE *@var{stream}) |
798 | The @code{fputc} function converts the character @var{c} to type | |
19c3f208 | 799 | @code{unsigned char}, and writes it to the stream @var{stream}. |
28f540f4 RM |
800 | @code{EOF} is returned if a write error occurs; otherwise the |
801 | character @var{c} is returned. | |
802 | @end deftypefun | |
803 | ||
b5e73f56 UD |
804 | @comment wchar.h |
805 | @comment ISO | |
806 | @deftypefun wint_t fputwc (wchar_t @var{wc}, FILE *@var{stream}) | |
807 | The @code{fputwc} function writes the wide character @var{wc} to the | |
808 | stream @var{stream}. @code{WEOF} is returned if a write error occurs; | |
809 | otherwise the character @var{wc} is returned. | |
810 | @end deftypefun | |
811 | ||
7b4161bb UD |
812 | @comment stdio.h |
813 | @comment POSIX | |
814 | @deftypefun int fputc_unlocked (int @var{c}, FILE *@var{stream}) | |
815 | The @code{fputc_unlocked} function is equivalent to the @code{fputc} | |
1dc843f7 | 816 | function except that it does not implicitly lock the stream. |
7b4161bb UD |
817 | @end deftypefun |
818 | ||
b5e73f56 UD |
819 | @comment wchar.h |
820 | @comment POSIX | |
821 | @deftypefun wint_t fputwc_unlocked (wint_t @var{wc}, FILE *@var{stream}) | |
822 | The @code{fputwc_unlocked} function is equivalent to the @code{fputwc} | |
1dc843f7 | 823 | function except that it does not implicitly lock the stream. |
b5e73f56 UD |
824 | |
825 | This function is a GNU extension. | |
826 | @end deftypefun | |
827 | ||
28f540f4 | 828 | @comment stdio.h |
f65fd747 | 829 | @comment ISO |
28f540f4 RM |
830 | @deftypefun int putc (int @var{c}, FILE *@var{stream}) |
831 | This is just like @code{fputc}, except that most systems implement it as | |
832 | a macro, making it faster. One consequence is that it may evaluate the | |
833 | @var{stream} argument more than once, which is an exception to the | |
834 | general rule for macros. @code{putc} is usually the best function to | |
835 | use for writing a single character. | |
836 | @end deftypefun | |
837 | ||
b5e73f56 UD |
838 | @comment wchar.h |
839 | @comment ISO | |
840 | @deftypefun wint_t putwc (wchar_t @var{wc}, FILE *@var{stream}) | |
841 | This is just like @code{fputwc}, except that it can be implement as | |
842 | a macro, making it faster. One consequence is that it may evaluate the | |
843 | @var{stream} argument more than once, which is an exception to the | |
844 | general rule for macros. @code{putwc} is usually the best function to | |
845 | use for writing a single wide character. | |
846 | @end deftypefun | |
847 | ||
7b4161bb UD |
848 | @comment stdio.h |
849 | @comment POSIX | |
850 | @deftypefun int putc_unlocked (int @var{c}, FILE *@var{stream}) | |
851 | The @code{putc_unlocked} function is equivalent to the @code{putc} | |
1dc843f7 | 852 | function except that it does not implicitly lock the stream. |
7b4161bb UD |
853 | @end deftypefun |
854 | ||
b5e73f56 UD |
855 | @comment wchar.h |
856 | @comment GNU | |
857 | @deftypefun wint_t putwc_unlocked (wchar_t @var{wc}, FILE *@var{stream}) | |
858 | The @code{putwc_unlocked} function is equivalent to the @code{putwc} | |
1dc843f7 | 859 | function except that it does not implicitly lock the stream. |
b5e73f56 UD |
860 | |
861 | This function is a GNU extension. | |
862 | @end deftypefun | |
863 | ||
28f540f4 | 864 | @comment stdio.h |
f65fd747 | 865 | @comment ISO |
28f540f4 RM |
866 | @deftypefun int putchar (int @var{c}) |
867 | The @code{putchar} function is equivalent to @code{putc} with | |
868 | @code{stdout} as the value of the @var{stream} argument. | |
869 | @end deftypefun | |
870 | ||
b5e73f56 UD |
871 | @comment wchar.h |
872 | @comment ISO | |
c44a663d | 873 | @deftypefun wint_t putwchar (wchar_t @var{wc}) |
b5e73f56 UD |
874 | The @code{putwchar} function is equivalent to @code{putwc} with |
875 | @code{stdout} as the value of the @var{stream} argument. | |
876 | @end deftypefun | |
877 | ||
7b4161bb UD |
878 | @comment stdio.h |
879 | @comment POSIX | |
880 | @deftypefun int putchar_unlocked (int @var{c}) | |
881 | The @code{putchar_unlocked} function is equivalent to the @code{putchar} | |
1dc843f7 | 882 | function except that it does not implicitly lock the stream. |
7b4161bb UD |
883 | @end deftypefun |
884 | ||
b5e73f56 UD |
885 | @comment wchar.h |
886 | @comment GNU | |
887 | @deftypefun wint_t putwchar_unlocked (wchar_t @var{wc}) | |
888 | The @code{putwchar_unlocked} function is equivalent to the @code{putwchar} | |
1dc843f7 | 889 | function except that it does not implicitly lock the stream. |
b5e73f56 UD |
890 | |
891 | This function is a GNU extension. | |
892 | @end deftypefun | |
893 | ||
28f540f4 | 894 | @comment stdio.h |
f65fd747 | 895 | @comment ISO |
28f540f4 RM |
896 | @deftypefun int fputs (const char *@var{s}, FILE *@var{stream}) |
897 | The function @code{fputs} writes the string @var{s} to the stream | |
898 | @var{stream}. The terminating null character is not written. | |
899 | This function does @emph{not} add a newline character, either. | |
900 | It outputs only the characters in the string. | |
901 | ||
902 | This function returns @code{EOF} if a write error occurs, and otherwise | |
903 | a non-negative value. | |
904 | ||
905 | For example: | |
906 | ||
907 | @smallexample | |
908 | fputs ("Are ", stdout); | |
909 | fputs ("you ", stdout); | |
910 | fputs ("hungry?\n", stdout); | |
911 | @end smallexample | |
912 | ||
913 | @noindent | |
914 | outputs the text @samp{Are you hungry?} followed by a newline. | |
915 | @end deftypefun | |
916 | ||
b5e73f56 UD |
917 | @comment wchar.h |
918 | @comment ISO | |
919 | @deftypefun int fputws (const wchar_t *@var{ws}, FILE *@var{stream}) | |
920 | The function @code{fputws} writes the wide character string @var{ws} to | |
921 | the stream @var{stream}. The terminating null character is not written. | |
922 | This function does @emph{not} add a newline character, either. It | |
923 | outputs only the characters in the string. | |
924 | ||
925 | This function returns @code{WEOF} if a write error occurs, and otherwise | |
926 | a non-negative value. | |
927 | @end deftypefun | |
928 | ||
7b4161bb UD |
929 | @comment stdio.h |
930 | @comment GNU | |
931 | @deftypefun int fputs_unlocked (const char *@var{s}, FILE *@var{stream}) | |
932 | The @code{fputs_unlocked} function is equivalent to the @code{fputs} | |
1dc843f7 | 933 | function except that it does not implicitly lock the stream. |
7b4161bb UD |
934 | |
935 | This function is a GNU extension. | |
936 | @end deftypefun | |
937 | ||
b5e73f56 UD |
938 | @comment wchar.h |
939 | @comment GNU | |
940 | @deftypefun int fputws_unlocked (const wchar_t *@var{ws}, FILE *@var{stream}) | |
941 | The @code{fputws_unlocked} function is equivalent to the @code{fputws} | |
1dc843f7 | 942 | function except that it does not implicitly lock the stream. |
b5e73f56 UD |
943 | |
944 | This function is a GNU extension. | |
945 | @end deftypefun | |
946 | ||
28f540f4 | 947 | @comment stdio.h |
f65fd747 | 948 | @comment ISO |
28f540f4 RM |
949 | @deftypefun int puts (const char *@var{s}) |
950 | The @code{puts} function writes the string @var{s} to the stream | |
951 | @code{stdout} followed by a newline. The terminating null character of | |
952 | the string is not written. (Note that @code{fputs} does @emph{not} | |
953 | write a newline as this function does.) | |
954 | ||
955 | @code{puts} is the most convenient function for printing simple | |
956 | messages. For example: | |
957 | ||
958 | @smallexample | |
959 | puts ("This is a message."); | |
960 | @end smallexample | |
0be8752b UD |
961 | |
962 | @noindent | |
963 | outputs the text @samp{This is a message.} followed by a newline. | |
28f540f4 RM |
964 | @end deftypefun |
965 | ||
966 | @comment stdio.h | |
967 | @comment SVID | |
968 | @deftypefun int putw (int @var{w}, FILE *@var{stream}) | |
969 | This function writes the word @var{w} (that is, an @code{int}) to | |
970 | @var{stream}. It is provided for compatibility with SVID, but we | |
971 | recommend you use @code{fwrite} instead (@pxref{Block Input/Output}). | |
972 | @end deftypefun | |
973 | ||
974 | @node Character Input | |
975 | @section Character Input | |
976 | ||
977 | @cindex reading from a stream, by characters | |
b5e73f56 UD |
978 | This section describes functions for performing character-oriented |
979 | input. These narrow streams functions are declared in the header file | |
980 | @file{stdio.h} and the wide character functions are declared in | |
981 | @file{wchar.h}. | |
28f540f4 | 982 | @pindex stdio.h |
b5e73f56 UD |
983 | @pindex wchar.h |
984 | ||
985 | These functions return an @code{int} or @code{wint_t} value (for narrow | |
986 | and wide stream functions respectively) that is either a character of | |
987 | input, or the special value @code{EOF}/@code{WEOF} (usually -1). For | |
988 | the narrow stream functions it is important to store the result of these | |
989 | functions in a variable of type @code{int} instead of @code{char}, even | |
990 | when you plan to use it only as a character. Storing @code{EOF} in a | |
991 | @code{char} variable truncates its value to the size of a character, so | |
992 | that it is no longer distinguishable from the valid character | |
993 | @samp{(char) -1}. So always use an @code{int} for the result of | |
994 | @code{getc} and friends, and check for @code{EOF} after the call; once | |
995 | you've verified that the result is not @code{EOF}, you can be sure that | |
996 | it will fit in a @samp{char} variable without loss of information. | |
28f540f4 RM |
997 | |
998 | @comment stdio.h | |
f65fd747 | 999 | @comment ISO |
28f540f4 RM |
1000 | @deftypefun int fgetc (FILE *@var{stream}) |
1001 | This function reads the next character as an @code{unsigned char} from | |
1002 | the stream @var{stream} and returns its value, converted to an | |
1003 | @code{int}. If an end-of-file condition or read error occurs, | |
19c3f208 | 1004 | @code{EOF} is returned instead. |
28f540f4 RM |
1005 | @end deftypefun |
1006 | ||
b5e73f56 UD |
1007 | @comment wchar.h |
1008 | @comment ISO | |
1009 | @deftypefun wint_t fgetwc (FILE *@var{stream}) | |
1010 | This function reads the next wide character from the stream @var{stream} | |
1011 | and returns its value. If an end-of-file condition or read error | |
1012 | occurs, @code{WEOF} is returned instead. | |
1013 | @end deftypefun | |
1014 | ||
7b4161bb UD |
1015 | @comment stdio.h |
1016 | @comment POSIX | |
1017 | @deftypefun int fgetc_unlocked (FILE *@var{stream}) | |
1018 | The @code{fgetc_unlocked} function is equivalent to the @code{fgetc} | |
1dc843f7 | 1019 | function except that it does not implicitly lock the stream. |
7b4161bb UD |
1020 | @end deftypefun |
1021 | ||
b5e73f56 UD |
1022 | @comment wchar.h |
1023 | @comment GNU | |
1024 | @deftypefun wint_t fgetwc_unlocked (FILE *@var{stream}) | |
1025 | The @code{fgetwc_unlocked} function is equivalent to the @code{fgetwc} | |
1dc843f7 | 1026 | function except that it does not implicitly lock the stream. |
b5e73f56 UD |
1027 | |
1028 | This function is a GNU extension. | |
1029 | @end deftypefun | |
1030 | ||
28f540f4 | 1031 | @comment stdio.h |
f65fd747 | 1032 | @comment ISO |
28f540f4 RM |
1033 | @deftypefun int getc (FILE *@var{stream}) |
1034 | This is just like @code{fgetc}, except that it is permissible (and | |
1035 | typical) for it to be implemented as a macro that evaluates the | |
1036 | @var{stream} argument more than once. @code{getc} is often highly | |
1037 | optimized, so it is usually the best function to use to read a single | |
1038 | character. | |
1039 | @end deftypefun | |
1040 | ||
b5e73f56 UD |
1041 | @comment wchar.h |
1042 | @comment ISO | |
1043 | @deftypefun wint_t getwc (FILE *@var{stream}) | |
1044 | This is just like @code{fgetwc}, except that it is permissible for it to | |
1045 | be implemented as a macro that evaluates the @var{stream} argument more | |
1046 | than once. @code{getwc} can be highly optimized, so it is usually the | |
1047 | best function to use to read a single wide character. | |
1048 | @end deftypefun | |
1049 | ||
7b4161bb UD |
1050 | @comment stdio.h |
1051 | @comment POSIX | |
1052 | @deftypefun int getc_unlocked (FILE *@var{stream}) | |
1053 | The @code{getc_unlocked} function is equivalent to the @code{getc} | |
1dc843f7 | 1054 | function except that it does not implicitly lock the stream. |
7b4161bb UD |
1055 | @end deftypefun |
1056 | ||
b5e73f56 UD |
1057 | @comment wchar.h |
1058 | @comment GNU | |
1059 | @deftypefun wint_t getwc_unlocked (FILE *@var{stream}) | |
1060 | The @code{getwc_unlocked} function is equivalent to the @code{getwc} | |
1dc843f7 | 1061 | function except that it does not implicitly lock the stream. |
b5e73f56 UD |
1062 | |
1063 | This function is a GNU extension. | |
1064 | @end deftypefun | |
1065 | ||
28f540f4 | 1066 | @comment stdio.h |
f65fd747 | 1067 | @comment ISO |
28f540f4 RM |
1068 | @deftypefun int getchar (void) |
1069 | The @code{getchar} function is equivalent to @code{getc} with @code{stdin} | |
1070 | as the value of the @var{stream} argument. | |
1071 | @end deftypefun | |
1072 | ||
b5e73f56 UD |
1073 | @comment wchar.h |
1074 | @comment ISO | |
1075 | @deftypefun wint_t getwchar (void) | |
1076 | The @code{getwchar} function is equivalent to @code{getwc} with @code{stdin} | |
1077 | as the value of the @var{stream} argument. | |
1078 | @end deftypefun | |
1079 | ||
7b4161bb UD |
1080 | @comment stdio.h |
1081 | @comment POSIX | |
1082 | @deftypefun int getchar_unlocked (void) | |
1083 | The @code{getchar_unlocked} function is equivalent to the @code{getchar} | |
1dc843f7 | 1084 | function except that it does not implicitly lock the stream. |
7b4161bb UD |
1085 | @end deftypefun |
1086 | ||
b5e73f56 UD |
1087 | @comment wchar.h |
1088 | @comment GNU | |
1089 | @deftypefun wint_t getwchar_unlocked (void) | |
1090 | The @code{getwchar_unlocked} function is equivalent to the @code{getwchar} | |
1dc843f7 | 1091 | function except that it does not implicitly lock the stream. |
b5e73f56 UD |
1092 | |
1093 | This function is a GNU extension. | |
1094 | @end deftypefun | |
1095 | ||
28f540f4 RM |
1096 | Here is an example of a function that does input using @code{fgetc}. It |
1097 | would work just as well using @code{getc} instead, or using | |
b5e73f56 UD |
1098 | @code{getchar ()} instead of @w{@code{fgetc (stdin)}}. The code would |
1099 | also work the same for the wide character stream functions. | |
28f540f4 RM |
1100 | |
1101 | @smallexample | |
1102 | int | |
1103 | y_or_n_p (const char *question) | |
1104 | @{ | |
1105 | fputs (question, stdout); | |
1106 | while (1) | |
1107 | @{ | |
1108 | int c, answer; | |
1109 | /* @r{Write a space to separate answer from question.} */ | |
1110 | fputc (' ', stdout); | |
1111 | /* @r{Read the first character of the line.} | |
1112 | @r{This should be the answer character, but might not be.} */ | |
1113 | c = tolower (fgetc (stdin)); | |
1114 | answer = c; | |
1115 | /* @r{Discard rest of input line.} */ | |
1116 | while (c != '\n' && c != EOF) | |
1117 | c = fgetc (stdin); | |
1118 | /* @r{Obey the answer if it was valid.} */ | |
1119 | if (answer == 'y') | |
1120 | return 1; | |
1121 | if (answer == 'n') | |
1122 | return 0; | |
1123 | /* @r{Answer was invalid: ask for valid answer.} */ | |
1124 | fputs ("Please answer y or n:", stdout); | |
1125 | @} | |
1126 | @} | |
1127 | @end smallexample | |
1128 | ||
1129 | @comment stdio.h | |
1130 | @comment SVID | |
1131 | @deftypefun int getw (FILE *@var{stream}) | |
1132 | This function reads a word (that is, an @code{int}) from @var{stream}. | |
1133 | It's provided for compatibility with SVID. We recommend you use | |
1134 | @code{fread} instead (@pxref{Block Input/Output}). Unlike @code{getc}, | |
1135 | any @code{int} value could be a valid result. @code{getw} returns | |
1136 | @code{EOF} when it encounters end-of-file or an error, but there is no | |
1137 | way to distinguish this from an input word with value -1. | |
1138 | @end deftypefun | |
1139 | ||
1140 | @node Line Input | |
1141 | @section Line-Oriented Input | |
1142 | ||
b5e73f56 | 1143 | Since many programs interpret input on the basis of lines, it is |
28f540f4 RM |
1144 | convenient to have functions to read a line of text from a stream. |
1145 | ||
1146 | Standard C has functions to do this, but they aren't very safe: null | |
1147 | characters and even (for @code{gets}) long lines can confuse them. So | |
1148 | the GNU library provides the nonstandard @code{getline} function that | |
1149 | makes it easy to read lines reliably. | |
1150 | ||
1151 | Another GNU extension, @code{getdelim}, generalizes @code{getline}. It | |
1152 | reads a delimited record, defined as everything through the next | |
1153 | occurrence of a specified delimiter character. | |
1154 | ||
1155 | All these functions are declared in @file{stdio.h}. | |
1156 | ||
1157 | @comment stdio.h | |
1158 | @comment GNU | |
1159 | @deftypefun ssize_t getline (char **@var{lineptr}, size_t *@var{n}, FILE *@var{stream}) | |
1160 | This function reads an entire line from @var{stream}, storing the text | |
1161 | (including the newline and a terminating null character) in a buffer | |
1162 | and storing the buffer address in @code{*@var{lineptr}}. | |
1163 | ||
1164 | Before calling @code{getline}, you should place in @code{*@var{lineptr}} | |
1165 | the address of a buffer @code{*@var{n}} bytes long, allocated with | |
1166 | @code{malloc}. If this buffer is long enough to hold the line, | |
1167 | @code{getline} stores the line in this buffer. Otherwise, | |
1168 | @code{getline} makes the buffer bigger using @code{realloc}, storing the | |
1169 | new buffer address back in @code{*@var{lineptr}} and the increased size | |
1170 | back in @code{*@var{n}}. | |
1171 | @xref{Unconstrained Allocation}. | |
1172 | ||
1173 | If you set @code{*@var{lineptr}} to a null pointer, and @code{*@var{n}} | |
1174 | to zero, before the call, then @code{getline} allocates the initial | |
1175 | buffer for you by calling @code{malloc}. | |
1176 | ||
1177 | In either case, when @code{getline} returns, @code{*@var{lineptr}} is | |
1178 | a @code{char *} which points to the text of the line. | |
1179 | ||
1180 | When @code{getline} is successful, it returns the number of characters | |
1181 | read (including the newline, but not including the terminating null). | |
1182 | This value enables you to distinguish null characters that are part of | |
1183 | the line from the null character inserted as a terminator. | |
1184 | ||
1185 | This function is a GNU extension, but it is the recommended way to read | |
1186 | lines from a stream. The alternative standard functions are unreliable. | |
1187 | ||
ec28fc7c UD |
1188 | If an error occurs or end of file is reached without any bytes read, |
1189 | @code{getline} returns @code{-1}. | |
28f540f4 RM |
1190 | @end deftypefun |
1191 | ||
1192 | @comment stdio.h | |
1193 | @comment GNU | |
1194 | @deftypefun ssize_t getdelim (char **@var{lineptr}, size_t *@var{n}, int @var{delimiter}, FILE *@var{stream}) | |
1195 | This function is like @code{getline} except that the character which | |
1196 | tells it to stop reading is not necessarily newline. The argument | |
1197 | @var{delimiter} specifies the delimiter character; @code{getdelim} keeps | |
1198 | reading until it sees that character (or end of file). | |
1199 | ||
1200 | The text is stored in @var{lineptr}, including the delimiter character | |
1201 | and a terminating null. Like @code{getline}, @code{getdelim} makes | |
1202 | @var{lineptr} bigger if it isn't big enough. | |
1203 | ||
1204 | @code{getline} is in fact implemented in terms of @code{getdelim}, just | |
1205 | like this: | |
1206 | ||
1207 | @smallexample | |
1208 | ssize_t | |
1209 | getline (char **lineptr, size_t *n, FILE *stream) | |
1210 | @{ | |
1211 | return getdelim (lineptr, n, '\n', stream); | |
1212 | @} | |
1213 | @end smallexample | |
1214 | @end deftypefun | |
1215 | ||
1216 | @comment stdio.h | |
f65fd747 | 1217 | @comment ISO |
28f540f4 RM |
1218 | @deftypefun {char *} fgets (char *@var{s}, int @var{count}, FILE *@var{stream}) |
1219 | The @code{fgets} function reads characters from the stream @var{stream} | |
1220 | up to and including a newline character and stores them in the string | |
1221 | @var{s}, adding a null character to mark the end of the string. You | |
1222 | must supply @var{count} characters worth of space in @var{s}, but the | |
1223 | number of characters read is at most @var{count} @minus{} 1. The extra | |
1224 | character space is used to hold the null character at the end of the | |
1225 | string. | |
1226 | ||
1227 | If the system is already at end of file when you call @code{fgets}, then | |
1228 | the contents of the array @var{s} are unchanged and a null pointer is | |
1229 | returned. A null pointer is also returned if a read error occurs. | |
1230 | Otherwise, the return value is the pointer @var{s}. | |
1231 | ||
1232 | @strong{Warning:} If the input data has a null character, you can't tell. | |
1233 | So don't use @code{fgets} unless you know the data cannot contain a null. | |
1234 | Don't use it to read files edited by the user because, if the user inserts | |
1235 | a null character, you should either handle it properly or print a clear | |
1236 | error message. We recommend using @code{getline} instead of @code{fgets}. | |
1237 | @end deftypefun | |
1238 | ||
b5e73f56 UD |
1239 | @comment wchar.h |
1240 | @comment ISO | |
1241 | @deftypefun {wchar_t *} fgetws (wchar_t *@var{ws}, int @var{count}, FILE *@var{stream}) | |
1242 | The @code{fgetws} function reads wide characters from the stream | |
1243 | @var{stream} up to and including a newline character and stores them in | |
1244 | the string @var{ws}, adding a null wide character to mark the end of the | |
1245 | string. You must supply @var{count} wide characters worth of space in | |
1246 | @var{ws}, but the number of characters read is at most @var{count} | |
1247 | @minus{} 1. The extra character space is used to hold the null wide | |
1248 | character at the end of the string. | |
1249 | ||
1250 | If the system is already at end of file when you call @code{fgetws}, then | |
1251 | the contents of the array @var{ws} are unchanged and a null pointer is | |
1252 | returned. A null pointer is also returned if a read error occurs. | |
1253 | Otherwise, the return value is the pointer @var{ws}. | |
1254 | ||
1255 | @strong{Warning:} If the input data has a null wide character (which are | |
1256 | null bytes in the input stream), you can't tell. So don't use | |
1257 | @code{fgetws} unless you know the data cannot contain a null. Don't use | |
1258 | it to read files edited by the user because, if the user inserts a null | |
1259 | character, you should either handle it properly or print a clear error | |
1260 | message. | |
1261 | @comment XXX We need getwline!!! | |
1262 | @end deftypefun | |
1263 | ||
7b4161bb UD |
1264 | @comment stdio.h |
1265 | @comment GNU | |
1266 | @deftypefun {char *} fgets_unlocked (char *@var{s}, int @var{count}, FILE *@var{stream}) | |
1267 | The @code{fgets_unlocked} function is equivalent to the @code{fgets} | |
1dc843f7 | 1268 | function except that it does not implicitly lock the stream. |
7b4161bb UD |
1269 | |
1270 | This function is a GNU extension. | |
1271 | @end deftypefun | |
1272 | ||
b5e73f56 UD |
1273 | @comment wchar.h |
1274 | @comment GNU | |
1275 | @deftypefun {wchar_t *} fgetws_unlocked (wchar_t *@var{ws}, int @var{count}, FILE *@var{stream}) | |
1276 | The @code{fgetws_unlocked} function is equivalent to the @code{fgetws} | |
1dc843f7 | 1277 | function except that it does not implicitly lock the stream. |
b5e73f56 UD |
1278 | |
1279 | This function is a GNU extension. | |
1280 | @end deftypefun | |
1281 | ||
28f540f4 | 1282 | @comment stdio.h |
f65fd747 | 1283 | @comment ISO |
28f540f4 RM |
1284 | @deftypefn {Deprecated function} {char *} gets (char *@var{s}) |
1285 | The function @code{gets} reads characters from the stream @code{stdin} | |
1286 | up to the next newline character, and stores them in the string @var{s}. | |
1287 | The newline character is discarded (note that this differs from the | |
1288 | behavior of @code{fgets}, which copies the newline character into the | |
1289 | string). If @code{gets} encounters a read error or end-of-file, it | |
1290 | returns a null pointer; otherwise it returns @var{s}. | |
1291 | ||
1292 | @strong{Warning:} The @code{gets} function is @strong{very dangerous} | |
1293 | because it provides no protection against overflowing the string | |
1294 | @var{s}. The GNU library includes it for compatibility only. You | |
1295 | should @strong{always} use @code{fgets} or @code{getline} instead. To | |
1296 | remind you of this, the linker (if using GNU @code{ld}) will issue a | |
1297 | warning whenever you use @code{gets}. | |
1298 | @end deftypefn | |
1299 | ||
1300 | @node Unreading | |
1301 | @section Unreading | |
1302 | @cindex peeking at input | |
1303 | @cindex unreading characters | |
1304 | @cindex pushing input back | |
1305 | ||
1306 | In parser programs it is often useful to examine the next character in | |
1307 | the input stream without removing it from the stream. This is called | |
1308 | ``peeking ahead'' at the input because your program gets a glimpse of | |
1309 | the input it will read next. | |
1310 | ||
1311 | Using stream I/O, you can peek ahead at input by first reading it and | |
19c3f208 | 1312 | then @dfn{unreading} it (also called @dfn{pushing it back} on the stream). |
28f540f4 RM |
1313 | Unreading a character makes it available to be input again from the stream, |
1314 | by the next call to @code{fgetc} or other input function on that stream. | |
1315 | ||
1316 | @menu | |
1317 | * Unreading Idea:: An explanation of unreading with pictures. | |
1318 | * How Unread:: How to call @code{ungetc} to do unreading. | |
1319 | @end menu | |
1320 | ||
1321 | @node Unreading Idea | |
1322 | @subsection What Unreading Means | |
1323 | ||
1324 | Here is a pictorial explanation of unreading. Suppose you have a | |
1325 | stream reading a file that contains just six characters, the letters | |
1326 | @samp{foobar}. Suppose you have read three characters so far. The | |
1327 | situation looks like this: | |
1328 | ||
1329 | @smallexample | |
1330 | f o o b a r | |
1331 | ^ | |
1332 | @end smallexample | |
1333 | ||
1334 | @noindent | |
1335 | so the next input character will be @samp{b}. | |
1336 | ||
1337 | @c @group Invalid outside @example | |
1338 | If instead of reading @samp{b} you unread the letter @samp{o}, you get a | |
1339 | situation like this: | |
1340 | ||
1341 | @smallexample | |
1342 | f o o b a r | |
1343 | | | |
1344 | o-- | |
1345 | ^ | |
1346 | @end smallexample | |
1347 | ||
1348 | @noindent | |
1349 | so that the next input characters will be @samp{o} and @samp{b}. | |
1350 | @c @end group | |
1351 | ||
1352 | @c @group | |
1353 | If you unread @samp{9} instead of @samp{o}, you get this situation: | |
1354 | ||
1355 | @smallexample | |
1356 | f o o b a r | |
1357 | | | |
1358 | 9-- | |
1359 | ^ | |
1360 | @end smallexample | |
1361 | ||
1362 | @noindent | |
1363 | so that the next input characters will be @samp{9} and @samp{b}. | |
1364 | @c @end group | |
1365 | ||
1366 | @node How Unread | |
1367 | @subsection Using @code{ungetc} To Do Unreading | |
19c3f208 | 1368 | |
28f540f4 RM |
1369 | The function to unread a character is called @code{ungetc}, because it |
1370 | reverses the action of @code{getc}. | |
1371 | ||
1372 | @comment stdio.h | |
f65fd747 | 1373 | @comment ISO |
28f540f4 RM |
1374 | @deftypefun int ungetc (int @var{c}, FILE *@var{stream}) |
1375 | The @code{ungetc} function pushes back the character @var{c} onto the | |
1376 | input stream @var{stream}. So the next input from @var{stream} will | |
1377 | read @var{c} before anything else. | |
1378 | ||
1379 | If @var{c} is @code{EOF}, @code{ungetc} does nothing and just returns | |
1380 | @code{EOF}. This lets you call @code{ungetc} with the return value of | |
1381 | @code{getc} without needing to check for an error from @code{getc}. | |
1382 | ||
1383 | The character that you push back doesn't have to be the same as the last | |
1384 | character that was actually read from the stream. In fact, it isn't | |
1385 | necessary to actually read any characters from the stream before | |
f4efd068 UD |
1386 | unreading them with @code{ungetc}! But that is a strange way to write a |
1387 | program; usually @code{ungetc} is used only to unread a character that | |
1388 | was just read from the same stream. The GNU C library supports this | |
1389 | even on files opened in binary mode, but other systems might not. | |
28f540f4 RM |
1390 | |
1391 | The GNU C library only supports one character of pushback---in other | |
1392 | words, it does not work to call @code{ungetc} twice without doing input | |
1393 | in between. Other systems might let you push back multiple characters; | |
1394 | then reading from the stream retrieves the characters in the reverse | |
1395 | order that they were pushed. | |
1396 | ||
1397 | Pushing back characters doesn't alter the file; only the internal | |
1398 | buffering for the stream is affected. If a file positioning function | |
a5a0310d UD |
1399 | (such as @code{fseek}, @code{fseeko} or @code{rewind}; @pxref{File |
1400 | Positioning}) is called, any pending pushed-back characters are | |
1401 | discarded. | |
28f540f4 RM |
1402 | |
1403 | Unreading a character on a stream that is at end of file clears the | |
1404 | end-of-file indicator for the stream, because it makes the character of | |
1405 | input available. After you read that character, trying to read again | |
1406 | will encounter end of file. | |
1407 | @end deftypefun | |
1408 | ||
b5e73f56 UD |
1409 | @comment wchar.h |
1410 | @comment ISO | |
1411 | @deftypefun wint_t ungetwc (wint_t @var{wc}, FILE *@var{stream}) | |
1412 | The @code{ungetwc} function behaves just like @code{ungetc} just that it | |
1413 | pushes back a wide character. | |
1414 | @end deftypefun | |
1415 | ||
28f540f4 RM |
1416 | Here is an example showing the use of @code{getc} and @code{ungetc} to |
1417 | skip over whitespace characters. When this function reaches a | |
1418 | non-whitespace character, it unreads that character to be seen again on | |
1419 | the next read operation on the stream. | |
1420 | ||
1421 | @smallexample | |
1422 | #include <stdio.h> | |
1423 | #include <ctype.h> | |
1424 | ||
1425 | void | |
1426 | skip_whitespace (FILE *stream) | |
1427 | @{ | |
1428 | int c; | |
1429 | do | |
1430 | /* @r{No need to check for @code{EOF} because it is not} | |
1431 | @r{@code{isspace}, and @code{ungetc} ignores @code{EOF}.} */ | |
1432 | c = getc (stream); | |
1433 | while (isspace (c)); | |
1434 | ungetc (c, stream); | |
1435 | @} | |
1436 | @end smallexample | |
1437 | ||
1438 | @node Block Input/Output | |
1439 | @section Block Input/Output | |
1440 | ||
1441 | This section describes how to do input and output operations on blocks | |
1442 | of data. You can use these functions to read and write binary data, as | |
1443 | well as to read and write text in fixed-size blocks instead of by | |
1444 | characters or lines. | |
1445 | @cindex binary I/O to a stream | |
1446 | @cindex block I/O to a stream | |
1447 | @cindex reading from a stream, by blocks | |
1448 | @cindex writing to a stream, by blocks | |
1449 | ||
1450 | Binary files are typically used to read and write blocks of data in the | |
1451 | same format as is used to represent the data in a running program. In | |
1452 | other words, arbitrary blocks of memory---not just character or string | |
1453 | objects---can be written to a binary file, and meaningfully read in | |
1454 | again by the same program. | |
1455 | ||
1456 | Storing data in binary form is often considerably more efficient than | |
1457 | using the formatted I/O functions. Also, for floating-point numbers, | |
1458 | the binary form avoids possible loss of precision in the conversion | |
1459 | process. On the other hand, binary files can't be examined or modified | |
1460 | easily using many standard file utilities (such as text editors), and | |
1461 | are not portable between different implementations of the language, or | |
1462 | different kinds of computers. | |
1463 | ||
1464 | These functions are declared in @file{stdio.h}. | |
1465 | @pindex stdio.h | |
1466 | ||
1467 | @comment stdio.h | |
f65fd747 | 1468 | @comment ISO |
28f540f4 RM |
1469 | @deftypefun size_t fread (void *@var{data}, size_t @var{size}, size_t @var{count}, FILE *@var{stream}) |
1470 | This function reads up to @var{count} objects of size @var{size} into | |
1471 | the array @var{data}, from the stream @var{stream}. It returns the | |
1472 | number of objects actually read, which might be less than @var{count} if | |
1473 | a read error occurs or the end of the file is reached. This function | |
1474 | returns a value of zero (and doesn't read anything) if either @var{size} | |
1475 | or @var{count} is zero. | |
1476 | ||
1477 | If @code{fread} encounters end of file in the middle of an object, it | |
1478 | returns the number of complete objects read, and discards the partial | |
1479 | object. Therefore, the stream remains at the actual end of the file. | |
1480 | @end deftypefun | |
1481 | ||
7b4161bb UD |
1482 | @comment stdio.h |
1483 | @comment GNU | |
1484 | @deftypefun size_t fread_unlocked (void *@var{data}, size_t @var{size}, size_t @var{count}, FILE *@var{stream}) | |
1485 | The @code{fread_unlocked} function is equivalent to the @code{fread} | |
1dc843f7 | 1486 | function except that it does not implicitly lock the stream. |
7b4161bb UD |
1487 | |
1488 | This function is a GNU extension. | |
1489 | @end deftypefun | |
1490 | ||
28f540f4 | 1491 | @comment stdio.h |
f65fd747 | 1492 | @comment ISO |
28f540f4 RM |
1493 | @deftypefun size_t fwrite (const void *@var{data}, size_t @var{size}, size_t @var{count}, FILE *@var{stream}) |
1494 | This function writes up to @var{count} objects of size @var{size} from | |
1495 | the array @var{data}, to the stream @var{stream}. The return value is | |
1496 | normally @var{count}, if the call succeeds. Any other value indicates | |
1497 | some sort of error, such as running out of space. | |
1498 | @end deftypefun | |
1499 | ||
7b4161bb UD |
1500 | @comment stdio.h |
1501 | @comment GNU | |
1502 | @deftypefun size_t fwrite_unlocked (const void *@var{data}, size_t @var{size}, size_t @var{count}, FILE *@var{stream}) | |
1503 | The @code{fwrite_unlocked} function is equivalent to the @code{fwrite} | |
1dc843f7 | 1504 | function except that it does not implicitly lock the stream. |
7b4161bb UD |
1505 | |
1506 | This function is a GNU extension. | |
1507 | @end deftypefun | |
1508 | ||
28f540f4 RM |
1509 | @node Formatted Output |
1510 | @section Formatted Output | |
1511 | ||
1512 | @cindex format string, for @code{printf} | |
1513 | @cindex template, for @code{printf} | |
1514 | @cindex formatted output to a stream | |
1515 | @cindex writing to a stream, formatted | |
1516 | The functions described in this section (@code{printf} and related | |
1517 | functions) provide a convenient way to perform formatted output. You | |
1518 | call @code{printf} with a @dfn{format string} or @dfn{template string} | |
1519 | that specifies how to format the values of the remaining arguments. | |
1520 | ||
1521 | Unless your program is a filter that specifically performs line- or | |
1522 | character-oriented processing, using @code{printf} or one of the other | |
1523 | related functions described in this section is usually the easiest and | |
1524 | most concise way to perform output. These functions are especially | |
1525 | useful for printing error messages, tables of data, and the like. | |
1526 | ||
1527 | @menu | |
1528 | * Formatted Output Basics:: Some examples to get you started. | |
1529 | * Output Conversion Syntax:: General syntax of conversion | |
1530 | specifications. | |
1531 | * Table of Output Conversions:: Summary of output conversions and | |
1532 | what they do. | |
1533 | * Integer Conversions:: Details about formatting of integers. | |
1534 | * Floating-Point Conversions:: Details about formatting of | |
1535 | floating-point numbers. | |
1536 | * Other Output Conversions:: Details about formatting of strings, | |
1537 | characters, pointers, and the like. | |
1538 | * Formatted Output Functions:: Descriptions of the actual functions. | |
1539 | * Dynamic Output:: Functions that allocate memory for the output. | |
1540 | * Variable Arguments Output:: @code{vprintf} and friends. | |
1541 | * Parsing a Template String:: What kinds of args does a given template | |
19c3f208 | 1542 | call for? |
28f540f4 RM |
1543 | * Example of Parsing:: Sample program using @code{parse_printf_format}. |
1544 | @end menu | |
1545 | ||
1546 | @node Formatted Output Basics | |
1547 | @subsection Formatted Output Basics | |
1548 | ||
1549 | The @code{printf} function can be used to print any number of arguments. | |
1550 | The template string argument you supply in a call provides | |
1551 | information not only about the number of additional arguments, but also | |
1552 | about their types and what style should be used for printing them. | |
1553 | ||
1554 | Ordinary characters in the template string are simply written to the | |
1555 | output stream as-is, while @dfn{conversion specifications} introduced by | |
1556 | a @samp{%} character in the template cause subsequent arguments to be | |
1557 | formatted and written to the output stream. For example, | |
1558 | @cindex conversion specifications (@code{printf}) | |
1559 | ||
1560 | @smallexample | |
1561 | int pct = 37; | |
1562 | char filename[] = "foo.txt"; | |
1563 | printf ("Processing of `%s' is %d%% finished.\nPlease be patient.\n", | |
1564 | filename, pct); | |
1565 | @end smallexample | |
1566 | ||
1567 | @noindent | |
1568 | produces output like | |
1569 | ||
1570 | @smallexample | |
1571 | Processing of `foo.txt' is 37% finished. | |
1572 | Please be patient. | |
1573 | @end smallexample | |
1574 | ||
1575 | This example shows the use of the @samp{%d} conversion to specify that | |
1576 | an @code{int} argument should be printed in decimal notation, the | |
1577 | @samp{%s} conversion to specify printing of a string argument, and | |
1578 | the @samp{%%} conversion to print a literal @samp{%} character. | |
1579 | ||
1580 | There are also conversions for printing an integer argument as an | |
1581 | unsigned value in octal, decimal, or hexadecimal radix (@samp{%o}, | |
1582 | @samp{%u}, or @samp{%x}, respectively); or as a character value | |
1583 | (@samp{%c}). | |
1584 | ||
1585 | Floating-point numbers can be printed in normal, fixed-point notation | |
1586 | using the @samp{%f} conversion or in exponential notation using the | |
1587 | @samp{%e} conversion. The @samp{%g} conversion uses either @samp{%e} | |
1588 | or @samp{%f} format, depending on what is more appropriate for the | |
1589 | magnitude of the particular number. | |
1590 | ||
1591 | You can control formatting more precisely by writing @dfn{modifiers} | |
1592 | between the @samp{%} and the character that indicates which conversion | |
1593 | to apply. These slightly alter the ordinary behavior of the conversion. | |
1594 | For example, most conversion specifications permit you to specify a | |
1595 | minimum field width and a flag indicating whether you want the result | |
1596 | left- or right-justified within the field. | |
1597 | ||
1598 | The specific flags and modifiers that are permitted and their | |
1599 | interpretation vary depending on the particular conversion. They're all | |
1600 | described in more detail in the following sections. Don't worry if this | |
1601 | all seems excessively complicated at first; you can almost always get | |
1602 | reasonable free-format output without using any of the modifiers at all. | |
1603 | The modifiers are mostly used to make the output look ``prettier'' in | |
1604 | tables. | |
1605 | ||
1606 | @node Output Conversion Syntax | |
1607 | @subsection Output Conversion Syntax | |
1608 | ||
1609 | This section provides details about the precise syntax of conversion | |
1610 | specifications that can appear in a @code{printf} template | |
1611 | string. | |
1612 | ||
390955cb UD |
1613 | Characters in the template string that are not part of a conversion |
1614 | specification are printed as-is to the output stream. Multibyte | |
1615 | character sequences (@pxref{Character Set Handling}) are permitted in a | |
1616 | template string. | |
28f540f4 RM |
1617 | |
1618 | The conversion specifications in a @code{printf} template string have | |
1619 | the general form: | |
1620 | ||
f89829da | 1621 | @smallexample |
dfd2257a | 1622 | % @r{[} @var{param-no} @r{$]} @var{flags} @var{width} @r{[} . @var{precision} @r{]} @var{type} @var{conversion} |
f89829da UD |
1623 | @end smallexample |
1624 | ||
1625 | @noindent | |
1626 | or | |
1627 | ||
1628 | @smallexample | |
1629 | % @r{[} @var{param-no} @r{$]} @var{flags} @var{width} . @r{*} @r{[} @var{param-no} @r{$]} @var{type} @var{conversion} | |
1630 | @end smallexample | |
28f540f4 RM |
1631 | |
1632 | For example, in the conversion specifier @samp{%-10.8ld}, the @samp{-} | |
1633 | is a flag, @samp{10} specifies the field width, the precision is | |
1634 | @samp{8}, the letter @samp{l} is a type modifier, and @samp{d} specifies | |
1635 | the conversion style. (This particular type specifier says to | |
1636 | print a @code{long int} argument in decimal notation, with a minimum of | |
1637 | 8 digits left-justified in a field at least 10 characters wide.) | |
1638 | ||
1639 | In more detail, output conversion specifications consist of an | |
1640 | initial @samp{%} character followed in sequence by: | |
1641 | ||
1642 | @itemize @bullet | |
dfd2257a UD |
1643 | @item |
1644 | An optional specification of the parameter used for this format. | |
e8b1163e | 1645 | Normally the parameters to the @code{printf} function are assigned to the |
f2ea0f5b | 1646 | formats in the order of appearance in the format string. But in some |
dfd2257a | 1647 | situations (such as message translation) this is not desirable and this |
e8b1163e | 1648 | extension allows an explicit parameter to be specified. |
dfd2257a | 1649 | |
f89829da | 1650 | The @var{param-no} parts of the format must be integers in the range of |
dfd2257a UD |
1651 | 1 to the maximum number of arguments present to the function call. Some |
1652 | implementations limit this number to a certainly upper bound. The exact | |
1653 | limit can be retrieved by the following constant. | |
1654 | ||
1655 | @defvr Macro NL_ARGMAX | |
f89829da | 1656 | The value of @code{NL_ARGMAX} is the maximum value allowed for the |
dfd2257a UD |
1657 | specification of an positional parameter in a @code{printf} call. The |
1658 | actual value in effect at runtime can be retrieved by using | |
1659 | @code{sysconf} using the @code{_SC_NL_ARGMAX} parameter @pxref{Sysconf | |
1660 | Definition}. | |
1661 | ||
1662 | Some system have a quite low limit such as @math{9} for @w{System V} | |
1663 | systems. The GNU C library has no real limit. | |
1664 | @end defvr | |
1665 | ||
1666 | If any of the formats has a specification for the parameter position all | |
0bc93a2f | 1667 | of them in the format string shall have one. Otherwise the behavior is |
dfd2257a UD |
1668 | undefined. |
1669 | ||
19c3f208 | 1670 | @item |
28f540f4 RM |
1671 | Zero or more @dfn{flag characters} that modify the normal behavior of |
1672 | the conversion specification. | |
1673 | @cindex flag character (@code{printf}) | |
1674 | ||
19c3f208 | 1675 | @item |
28f540f4 RM |
1676 | An optional decimal integer specifying the @dfn{minimum field width}. |
1677 | If the normal conversion produces fewer characters than this, the field | |
1678 | is padded with spaces to the specified width. This is a @emph{minimum} | |
1679 | value; if the normal conversion produces more characters than this, the | |
1680 | field is @emph{not} truncated. Normally, the output is right-justified | |
1681 | within the field. | |
1682 | @cindex minimum field width (@code{printf}) | |
1683 | ||
1684 | You can also specify a field width of @samp{*}. This means that the | |
1685 | next argument in the argument list (before the actual value to be | |
1686 | printed) is used as the field width. The value must be an @code{int}. | |
1687 | If the value is negative, this means to set the @samp{-} flag (see | |
1688 | below) and to use the absolute value as the field width. | |
1689 | ||
19c3f208 | 1690 | @item |
28f540f4 RM |
1691 | An optional @dfn{precision} to specify the number of digits to be |
1692 | written for the numeric conversions. If the precision is specified, it | |
1693 | consists of a period (@samp{.}) followed optionally by a decimal integer | |
1694 | (which defaults to zero if omitted). | |
1695 | @cindex precision (@code{printf}) | |
1696 | ||
1697 | You can also specify a precision of @samp{*}. This means that the next | |
1698 | argument in the argument list (before the actual value to be printed) is | |
1699 | used as the precision. The value must be an @code{int}, and is ignored | |
1700 | if it is negative. If you specify @samp{*} for both the field width and | |
1701 | precision, the field width argument precedes the precision argument. | |
1702 | Other C library versions may not recognize this syntax. | |
1703 | ||
1704 | @item | |
1705 | An optional @dfn{type modifier character}, which is used to specify the | |
1706 | data type of the corresponding argument if it differs from the default | |
1707 | type. (For example, the integer conversions assume a type of @code{int}, | |
1708 | but you can specify @samp{h}, @samp{l}, or @samp{L} for other integer | |
1709 | types.) | |
1710 | @cindex type modifier character (@code{printf}) | |
1711 | ||
1712 | @item | |
1713 | A character that specifies the conversion to be applied. | |
1714 | @end itemize | |
1715 | ||
19c3f208 | 1716 | The exact options that are permitted and how they are interpreted vary |
28f540f4 RM |
1717 | between the different conversion specifiers. See the descriptions of the |
1718 | individual conversions for information about the particular options that | |
1719 | they use. | |
1720 | ||
1721 | With the @samp{-Wformat} option, the GNU C compiler checks calls to | |
1722 | @code{printf} and related functions. It examines the format string and | |
1723 | verifies that the correct number and types of arguments are supplied. | |
1724 | There is also a GNU C syntax to tell the compiler that a function you | |
19c3f208 | 1725 | write uses a @code{printf}-style format string. |
28f540f4 RM |
1726 | @xref{Function Attributes, , Declaring Attributes of Functions, |
1727 | gcc.info, Using GNU CC}, for more information. | |
1728 | ||
1729 | @node Table of Output Conversions | |
1730 | @subsection Table of Output Conversions | |
1731 | @cindex output conversions, for @code{printf} | |
1732 | ||
1733 | Here is a table summarizing what all the different conversions do: | |
1734 | ||
1735 | @table @asis | |
1736 | @item @samp{%d}, @samp{%i} | |
1737 | Print an integer as a signed decimal number. @xref{Integer | |
1738 | Conversions}, for details. @samp{%d} and @samp{%i} are synonymous for | |
1739 | output, but are different when used with @code{scanf} for input | |
1740 | (@pxref{Table of Input Conversions}). | |
1741 | ||
1742 | @item @samp{%o} | |
1743 | Print an integer as an unsigned octal number. @xref{Integer | |
1744 | Conversions}, for details. | |
1745 | ||
1746 | @item @samp{%u} | |
1747 | Print an integer as an unsigned decimal number. @xref{Integer | |
1748 | Conversions}, for details. | |
1749 | ||
1750 | @item @samp{%x}, @samp{%X} | |
1751 | Print an integer as an unsigned hexadecimal number. @samp{%x} uses | |
1752 | lower-case letters and @samp{%X} uses upper-case. @xref{Integer | |
1753 | Conversions}, for details. | |
1754 | ||
1755 | @item @samp{%f} | |
1756 | Print a floating-point number in normal (fixed-point) notation. | |
1757 | @xref{Floating-Point Conversions}, for details. | |
1758 | ||
1759 | @item @samp{%e}, @samp{%E} | |
1760 | Print a floating-point number in exponential notation. @samp{%e} uses | |
1761 | lower-case letters and @samp{%E} uses upper-case. @xref{Floating-Point | |
1762 | Conversions}, for details. | |
1763 | ||
1764 | @item @samp{%g}, @samp{%G} | |
1765 | Print a floating-point number in either normal or exponential notation, | |
1766 | whichever is more appropriate for its magnitude. @samp{%g} uses | |
1767 | lower-case letters and @samp{%G} uses upper-case. @xref{Floating-Point | |
1768 | Conversions}, for details. | |
1769 | ||
2f6d1f1b UD |
1770 | @item @samp{%a}, @samp{%A} |
1771 | Print a floating-point number in a hexadecimal fractional notation which | |
1772 | the exponent to base 2 represented in decimal digits. @samp{%a} uses | |
1773 | lower-case letters and @samp{%A} uses upper-case. @xref{Floating-Point | |
1774 | Conversions}, for details. | |
1775 | ||
28f540f4 RM |
1776 | @item @samp{%c} |
1777 | Print a single character. @xref{Other Output Conversions}. | |
1778 | ||
b5e73f56 UD |
1779 | @item @samp{%C} |
1780 | This is an alias for @samp{%lc} which is supported for compatibility | |
1781 | with the Unix standard. | |
1782 | ||
28f540f4 RM |
1783 | @item @samp{%s} |
1784 | Print a string. @xref{Other Output Conversions}. | |
1785 | ||
b5e73f56 UD |
1786 | @item @samp{%S} |
1787 | This is an alias for @samp{%ls} which is supported for compatibility | |
1788 | with the Unix standard. | |
1789 | ||
28f540f4 RM |
1790 | @item @samp{%p} |
1791 | Print the value of a pointer. @xref{Other Output Conversions}. | |
1792 | ||
1793 | @item @samp{%n} | |
1794 | Get the number of characters printed so far. @xref{Other Output Conversions}. | |
1795 | Note that this conversion specification never produces any output. | |
1796 | ||
1797 | @item @samp{%m} | |
1798 | Print the string corresponding to the value of @code{errno}. | |
1799 | (This is a GNU extension.) | |
1800 | @xref{Other Output Conversions}. | |
1801 | ||
1802 | @item @samp{%%} | |
1803 | Print a literal @samp{%} character. @xref{Other Output Conversions}. | |
1804 | @end table | |
1805 | ||
1806 | If the syntax of a conversion specification is invalid, unpredictable | |
1807 | things will happen, so don't do this. If there aren't enough function | |
1808 | arguments provided to supply values for all the conversion | |
1809 | specifications in the template string, or if the arguments are not of | |
1810 | the correct types, the results are unpredictable. If you supply more | |
1811 | arguments than conversion specifications, the extra argument values are | |
1812 | simply ignored; this is sometimes useful. | |
1813 | ||
1814 | @node Integer Conversions | |
1815 | @subsection Integer Conversions | |
1816 | ||
1817 | This section describes the options for the @samp{%d}, @samp{%i}, | |
1818 | @samp{%o}, @samp{%u}, @samp{%x}, and @samp{%X} conversion | |
1819 | specifications. These conversions print integers in various formats. | |
1820 | ||
1821 | The @samp{%d} and @samp{%i} conversion specifications both print an | |
1822 | @code{int} argument as a signed decimal number; while @samp{%o}, | |
1823 | @samp{%u}, and @samp{%x} print the argument as an unsigned octal, | |
1824 | decimal, or hexadecimal number (respectively). The @samp{%X} conversion | |
1825 | specification is just like @samp{%x} except that it uses the characters | |
1826 | @samp{ABCDEF} as digits instead of @samp{abcdef}. | |
1827 | ||
1828 | The following flags are meaningful: | |
1829 | ||
1830 | @table @asis | |
1831 | @item @samp{-} | |
1832 | Left-justify the result in the field (instead of the normal | |
1833 | right-justification). | |
1834 | ||
1835 | @item @samp{+} | |
1836 | For the signed @samp{%d} and @samp{%i} conversions, print a | |
1837 | plus sign if the value is positive. | |
1838 | ||
1839 | @item @samp{ } | |
1840 | For the signed @samp{%d} and @samp{%i} conversions, if the result | |
1841 | doesn't start with a plus or minus sign, prefix it with a space | |
1842 | character instead. Since the @samp{+} flag ensures that the result | |
1843 | includes a sign, this flag is ignored if you supply both of them. | |
1844 | ||
1845 | @item @samp{#} | |
1846 | For the @samp{%o} conversion, this forces the leading digit to be | |
1847 | @samp{0}, as if by increasing the precision. For @samp{%x} or | |
1848 | @samp{%X}, this prefixes a leading @samp{0x} or @samp{0X} (respectively) | |
1849 | to the result. This doesn't do anything useful for the @samp{%d}, | |
1850 | @samp{%i}, or @samp{%u} conversions. Using this flag produces output | |
1851 | which can be parsed by the @code{strtoul} function (@pxref{Parsing of | |
1852 | Integers}) and @code{scanf} with the @samp{%i} conversion | |
1853 | (@pxref{Numeric Input Conversions}). | |
1854 | ||
1855 | @item @samp{'} | |
1856 | Separate the digits into groups as specified by the locale specified for | |
1857 | the @code{LC_NUMERIC} category; @pxref{General Numeric}. This flag is a | |
1858 | GNU extension. | |
1859 | ||
1860 | @item @samp{0} | |
1861 | Pad the field with zeros instead of spaces. The zeros are placed after | |
1862 | any indication of sign or base. This flag is ignored if the @samp{-} | |
1863 | flag is also specified, or if a precision is specified. | |
1864 | @end table | |
1865 | ||
1866 | If a precision is supplied, it specifies the minimum number of digits to | |
1867 | appear; leading zeros are produced if necessary. If you don't specify a | |
1868 | precision, the number is printed with as many digits as it needs. If | |
1869 | you convert a value of zero with an explicit precision of zero, then no | |
1870 | characters at all are produced. | |
1871 | ||
1872 | Without a type modifier, the corresponding argument is treated as an | |
1873 | @code{int} (for the signed conversions @samp{%i} and @samp{%d}) or | |
1874 | @code{unsigned int} (for the unsigned conversions @samp{%o}, @samp{%u}, | |
1875 | @samp{%x}, and @samp{%X}). Recall that since @code{printf} and friends | |
1876 | are variadic, any @code{char} and @code{short} arguments are | |
1877 | automatically converted to @code{int} by the default argument | |
1878 | promotions. For arguments of other integer types, you can use these | |
1879 | modifiers: | |
1880 | ||
1881 | @table @samp | |
cc3fa755 UD |
1882 | @item hh |
1883 | Specifies that the argument is a @code{signed char} or @code{unsigned | |
1884 | char}, as appropriate. A @code{char} argument is converted to an | |
1885 | @code{int} or @code{unsigned int} by the default argument promotions | |
1886 | anyway, but the @samp{h} modifier says to convert it back to a | |
1887 | @code{char} again. | |
1888 | ||
ec751a23 | 1889 | This modifier was introduced in @w{ISO C99}. |
e852e889 | 1890 | |
28f540f4 RM |
1891 | @item h |
1892 | Specifies that the argument is a @code{short int} or @code{unsigned | |
1893 | short int}, as appropriate. A @code{short} argument is converted to an | |
1894 | @code{int} or @code{unsigned int} by the default argument promotions | |
1895 | anyway, but the @samp{h} modifier says to convert it back to a | |
1896 | @code{short} again. | |
1897 | ||
e852e889 UD |
1898 | @item j |
1899 | Specifies that the argument is a @code{intmax_t} or @code{uintmax_t}, as | |
1900 | appropriate. | |
1901 | ||
ec751a23 | 1902 | This modifier was introduced in @w{ISO C99}. |
e852e889 | 1903 | |
28f540f4 RM |
1904 | @item l |
1905 | Specifies that the argument is a @code{long int} or @code{unsigned long | |
1906 | int}, as appropriate. Two @samp{l} characters is like the @samp{L} | |
1907 | modifier, below. | |
1908 | ||
b5e73f56 UD |
1909 | If used with @samp{%c} or @samp{%s} the corresponding parameter is |
1910 | considered as a wide character or wide character string respectively. | |
1911 | This use of @samp{l} was introduced in @w{Amendment 1} to @w{ISO C90}. | |
1912 | ||
28f540f4 RM |
1913 | @item L |
1914 | @itemx ll | |
1915 | @itemx q | |
1916 | Specifies that the argument is a @code{long long int}. (This type is | |
1917 | an extension supported by the GNU C compiler. On systems that don't | |
1918 | support extra-long integers, this is the same as @code{long int}.) | |
1919 | ||
1920 | The @samp{q} modifier is another name for the same thing, which comes | |
1921 | from 4.4 BSD; a @w{@code{long long int}} is sometimes called a ``quad'' | |
1922 | @code{int}. | |
1923 | ||
e852e889 UD |
1924 | @item t |
1925 | Specifies that the argument is a @code{ptrdiff_t}. | |
1926 | ||
ec751a23 | 1927 | This modifier was introduced in @w{ISO C99}. |
e852e889 UD |
1928 | |
1929 | @item z | |
1930 | @itemx Z | |
1931 | Specifies that the argument is a @code{size_t}. | |
1932 | ||
ec751a23 | 1933 | @samp{z} was introduced in @w{ISO C99}. @samp{Z} is a GNU extension |
0be8752b | 1934 | predating this addition and should not be used in new code. |
28f540f4 RM |
1935 | @end table |
1936 | ||
1937 | Here is an example. Using the template string: | |
1938 | ||
1939 | @smallexample | |
1940 | "|%5d|%-5d|%+5d|%+-5d|% 5d|%05d|%5.0d|%5.2d|%d|\n" | |
1941 | @end smallexample | |
1942 | ||
1943 | @noindent | |
1944 | to print numbers using the different options for the @samp{%d} | |
1945 | conversion gives results like: | |
1946 | ||
1947 | @smallexample | |
1948 | | 0|0 | +0|+0 | 0|00000| | 00|0| | |
1949 | | 1|1 | +1|+1 | 1|00001| 1| 01|1| | |
1950 | | -1|-1 | -1|-1 | -1|-0001| -1| -01|-1| | |
776e8492 | 1951 | |100000|100000|+100000|+100000| 100000|100000|100000|100000|100000| |
28f540f4 RM |
1952 | @end smallexample |
1953 | ||
1954 | In particular, notice what happens in the last case where the number | |
1955 | is too large to fit in the minimum field width specified. | |
1956 | ||
1957 | Here are some more examples showing how unsigned integers print under | |
1958 | various format options, using the template string: | |
1959 | ||
1960 | @smallexample | |
1961 | "|%5u|%5o|%5x|%5X|%#5o|%#5x|%#5X|%#10.8x|\n" | |
1962 | @end smallexample | |
1963 | ||
1964 | @smallexample | |
776e8492 | 1965 | | 0| 0| 0| 0| 0| 0| 0| 00000000| |
28f540f4 RM |
1966 | | 1| 1| 1| 1| 01| 0x1| 0X1|0x00000001| |
1967 | |100000|303240|186a0|186A0|0303240|0x186a0|0X186A0|0x000186a0| | |
1968 | @end smallexample | |
1969 | ||
1970 | ||
1971 | @node Floating-Point Conversions | |
1972 | @subsection Floating-Point Conversions | |
1973 | ||
1974 | This section discusses the conversion specifications for floating-point | |
1975 | numbers: the @samp{%f}, @samp{%e}, @samp{%E}, @samp{%g}, and @samp{%G} | |
1976 | conversions. | |
1977 | ||
1978 | The @samp{%f} conversion prints its argument in fixed-point notation, | |
1979 | producing output of the form | |
1980 | @w{[@code{-}]@var{ddd}@code{.}@var{ddd}}, | |
1981 | where the number of digits following the decimal point is controlled | |
1982 | by the precision you specify. | |
1983 | ||
1984 | The @samp{%e} conversion prints its argument in exponential notation, | |
1985 | producing output of the form | |
1986 | @w{[@code{-}]@var{d}@code{.}@var{ddd}@code{e}[@code{+}|@code{-}]@var{dd}}. | |
1987 | Again, the number of digits following the decimal point is controlled by | |
1988 | the precision. The exponent always contains at least two digits. The | |
1989 | @samp{%E} conversion is similar but the exponent is marked with the letter | |
1990 | @samp{E} instead of @samp{e}. | |
1991 | ||
1992 | The @samp{%g} and @samp{%G} conversions print the argument in the style | |
1993 | of @samp{%e} or @samp{%E} (respectively) if the exponent would be less | |
776e8492 | 1994 | than -4 or greater than or equal to the precision; otherwise they use |
11bf311e | 1995 | the @samp{%f} style. A precision of @code{0}, is taken as 1. |
776e8492 AJ |
1996 | Trailing zeros are removed from the fractional portion of the result and |
1997 | a decimal-point character appears only if it is followed by a digit. | |
28f540f4 | 1998 | |
2f6d1f1b | 1999 | The @samp{%a} and @samp{%A} conversions are meant for representing |
0be8752b | 2000 | floating-point numbers exactly in textual form so that they can be |
2f6d1f1b UD |
2001 | exchanged as texts between different programs and/or machines. The |
2002 | numbers are represented is the form | |
2003 | @w{[@code{-}]@code{0x}@var{h}@code{.}@var{hhh}@code{p}[@code{+}|@code{-}]@var{dd}}. | |
2004 | At the left of the decimal-point character exactly one digit is print. | |
91ea72b7 | 2005 | This character is only @code{0} if the number is denormalized. |
49c091e5 | 2006 | Otherwise the value is unspecified; it is implementation dependent how many |
2f6d1f1b UD |
2007 | bits are used. The number of hexadecimal digits on the right side of |
2008 | the decimal-point character is equal to the precision. If the precision | |
2009 | is zero it is determined to be large enough to provide an exact | |
2010 | representation of the number (or it is large enough to distinguish two | |
2011 | adjacent values if the @code{FLT_RADIX} is not a power of 2, | |
91ea72b7 | 2012 | @pxref{Floating Point Parameters}). For the @samp{%a} conversion |
2f6d1f1b UD |
2013 | lower-case characters are used to represent the hexadecimal number and |
2014 | the prefix and exponent sign are printed as @code{0x} and @code{p} | |
2015 | respectively. Otherwise upper-case characters are used and @code{0X} | |
2016 | and @code{P} are used for the representation of prefix and exponent | |
2017 | string. The exponent to the base of two is printed as a decimal number | |
2018 | using at least one digit but at most as many digits as necessary to | |
2019 | represent the value exactly. | |
2020 | ||
2021 | If the value to be printed represents infinity or a NaN, the output is | |
2022 | @w{[@code{-}]@code{inf}} or @code{nan} respectively if the conversion | |
2023 | specifier is @samp{%a}, @samp{%e}, @samp{%f}, or @samp{%g} and it is | |
2024 | @w{[@code{-}]@code{INF}} or @code{NAN} respectively if the conversion is | |
2025 | @samp{%A}, @samp{%E}, or @samp{%G}. | |
2026 | ||
28f540f4 RM |
2027 | The following flags can be used to modify the behavior: |
2028 | ||
2029 | @comment We use @asis instead of @samp so we can have ` ' as an item. | |
2030 | @table @asis | |
2031 | @item @samp{-} | |
2032 | Left-justify the result in the field. Normally the result is | |
2033 | right-justified. | |
2034 | ||
2035 | @item @samp{+} | |
2036 | Always include a plus or minus sign in the result. | |
2037 | ||
2038 | @item @samp{ } | |
2039 | If the result doesn't start with a plus or minus sign, prefix it with a | |
2040 | space instead. Since the @samp{+} flag ensures that the result includes | |
2041 | a sign, this flag is ignored if you supply both of them. | |
2042 | ||
2043 | @item @samp{#} | |
2044 | Specifies that the result should always include a decimal point, even | |
2045 | if no digits follow it. For the @samp{%g} and @samp{%G} conversions, | |
2046 | this also forces trailing zeros after the decimal point to be left | |
2047 | in place where they would otherwise be removed. | |
2048 | ||
2049 | @item @samp{'} | |
2050 | Separate the digits of the integer part of the result into groups as | |
2051 | specified by the locale specified for the @code{LC_NUMERIC} category; | |
2052 | @pxref{General Numeric}. This flag is a GNU extension. | |
2053 | ||
2054 | @item @samp{0} | |
2055 | Pad the field with zeros instead of spaces; the zeros are placed | |
2056 | after any sign. This flag is ignored if the @samp{-} flag is also | |
2057 | specified. | |
2058 | @end table | |
2059 | ||
2060 | The precision specifies how many digits follow the decimal-point | |
2061 | character for the @samp{%f}, @samp{%e}, and @samp{%E} conversions. For | |
2062 | these conversions, the default precision is @code{6}. If the precision | |
2063 | is explicitly @code{0}, this suppresses the decimal point character | |
2064 | entirely. For the @samp{%g} and @samp{%G} conversions, the precision | |
2065 | specifies how many significant digits to print. Significant digits are | |
2066 | the first digit before the decimal point, and all the digits after it. | |
91ea72b7 | 2067 | If the precision is @code{0} or not specified for @samp{%g} or @samp{%G}, |
28f540f4 RM |
2068 | it is treated like a value of @code{1}. If the value being printed |
2069 | cannot be expressed accurately in the specified number of digits, the | |
2070 | value is rounded to the nearest number that fits. | |
2071 | ||
2072 | Without a type modifier, the floating-point conversions use an argument | |
2073 | of type @code{double}. (By the default argument promotions, any | |
2074 | @code{float} arguments are automatically converted to @code{double}.) | |
2075 | The following type modifier is supported: | |
2076 | ||
2077 | @table @samp | |
2078 | @item L | |
2079 | An uppercase @samp{L} specifies that the argument is a @code{long | |
2080 | double}. | |
2081 | @end table | |
2082 | ||
2083 | Here are some examples showing how numbers print using the various | |
2084 | floating-point conversions. All of the numbers were printed using | |
2085 | this template string: | |
2086 | ||
2087 | @smallexample | |
2f6d1f1b | 2088 | "|%13.4a|%13.4f|%13.4e|%13.4g|\n" |
28f540f4 RM |
2089 | @end smallexample |
2090 | ||
2091 | Here is the output: | |
2092 | ||
2093 | @smallexample | |
2f6d1f1b UD |
2094 | | 0x0.0000p+0| 0.0000| 0.0000e+00| 0| |
2095 | | 0x1.0000p-1| 0.5000| 5.0000e-01| 0.5| | |
2096 | | 0x1.0000p+0| 1.0000| 1.0000e+00| 1| | |
2097 | | -0x1.0000p+0| -1.0000| -1.0000e+00| -1| | |
2098 | | 0x1.9000p+6| 100.0000| 1.0000e+02| 100| | |
2099 | | 0x1.f400p+9| 1000.0000| 1.0000e+03| 1000| | |
2100 | | 0x1.3880p+13| 10000.0000| 1.0000e+04| 1e+04| | |
2101 | | 0x1.81c8p+13| 12345.0000| 1.2345e+04| 1.234e+04| | |
2102 | | 0x1.86a0p+16| 100000.0000| 1.0000e+05| 1e+05| | |
2103 | | 0x1.e240p+16| 123456.0000| 1.2346e+05| 1.235e+05| | |
28f540f4 RM |
2104 | @end smallexample |
2105 | ||
2106 | Notice how the @samp{%g} conversion drops trailing zeros. | |
2107 | ||
2108 | @node Other Output Conversions | |
2109 | @subsection Other Output Conversions | |
2110 | ||
2111 | This section describes miscellaneous conversions for @code{printf}. | |
2112 | ||
b5e73f56 UD |
2113 | The @samp{%c} conversion prints a single character. In case there is no |
2114 | @samp{l} modifier the @code{int} argument is first converted to an | |
2115 | @code{unsigned char}. Then, if used in a wide stream function, the | |
2116 | character is converted into the corresponding wide character. The | |
2117 | @samp{-} flag can be used to specify left-justification in the field, | |
2118 | but no other flags are defined, and no precision or type modifier can be | |
2119 | given. For example: | |
28f540f4 RM |
2120 | |
2121 | @smallexample | |
2122 | printf ("%c%c%c%c%c", 'h', 'e', 'l', 'l', 'o'); | |
2123 | @end smallexample | |
2124 | ||
2125 | @noindent | |
2126 | prints @samp{hello}. | |
2127 | ||
b5e73f56 UD |
2128 | If there is a @samp{l} modifier present the argument is expected to be |
2129 | of type @code{wint_t}. If used in a multibyte function the wide | |
2130 | character is converted into a multibyte character before being added to | |
2131 | the output. In this case more than one output byte can be produced. | |
2132 | ||
2133 | The @samp{%s} conversion prints a string. If no @samp{l} modifier is | |
2134 | present the corresponding argument must be of type @code{char *} (or | |
2135 | @code{const char *}). If used in a wide stream function the string is | |
2136 | first converted in a wide character string. A precision can be | |
2137 | specified to indicate the maximum number of characters to write; | |
28f540f4 RM |
2138 | otherwise characters in the string up to but not including the |
2139 | terminating null character are written to the output stream. The | |
2140 | @samp{-} flag can be used to specify left-justification in the field, | |
2141 | but no other flags or type modifiers are defined for this conversion. | |
2142 | For example: | |
2143 | ||
2144 | @smallexample | |
2145 | printf ("%3s%-6s", "no", "where"); | |
2146 | @end smallexample | |
2147 | ||
2148 | @noindent | |
2149 | prints @samp{ nowhere }. | |
2150 | ||
b5e73f56 UD |
2151 | If there is a @samp{l} modifier present the argument is expected to be of type @code{wchar_t} (or @code{const wchar_t *}). |
2152 | ||
28f540f4 RM |
2153 | If you accidentally pass a null pointer as the argument for a @samp{%s} |
2154 | conversion, the GNU library prints it as @samp{(null)}. We think this | |
2155 | is more useful than crashing. But it's not good practice to pass a null | |
2156 | argument intentionally. | |
2157 | ||
2158 | The @samp{%m} conversion prints the string corresponding to the error | |
2159 | code in @code{errno}. @xref{Error Messages}. Thus: | |
2160 | ||
2161 | @smallexample | |
2162 | fprintf (stderr, "can't open `%s': %m\n", filename); | |
2163 | @end smallexample | |
2164 | ||
2165 | @noindent | |
2166 | is equivalent to: | |
2167 | ||
2168 | @smallexample | |
2169 | fprintf (stderr, "can't open `%s': %s\n", filename, strerror (errno)); | |
2170 | @end smallexample | |
2171 | ||
2172 | @noindent | |
2173 | The @samp{%m} conversion is a GNU C library extension. | |
2174 | ||
2175 | The @samp{%p} conversion prints a pointer value. The corresponding | |
2176 | argument must be of type @code{void *}. In practice, you can use any | |
2177 | type of pointer. | |
2178 | ||
2179 | In the GNU system, non-null pointers are printed as unsigned integers, | |
2180 | as if a @samp{%#x} conversion were used. Null pointers print as | |
2181 | @samp{(nil)}. (Pointers might print differently in other systems.) | |
2182 | ||
2183 | For example: | |
2184 | ||
2185 | @smallexample | |
2186 | printf ("%p", "testing"); | |
2187 | @end smallexample | |
2188 | ||
2189 | @noindent | |
2190 | prints @samp{0x} followed by a hexadecimal number---the address of the | |
2191 | string constant @code{"testing"}. It does not print the word | |
2192 | @samp{testing}. | |
2193 | ||
2194 | You can supply the @samp{-} flag with the @samp{%p} conversion to | |
2195 | specify left-justification, but no other flags, precision, or type | |
2196 | modifiers are defined. | |
2197 | ||
2198 | The @samp{%n} conversion is unlike any of the other output conversions. | |
2199 | It uses an argument which must be a pointer to an @code{int}, but | |
2200 | instead of printing anything it stores the number of characters printed | |
2201 | so far by this call at that location. The @samp{h} and @samp{l} type | |
2202 | modifiers are permitted to specify that the argument is of type | |
2203 | @code{short int *} or @code{long int *} instead of @code{int *}, but no | |
2204 | flags, field width, or precision are permitted. | |
2205 | ||
2206 | For example, | |
2207 | ||
2208 | @smallexample | |
2209 | int nchar; | |
2210 | printf ("%d %s%n\n", 3, "bears", &nchar); | |
2211 | @end smallexample | |
2212 | ||
2213 | @noindent | |
2214 | prints: | |
2215 | ||
2216 | @smallexample | |
2217 | 3 bears | |
2218 | @end smallexample | |
2219 | ||
2220 | @noindent | |
19c3f208 | 2221 | and sets @code{nchar} to @code{7}, because @samp{3 bears} is seven |
28f540f4 RM |
2222 | characters. |
2223 | ||
2224 | ||
2225 | The @samp{%%} conversion prints a literal @samp{%} character. This | |
2226 | conversion doesn't use an argument, and no flags, field width, | |
2227 | precision, or type modifiers are permitted. | |
2228 | ||
2229 | ||
2230 | @node Formatted Output Functions | |
2231 | @subsection Formatted Output Functions | |
2232 | ||
2233 | This section describes how to call @code{printf} and related functions. | |
2234 | Prototypes for these functions are in the header file @file{stdio.h}. | |
2235 | Because these functions take a variable number of arguments, you | |
2236 | @emph{must} declare prototypes for them before using them. Of course, | |
2237 | the easiest way to make sure you have all the right prototypes is to | |
2238 | just include @file{stdio.h}. | |
2239 | @pindex stdio.h | |
2240 | ||
2241 | @comment stdio.h | |
f65fd747 | 2242 | @comment ISO |
28f540f4 RM |
2243 | @deftypefun int printf (const char *@var{template}, @dots{}) |
2244 | The @code{printf} function prints the optional arguments under the | |
2245 | control of the template string @var{template} to the stream | |
2246 | @code{stdout}. It returns the number of characters printed, or a | |
2247 | negative value if there was an output error. | |
2248 | @end deftypefun | |
2249 | ||
b5e73f56 UD |
2250 | @comment wchar.h |
2251 | @comment ISO | |
2252 | @deftypefun int wprintf (const wchar_t *@var{template}, @dots{}) | |
2253 | The @code{wprintf} function prints the optional arguments under the | |
2254 | control of the wide template string @var{template} to the stream | |
2255 | @code{stdout}. It returns the number of wide characters printed, or a | |
2256 | negative value if there was an output error. | |
2257 | @end deftypefun | |
2258 | ||
28f540f4 | 2259 | @comment stdio.h |
f65fd747 | 2260 | @comment ISO |
28f540f4 RM |
2261 | @deftypefun int fprintf (FILE *@var{stream}, const char *@var{template}, @dots{}) |
2262 | This function is just like @code{printf}, except that the output is | |
2263 | written to the stream @var{stream} instead of @code{stdout}. | |
2264 | @end deftypefun | |
2265 | ||
b5e73f56 UD |
2266 | @comment wchar.h |
2267 | @comment ISO | |
2268 | @deftypefun int fwprintf (FILE *@var{stream}, const wchar_t *@var{template}, @dots{}) | |
2269 | This function is just like @code{wprintf}, except that the output is | |
2270 | written to the stream @var{stream} instead of @code{stdout}. | |
2271 | @end deftypefun | |
2272 | ||
28f540f4 | 2273 | @comment stdio.h |
f65fd747 | 2274 | @comment ISO |
28f540f4 RM |
2275 | @deftypefun int sprintf (char *@var{s}, const char *@var{template}, @dots{}) |
2276 | This is like @code{printf}, except that the output is stored in the character | |
2277 | array @var{s} instead of written to a stream. A null character is written | |
2278 | to mark the end of the string. | |
2279 | ||
2280 | The @code{sprintf} function returns the number of characters stored in | |
2281 | the array @var{s}, not including the terminating null character. | |
2282 | ||
2283 | The behavior of this function is undefined if copying takes place | |
2284 | between objects that overlap---for example, if @var{s} is also given | |
2285 | as an argument to be printed under control of the @samp{%s} conversion. | |
2286 | @xref{Copying and Concatenation}. | |
2287 | ||
2288 | @strong{Warning:} The @code{sprintf} function can be @strong{dangerous} | |
2289 | because it can potentially output more characters than can fit in the | |
2290 | allocation size of the string @var{s}. Remember that the field width | |
2291 | given in a conversion specification is only a @emph{minimum} value. | |
2292 | ||
2293 | To avoid this problem, you can use @code{snprintf} or @code{asprintf}, | |
2294 | described below. | |
2295 | @end deftypefun | |
2296 | ||
b5e73f56 UD |
2297 | @comment wchar.h |
2298 | @comment GNU | |
2299 | @deftypefun int swprintf (wchar_t *@var{s}, size_t @var{size}, const wchar_t *@var{template}, @dots{}) | |
2300 | This is like @code{wprintf}, except that the output is stored in the | |
2301 | wide character array @var{ws} instead of written to a stream. A null | |
2302 | wide character is written to mark the end of the string. The @var{size} | |
2303 | argument specifies the maximum number of characters to produce. The | |
2304 | trailing null character is counted towards this limit, so you should | |
2305 | allocate at least @var{size} wide characters for the string @var{ws}. | |
2306 | ||
2f278c94 UD |
2307 | The return value is the number of characters generated for the given |
2308 | input, excluding the trailing null. If not all output fits into the | |
2309 | provided buffer a negative value is returned. You should try again with | |
2310 | a bigger output string. @emph{Note:} this is different from how | |
2311 | @code{snprintf} handles this situation. | |
b5e73f56 UD |
2312 | |
2313 | Note that the corresponding narrow stream function takes fewer | |
2314 | parameters. @code{swprintf} in fact corresponds to the @code{snprintf} | |
2315 | function. Since the @code{sprintf} function can be dangerous and should | |
2316 | be avoided the @w{ISO C} committee refused to make the same mistake | |
2317 | again and decided to not define an function exactly corresponding to | |
2318 | @code{sprintf}. | |
2319 | @end deftypefun | |
2320 | ||
28f540f4 RM |
2321 | @comment stdio.h |
2322 | @comment GNU | |
2323 | @deftypefun int snprintf (char *@var{s}, size_t @var{size}, const char *@var{template}, @dots{}) | |
2324 | The @code{snprintf} function is similar to @code{sprintf}, except that | |
2325 | the @var{size} argument specifies the maximum number of characters to | |
2326 | produce. The trailing null character is counted towards this limit, so | |
2327 | you should allocate at least @var{size} characters for the string @var{s}. | |
2328 | ||
fe7bdd63 | 2329 | The return value is the number of characters which would be generated |
da2d1bc5 UD |
2330 | for the given input, excluding the trailing null. If this value is |
2331 | greater or equal to @var{size}, not all characters from the result have | |
2332 | been stored in @var{s}. You should try again with a bigger output | |
2333 | string. Here is an example of doing this: | |
28f540f4 RM |
2334 | |
2335 | @smallexample | |
2336 | @group | |
2337 | /* @r{Construct a message describing the value of a variable} | |
2338 | @r{whose name is @var{name} and whose value is @var{value}.} */ | |
2339 | char * | |
2340 | make_message (char *name, char *value) | |
2341 | @{ | |
2342 | /* @r{Guess we need no more than 100 chars of space.} */ | |
2343 | int size = 100; | |
2344 | char *buffer = (char *) xmalloc (size); | |
4cca6b86 | 2345 | int nchars; |
28f540f4 RM |
2346 | @end group |
2347 | @group | |
7ba73c63 UD |
2348 | if (buffer == NULL) |
2349 | return NULL; | |
2350 | ||
4cca6b86 UD |
2351 | /* @r{Try to print in the allocated space.} */ |
2352 | nchars = snprintf (buffer, size, "value of %s is %s", | |
2353 | name, value); | |
2354 | @end group | |
2355 | @group | |
fe7bdd63 | 2356 | if (nchars >= size) |
28f540f4 | 2357 | @{ |
4b8f94d3 | 2358 | /* @r{Reallocate buffer now that we know |
838e5ffe | 2359 | how much space is needed.} */ |
53631fa8 UD |
2360 | size = nchars + 1; |
2361 | buffer = (char *) xrealloc (buffer, size); | |
4cca6b86 | 2362 | |
7ba73c63 UD |
2363 | if (buffer != NULL) |
2364 | /* @r{Try again.} */ | |
2365 | snprintf (buffer, size, "value of %s is %s", | |
2366 | name, value); | |
28f540f4 | 2367 | @} |
4cca6b86 UD |
2368 | /* @r{The last call worked, return the string.} */ |
2369 | return buffer; | |
28f540f4 RM |
2370 | @} |
2371 | @end group | |
2372 | @end smallexample | |
2373 | ||
2374 | In practice, it is often easier just to use @code{asprintf}, below. | |
fb971363 | 2375 | |
5a74e68a UD |
2376 | @strong{Attention:} In versions of the GNU C library prior to 2.1 the |
2377 | return value is the number of characters stored, not including the | |
2378 | terminating null; unless there was not enough space in @var{s} to | |
2379 | store the result in which case @code{-1} is returned. This was | |
2380 | changed in order to comply with the @w{ISO C99} standard. | |
28f540f4 RM |
2381 | @end deftypefun |
2382 | ||
2383 | @node Dynamic Output | |
2384 | @subsection Dynamically Allocating Formatted Output | |
2385 | ||
2386 | The functions in this section do formatted output and place the results | |
2387 | in dynamically allocated memory. | |
2388 | ||
2389 | @comment stdio.h | |
2390 | @comment GNU | |
2391 | @deftypefun int asprintf (char **@var{ptr}, const char *@var{template}, @dots{}) | |
2392 | This function is similar to @code{sprintf}, except that it dynamically | |
2393 | allocates a string (as with @code{malloc}; @pxref{Unconstrained | |
2394 | Allocation}) to hold the output, instead of putting the output in a | |
2395 | buffer you allocate in advance. The @var{ptr} argument should be the | |
3a4e0609 UD |
2396 | address of a @code{char *} object, and a successful call to |
2397 | @code{asprintf} stores a pointer to the newly allocated string at that | |
2398 | location. | |
28f540f4 | 2399 | |
7ba73c63 | 2400 | The return value is the number of characters allocated for the buffer, or |
0bc93a2f | 2401 | less than zero if an error occurred. Usually this means that the buffer |
7ba73c63 UD |
2402 | could not be allocated. |
2403 | ||
28f540f4 RM |
2404 | Here is how to use @code{asprintf} to get the same result as the |
2405 | @code{snprintf} example, but more easily: | |
2406 | ||
2407 | @smallexample | |
2408 | /* @r{Construct a message describing the value of a variable} | |
2409 | @r{whose name is @var{name} and whose value is @var{value}.} */ | |
2410 | char * | |
2411 | make_message (char *name, char *value) | |
2412 | @{ | |
2413 | char *result; | |
7ba73c63 UD |
2414 | if (asprintf (&result, "value of %s is %s", name, value) < 0) |
2415 | return NULL; | |
28f540f4 RM |
2416 | return result; |
2417 | @} | |
2418 | @end smallexample | |
2419 | @end deftypefun | |
2420 | ||
2421 | @comment stdio.h | |
2422 | @comment GNU | |
2423 | @deftypefun int obstack_printf (struct obstack *@var{obstack}, const char *@var{template}, @dots{}) | |
2424 | This function is similar to @code{asprintf}, except that it uses the | |
2425 | obstack @var{obstack} to allocate the space. @xref{Obstacks}. | |
2426 | ||
2427 | The characters are written onto the end of the current object. | |
2428 | To get at them, you must finish the object with @code{obstack_finish} | |
2429 | (@pxref{Growing Objects}).@refill | |
2430 | @end deftypefun | |
2431 | ||
2432 | @node Variable Arguments Output | |
2433 | @subsection Variable Arguments Output Functions | |
2434 | ||
2435 | The functions @code{vprintf} and friends are provided so that you can | |
2436 | define your own variadic @code{printf}-like functions that make use of | |
2437 | the same internals as the built-in formatted output functions. | |
2438 | ||
2439 | The most natural way to define such functions would be to use a language | |
2440 | construct to say, ``Call @code{printf} and pass this template plus all | |
2441 | of my arguments after the first five.'' But there is no way to do this | |
2442 | in C, and it would be hard to provide a way, since at the C language | |
2443 | level there is no way to tell how many arguments your function received. | |
2444 | ||
2445 | Since that method is impossible, we provide alternative functions, the | |
2446 | @code{vprintf} series, which lets you pass a @code{va_list} to describe | |
2447 | ``all of my arguments after the first five.'' | |
2448 | ||
19c3f208 | 2449 | When it is sufficient to define a macro rather than a real function, |
28f540f4 RM |
2450 | the GNU C compiler provides a way to do this much more easily with macros. |
2451 | For example: | |
2452 | ||
2453 | @smallexample | |
838e5ffe | 2454 | #define myprintf(a, b, c, d, e, rest...) \ |
95fdc6a0 | 2455 | printf (mytemplate , ## rest) |
28f540f4 RM |
2456 | @end smallexample |
2457 | ||
2458 | @noindent | |
9229b83e RM |
2459 | @xref{Variadic Macros,,, cpp, The C preprocessor}, for details. |
2460 | But this is limited to macros, and does not apply to real functions at all. | |
28f540f4 RM |
2461 | |
2462 | Before calling @code{vprintf} or the other functions listed in this | |
2463 | section, you @emph{must} call @code{va_start} (@pxref{Variadic | |
2464 | Functions}) to initialize a pointer to the variable arguments. Then you | |
2465 | can call @code{va_arg} to fetch the arguments that you want to handle | |
2466 | yourself. This advances the pointer past those arguments. | |
2467 | ||
2468 | Once your @code{va_list} pointer is pointing at the argument of your | |
2469 | choice, you are ready to call @code{vprintf}. That argument and all | |
2470 | subsequent arguments that were passed to your function are used by | |
2471 | @code{vprintf} along with the template that you specified separately. | |
2472 | ||
2473 | In some other systems, the @code{va_list} pointer may become invalid | |
2474 | after the call to @code{vprintf}, so you must not use @code{va_arg} | |
2475 | after you call @code{vprintf}. Instead, you should call @code{va_end} | |
2476 | to retire the pointer from service. However, you can safely call | |
2477 | @code{va_start} on another pointer variable and begin fetching the | |
2478 | arguments again through that pointer. Calling @code{vprintf} does not | |
2479 | destroy the argument list of your function, merely the particular | |
2480 | pointer that you passed to it. | |
2481 | ||
2482 | GNU C does not have such restrictions. You can safely continue to fetch | |
2483 | arguments from a @code{va_list} pointer after passing it to | |
2484 | @code{vprintf}, and @code{va_end} is a no-op. (Note, however, that | |
2485 | subsequent @code{va_arg} calls will fetch the same arguments which | |
2486 | @code{vprintf} previously used.) | |
2487 | ||
2488 | Prototypes for these functions are declared in @file{stdio.h}. | |
2489 | @pindex stdio.h | |
2490 | ||
2491 | @comment stdio.h | |
f65fd747 | 2492 | @comment ISO |
28f540f4 RM |
2493 | @deftypefun int vprintf (const char *@var{template}, va_list @var{ap}) |
2494 | This function is similar to @code{printf} except that, instead of taking | |
2495 | a variable number of arguments directly, it takes an argument list | |
2496 | pointer @var{ap}. | |
2497 | @end deftypefun | |
2498 | ||
b5e73f56 UD |
2499 | @comment wchar.h |
2500 | @comment ISO | |
2501 | @deftypefun int vwprintf (const wchar_t *@var{template}, va_list @var{ap}) | |
2502 | This function is similar to @code{wprintf} except that, instead of taking | |
2503 | a variable number of arguments directly, it takes an argument list | |
2504 | pointer @var{ap}. | |
2505 | @end deftypefun | |
2506 | ||
28f540f4 | 2507 | @comment stdio.h |
f65fd747 | 2508 | @comment ISO |
28f540f4 RM |
2509 | @deftypefun int vfprintf (FILE *@var{stream}, const char *@var{template}, va_list @var{ap}) |
2510 | This is the equivalent of @code{fprintf} with the variable argument list | |
2511 | specified directly as for @code{vprintf}. | |
2512 | @end deftypefun | |
2513 | ||
b5e73f56 UD |
2514 | @comment wchar.h |
2515 | @comment ISO | |
2516 | @deftypefun int vfwprintf (FILE *@var{stream}, const wchar_t *@var{template}, va_list @var{ap}) | |
2517 | This is the equivalent of @code{fwprintf} with the variable argument list | |
2518 | specified directly as for @code{vwprintf}. | |
2519 | @end deftypefun | |
2520 | ||
28f540f4 | 2521 | @comment stdio.h |
f65fd747 | 2522 | @comment ISO |
28f540f4 RM |
2523 | @deftypefun int vsprintf (char *@var{s}, const char *@var{template}, va_list @var{ap}) |
2524 | This is the equivalent of @code{sprintf} with the variable argument list | |
2525 | specified directly as for @code{vprintf}. | |
2526 | @end deftypefun | |
2527 | ||
b5e73f56 UD |
2528 | @comment wchar.h |
2529 | @comment GNU | |
2530 | @deftypefun int vswprintf (wchar_t *@var{s}, size_t @var{size}, const wchar_t *@var{template}, va_list @var{ap}) | |
2531 | This is the equivalent of @code{swprintf} with the variable argument list | |
2532 | specified directly as for @code{vwprintf}. | |
2533 | @end deftypefun | |
2534 | ||
28f540f4 RM |
2535 | @comment stdio.h |
2536 | @comment GNU | |
2537 | @deftypefun int vsnprintf (char *@var{s}, size_t @var{size}, const char *@var{template}, va_list @var{ap}) | |
2538 | This is the equivalent of @code{snprintf} with the variable argument list | |
2539 | specified directly as for @code{vprintf}. | |
2540 | @end deftypefun | |
2541 | ||
2542 | @comment stdio.h | |
2543 | @comment GNU | |
2544 | @deftypefun int vasprintf (char **@var{ptr}, const char *@var{template}, va_list @var{ap}) | |
2545 | The @code{vasprintf} function is the equivalent of @code{asprintf} with the | |
2546 | variable argument list specified directly as for @code{vprintf}. | |
2547 | @end deftypefun | |
2548 | ||
2549 | @comment stdio.h | |
2550 | @comment GNU | |
2551 | @deftypefun int obstack_vprintf (struct obstack *@var{obstack}, const char *@var{template}, va_list @var{ap}) | |
2552 | The @code{obstack_vprintf} function is the equivalent of | |
2553 | @code{obstack_printf} with the variable argument list specified directly | |
2554 | as for @code{vprintf}.@refill | |
2555 | @end deftypefun | |
2556 | ||
2557 | Here's an example showing how you might use @code{vfprintf}. This is a | |
2558 | function that prints error messages to the stream @code{stderr}, along | |
2559 | with a prefix indicating the name of the program | |
19c3f208 | 2560 | (@pxref{Error Messages}, for a description of |
28f540f4 RM |
2561 | @code{program_invocation_short_name}). |
2562 | ||
2563 | @smallexample | |
2564 | @group | |
2565 | #include <stdio.h> | |
2566 | #include <stdarg.h> | |
2567 | ||
2568 | void | |
2569 | eprintf (const char *template, ...) | |
2570 | @{ | |
2571 | va_list ap; | |
2572 | extern char *program_invocation_short_name; | |
2573 | ||
2574 | fprintf (stderr, "%s: ", program_invocation_short_name); | |
4b8f94d3 | 2575 | va_start (ap, template); |
28f540f4 RM |
2576 | vfprintf (stderr, template, ap); |
2577 | va_end (ap); | |
2578 | @} | |
2579 | @end group | |
2580 | @end smallexample | |
2581 | ||
2582 | @noindent | |
2583 | You could call @code{eprintf} like this: | |
2584 | ||
2585 | @smallexample | |
2586 | eprintf ("file `%s' does not exist\n", filename); | |
2587 | @end smallexample | |
2588 | ||
2589 | In GNU C, there is a special construct you can use to let the compiler | |
2590 | know that a function uses a @code{printf}-style format string. Then it | |
2591 | can check the number and types of arguments in each call to the | |
2592 | function, and warn you when they do not match the format string. | |
2593 | For example, take this declaration of @code{eprintf}: | |
2594 | ||
2595 | @smallexample | |
2596 | void eprintf (const char *template, ...) | |
2597 | __attribute__ ((format (printf, 1, 2))); | |
2598 | @end smallexample | |
2599 | ||
2600 | @noindent | |
2601 | This tells the compiler that @code{eprintf} uses a format string like | |
2602 | @code{printf} (as opposed to @code{scanf}; @pxref{Formatted Input}); | |
2603 | the format string appears as the first argument; | |
2604 | and the arguments to satisfy the format begin with the second. | |
2605 | @xref{Function Attributes, , Declaring Attributes of Functions, | |
2606 | gcc.info, Using GNU CC}, for more information. | |
2607 | ||
2608 | @node Parsing a Template String | |
2609 | @subsection Parsing a Template String | |
2610 | @cindex parsing a template string | |
2611 | ||
2612 | You can use the function @code{parse_printf_format} to obtain | |
2613 | information about the number and types of arguments that are expected by | |
2614 | a given template string. This function permits interpreters that | |
2615 | provide interfaces to @code{printf} to avoid passing along invalid | |
2616 | arguments from the user's program, which could cause a crash. | |
2617 | ||
2618 | All the symbols described in this section are declared in the header | |
2619 | file @file{printf.h}. | |
2620 | ||
2621 | @comment printf.h | |
2622 | @comment GNU | |
2623 | @deftypefun size_t parse_printf_format (const char *@var{template}, size_t @var{n}, int *@var{argtypes}) | |
2624 | This function returns information about the number and types of | |
2625 | arguments expected by the @code{printf} template string @var{template}. | |
2626 | The information is stored in the array @var{argtypes}; each element of | |
2627 | this array describes one argument. This information is encoded using | |
2628 | the various @samp{PA_} macros, listed below. | |
2629 | ||
04b9968b UD |
2630 | The argument @var{n} specifies the number of elements in the array |
2631 | @var{argtypes}. This is the maximum number of elements that | |
28f540f4 RM |
2632 | @code{parse_printf_format} will try to write. |
2633 | ||
2634 | @code{parse_printf_format} returns the total number of arguments required | |
2635 | by @var{template}. If this number is greater than @var{n}, then the | |
2636 | information returned describes only the first @var{n} arguments. If you | |
04b9968b | 2637 | want information about additional arguments, allocate a bigger |
28f540f4 RM |
2638 | array and call @code{parse_printf_format} again. |
2639 | @end deftypefun | |
2640 | ||
2641 | The argument types are encoded as a combination of a basic type and | |
2642 | modifier flag bits. | |
2643 | ||
2644 | @comment printf.h | |
2645 | @comment GNU | |
2646 | @deftypevr Macro int PA_FLAG_MASK | |
2647 | This macro is a bitmask for the type modifier flag bits. You can write | |
2648 | the expression @code{(argtypes[i] & PA_FLAG_MASK)} to extract just the | |
2649 | flag bits for an argument, or @code{(argtypes[i] & ~PA_FLAG_MASK)} to | |
2650 | extract just the basic type code. | |
2651 | @end deftypevr | |
2652 | ||
2653 | Here are symbolic constants that represent the basic types; they stand | |
2654 | for integer values. | |
2655 | ||
779ae82e | 2656 | @vtable @code |
28f540f4 RM |
2657 | @comment printf.h |
2658 | @comment GNU | |
2659 | @item PA_INT | |
28f540f4 RM |
2660 | This specifies that the base type is @code{int}. |
2661 | ||
2662 | @comment printf.h | |
2663 | @comment GNU | |
2664 | @item PA_CHAR | |
28f540f4 RM |
2665 | This specifies that the base type is @code{int}, cast to @code{char}. |
2666 | ||
2667 | @comment printf.h | |
2668 | @comment GNU | |
2669 | @item PA_STRING | |
28f540f4 RM |
2670 | This specifies that the base type is @code{char *}, a null-terminated string. |
2671 | ||
2672 | @comment printf.h | |
2673 | @comment GNU | |
2674 | @item PA_POINTER | |
28f540f4 RM |
2675 | This specifies that the base type is @code{void *}, an arbitrary pointer. |
2676 | ||
2677 | @comment printf.h | |
2678 | @comment GNU | |
2679 | @item PA_FLOAT | |
28f540f4 RM |
2680 | This specifies that the base type is @code{float}. |
2681 | ||
2682 | @comment printf.h | |
2683 | @comment GNU | |
2684 | @item PA_DOUBLE | |
28f540f4 RM |
2685 | This specifies that the base type is @code{double}. |
2686 | ||
2687 | @comment printf.h | |
2688 | @comment GNU | |
2689 | @item PA_LAST | |
28f540f4 RM |
2690 | You can define additional base types for your own programs as offsets |
2691 | from @code{PA_LAST}. For example, if you have data types @samp{foo} | |
2692 | and @samp{bar} with their own specialized @code{printf} conversions, | |
2693 | you could define encodings for these types as: | |
2694 | ||
2695 | @smallexample | |
2696 | #define PA_FOO PA_LAST | |
2697 | #define PA_BAR (PA_LAST + 1) | |
2698 | @end smallexample | |
779ae82e | 2699 | @end vtable |
28f540f4 RM |
2700 | |
2701 | Here are the flag bits that modify a basic type. They are combined with | |
2702 | the code for the basic type using inclusive-or. | |
2703 | ||
779ae82e | 2704 | @vtable @code |
28f540f4 RM |
2705 | @comment printf.h |
2706 | @comment GNU | |
2707 | @item PA_FLAG_PTR | |
28f540f4 RM |
2708 | If this bit is set, it indicates that the encoded type is a pointer to |
2709 | the base type, rather than an immediate value. | |
2710 | For example, @samp{PA_INT|PA_FLAG_PTR} represents the type @samp{int *}. | |
2711 | ||
2712 | @comment printf.h | |
2713 | @comment GNU | |
2714 | @item PA_FLAG_SHORT | |
28f540f4 RM |
2715 | If this bit is set, it indicates that the base type is modified with |
2716 | @code{short}. (This corresponds to the @samp{h} type modifier.) | |
2717 | ||
2718 | @comment printf.h | |
2719 | @comment GNU | |
2720 | @item PA_FLAG_LONG | |
28f540f4 RM |
2721 | If this bit is set, it indicates that the base type is modified with |
2722 | @code{long}. (This corresponds to the @samp{l} type modifier.) | |
2723 | ||
2724 | @comment printf.h | |
2725 | @comment GNU | |
2726 | @item PA_FLAG_LONG_LONG | |
28f540f4 RM |
2727 | If this bit is set, it indicates that the base type is modified with |
2728 | @code{long long}. (This corresponds to the @samp{L} type modifier.) | |
2729 | ||
2730 | @comment printf.h | |
2731 | @comment GNU | |
2732 | @item PA_FLAG_LONG_DOUBLE | |
28f540f4 RM |
2733 | This is a synonym for @code{PA_FLAG_LONG_LONG}, used by convention with |
2734 | a base type of @code{PA_DOUBLE} to indicate a type of @code{long double}. | |
779ae82e | 2735 | @end vtable |
28f540f4 RM |
2736 | |
2737 | @ifinfo | |
6d52618b | 2738 | For an example of using these facilities, see @ref{Example of Parsing}. |
28f540f4 RM |
2739 | @end ifinfo |
2740 | ||
2741 | @node Example of Parsing | |
2742 | @subsection Example of Parsing a Template String | |
2743 | ||
2744 | Here is an example of decoding argument types for a format string. We | |
2745 | assume this is part of an interpreter which contains arguments of type | |
2746 | @code{NUMBER}, @code{CHAR}, @code{STRING} and @code{STRUCTURE} (and | |
2747 | perhaps others which are not valid here). | |
2748 | ||
2749 | @smallexample | |
2750 | /* @r{Test whether the @var{nargs} specified objects} | |
2751 | @r{in the vector @var{args} are valid} | |
2752 | @r{for the format string @var{format}:} | |
2753 | @r{if so, return 1.} | |
2754 | @r{If not, return 0 after printing an error message.} */ | |
2755 | ||
2756 | int | |
2757 | validate_args (char *format, int nargs, OBJECT *args) | |
2758 | @{ | |
2759 | int *argtypes; | |
2760 | int nwanted; | |
2761 | ||
2762 | /* @r{Get the information about the arguments.} | |
2763 | @r{Each conversion specification must be at least two characters} | |
2764 | @r{long, so there cannot be more specifications than half the} | |
2765 | @r{length of the string.} */ | |
2766 | ||
2767 | argtypes = (int *) alloca (strlen (format) / 2 * sizeof (int)); | |
2768 | nwanted = parse_printf_format (string, nelts, argtypes); | |
2769 | ||
2770 | /* @r{Check the number of arguments.} */ | |
2771 | if (nwanted > nargs) | |
2772 | @{ | |
2773 | error ("too few arguments (at least %d required)", nwanted); | |
2774 | return 0; | |
2775 | @} | |
19c3f208 | 2776 | |
28f540f4 RM |
2777 | /* @r{Check the C type wanted for each argument} |
2778 | @r{and see if the object given is suitable.} */ | |
2779 | for (i = 0; i < nwanted; i++) | |
2780 | @{ | |
2781 | int wanted; | |
2782 | ||
2783 | if (argtypes[i] & PA_FLAG_PTR) | |
2784 | wanted = STRUCTURE; | |
2785 | else | |
2786 | switch (argtypes[i] & ~PA_FLAG_MASK) | |
2787 | @{ | |
2788 | case PA_INT: | |
2789 | case PA_FLOAT: | |
2790 | case PA_DOUBLE: | |
2791 | wanted = NUMBER; | |
2792 | break; | |
2793 | case PA_CHAR: | |
2794 | wanted = CHAR; | |
2795 | break; | |
2796 | case PA_STRING: | |
2797 | wanted = STRING; | |
2798 | break; | |
2799 | case PA_POINTER: | |
2800 | wanted = STRUCTURE; | |
2801 | break; | |
2802 | @} | |
2803 | if (TYPE (args[i]) != wanted) | |
2804 | @{ | |
2805 | error ("type mismatch for arg number %d", i); | |
2806 | return 0; | |
2807 | @} | |
2808 | @} | |
2809 | return 1; | |
2810 | @} | |
2811 | @end smallexample | |
2812 | ||
2813 | @node Customizing Printf | |
2814 | @section Customizing @code{printf} | |
2815 | @cindex customizing @code{printf} | |
2816 | @cindex defining new @code{printf} conversions | |
2817 | @cindex extending @code{printf} | |
2818 | ||
2819 | The GNU C library lets you define your own custom conversion specifiers | |
2820 | for @code{printf} template strings, to teach @code{printf} clever ways | |
2821 | to print the important data structures of your program. | |
2822 | ||
2823 | The way you do this is by registering the conversion with the function | |
2824 | @code{register_printf_function}; see @ref{Registering New Conversions}. | |
2825 | One of the arguments you pass to this function is a pointer to a handler | |
2826 | function that produces the actual output; see @ref{Defining the Output | |
2827 | Handler}, for information on how to write this function. | |
2828 | ||
2829 | You can also install a function that just returns information about the | |
2830 | number and type of arguments expected by the conversion specifier. | |
2831 | @xref{Parsing a Template String}, for information about this. | |
2832 | ||
2833 | The facilities of this section are declared in the header file | |
2834 | @file{printf.h}. | |
2835 | ||
2836 | @menu | |
19c3f208 | 2837 | * Registering New Conversions:: Using @code{register_printf_function} |
28f540f4 RM |
2838 | to register a new output conversion. |
2839 | * Conversion Specifier Options:: The handler must be able to get | |
2840 | the options specified in the | |
19c3f208 | 2841 | template when it is called. |
28f540f4 RM |
2842 | * Defining the Output Handler:: Defining the handler and arginfo |
2843 | functions that are passed as arguments | |
19c3f208 | 2844 | to @code{register_printf_function}. |
28f540f4 | 2845 | * Printf Extension Example:: How to define a @code{printf} |
19c3f208 | 2846 | handler function. |
29bb8719 | 2847 | * Predefined Printf Handlers:: Predefined @code{printf} handlers. |
28f540f4 RM |
2848 | @end menu |
2849 | ||
2850 | @strong{Portability Note:} The ability to extend the syntax of | |
f65fd747 | 2851 | @code{printf} template strings is a GNU extension. ISO standard C has |
28f540f4 RM |
2852 | nothing similar. |
2853 | ||
2854 | @node Registering New Conversions | |
2855 | @subsection Registering New Conversions | |
2856 | ||
2857 | The function to register a new output conversion is | |
2858 | @code{register_printf_function}, declared in @file{printf.h}. | |
2859 | @pindex printf.h | |
2860 | ||
2861 | @comment printf.h | |
2862 | @comment GNU | |
2863 | @deftypefun int register_printf_function (int @var{spec}, printf_function @var{handler-function}, printf_arginfo_function @var{arginfo-function}) | |
2864 | This function defines the conversion specifier character @var{spec}. | |
42be70d4 | 2865 | Thus, if @var{spec} is @code{'Y'}, it defines the conversion @samp{%Y}. |
28f540f4 RM |
2866 | You can redefine the built-in conversions like @samp{%s}, but flag |
2867 | characters like @samp{#} and type modifiers like @samp{l} can never be | |
2868 | used as conversions; calling @code{register_printf_function} for those | |
42be70d4 UD |
2869 | characters has no effect. It is advisable not to use lowercase letters, |
2870 | since the ISO C standard warns that additional lowercase letters may be | |
2871 | standardized in future editions of the standard. | |
28f540f4 RM |
2872 | |
2873 | The @var{handler-function} is the function called by @code{printf} and | |
2874 | friends when this conversion appears in a template string. | |
2875 | @xref{Defining the Output Handler}, for information about how to define | |
2876 | a function to pass as this argument. If you specify a null pointer, any | |
2877 | existing handler function for @var{spec} is removed. | |
2878 | ||
2879 | The @var{arginfo-function} is the function called by | |
2880 | @code{parse_printf_format} when this conversion appears in a | |
2881 | template string. @xref{Parsing a Template String}, for information | |
2882 | about this. | |
2883 | ||
54d79e99 UD |
2884 | @c The following is not true anymore. The `parse_printf_format' function |
2885 | @c is now also called from `vfprintf' via `parse_one_spec'. | |
2886 | @c --drepper@gnu, 1996/11/14 | |
2887 | @c | |
2888 | @c Normally, you install both functions for a conversion at the same time, | |
2889 | @c but if you are never going to call @code{parse_printf_format}, you do | |
2890 | @c not need to define an arginfo function. | |
2891 | ||
04b9968b | 2892 | @strong{Attention:} In the GNU C library versions before 2.0 the |
54d79e99 | 2893 | @var{arginfo-function} function did not need to be installed unless |
04b9968b | 2894 | the user used the @code{parse_printf_format} function. This has changed. |
54d79e99 UD |
2895 | Now a call to any of the @code{printf} functions will call this |
2896 | function when this format specifier appears in the format string. | |
28f540f4 RM |
2897 | |
2898 | The return value is @code{0} on success, and @code{-1} on failure | |
2899 | (which occurs if @var{spec} is out of range). | |
2900 | ||
2901 | You can redefine the standard output conversions, but this is probably | |
2902 | not a good idea because of the potential for confusion. Library routines | |
2903 | written by other people could break if you do this. | |
2904 | @end deftypefun | |
2905 | ||
2906 | @node Conversion Specifier Options | |
2907 | @subsection Conversion Specifier Options | |
2908 | ||
40deae08 RM |
2909 | If you define a meaning for @samp{%A}, what if the template contains |
2910 | @samp{%+23A} or @samp{%-#A}? To implement a sensible meaning for these, | |
28f540f4 RM |
2911 | the handler when called needs to be able to get the options specified in |
2912 | the template. | |
2913 | ||
838e5ffe UD |
2914 | Both the @var{handler-function} and @var{arginfo-function} accept an |
2915 | argument that points to a @code{struct printf_info}, which contains | |
2916 | information about the options appearing in an instance of the conversion | |
2917 | specifier. This data type is declared in the header file | |
2918 | @file{printf.h}. | |
28f540f4 RM |
2919 | @pindex printf.h |
2920 | ||
2921 | @comment printf.h | |
2922 | @comment GNU | |
2923 | @deftp {Type} {struct printf_info} | |
2924 | This structure is used to pass information about the options appearing | |
2925 | in an instance of a conversion specifier in a @code{printf} template | |
2926 | string to the handler and arginfo functions for that specifier. It | |
2927 | contains the following members: | |
2928 | ||
2929 | @table @code | |
2930 | @item int prec | |
2931 | This is the precision specified. The value is @code{-1} if no precision | |
2932 | was specified. If the precision was given as @samp{*}, the | |
2933 | @code{printf_info} structure passed to the handler function contains the | |
2934 | actual value retrieved from the argument list. But the structure passed | |
2935 | to the arginfo function contains a value of @code{INT_MIN}, since the | |
2936 | actual value is not known. | |
2937 | ||
2938 | @item int width | |
2939 | This is the minimum field width specified. The value is @code{0} if no | |
2940 | width was specified. If the field width was given as @samp{*}, the | |
2941 | @code{printf_info} structure passed to the handler function contains the | |
2942 | actual value retrieved from the argument list. But the structure passed | |
2943 | to the arginfo function contains a value of @code{INT_MIN}, since the | |
2944 | actual value is not known. | |
2945 | ||
54d79e99 | 2946 | @item wchar_t spec |
28f540f4 RM |
2947 | This is the conversion specifier character specified. It's stored in |
2948 | the structure so that you can register the same handler function for | |
2949 | multiple characters, but still have a way to tell them apart when the | |
2950 | handler function is called. | |
2951 | ||
2952 | @item unsigned int is_long_double | |
2953 | This is a boolean that is true if the @samp{L}, @samp{ll}, or @samp{q} | |
2954 | type modifier was specified. For integer conversions, this indicates | |
2955 | @code{long long int}, as opposed to @code{long double} for floating | |
2956 | point conversions. | |
2957 | ||
d64b6ad0 UD |
2958 | @item unsigned int is_char |
2959 | This is a boolean that is true if the @samp{hh} type modifier was specified. | |
2960 | ||
28f540f4 RM |
2961 | @item unsigned int is_short |
2962 | This is a boolean that is true if the @samp{h} type modifier was specified. | |
2963 | ||
2964 | @item unsigned int is_long | |
2965 | This is a boolean that is true if the @samp{l} type modifier was specified. | |
2966 | ||
2967 | @item unsigned int alt | |
2968 | This is a boolean that is true if the @samp{#} flag was specified. | |
2969 | ||
2970 | @item unsigned int space | |
2971 | This is a boolean that is true if the @samp{ } flag was specified. | |
2972 | ||
2973 | @item unsigned int left | |
2974 | This is a boolean that is true if the @samp{-} flag was specified. | |
2975 | ||
2976 | @item unsigned int showsign | |
2977 | This is a boolean that is true if the @samp{+} flag was specified. | |
2978 | ||
2979 | @item unsigned int group | |
2980 | This is a boolean that is true if the @samp{'} flag was specified. | |
2981 | ||
54d79e99 UD |
2982 | @item unsigned int extra |
2983 | This flag has a special meaning depending on the context. It could | |
2984 | be used freely by the user-defined handlers but when called from | |
2985 | the @code{printf} function this variable always contains the value | |
2986 | @code{0}. | |
2987 | ||
d64b6ad0 UD |
2988 | @item unsigned int wide |
2989 | This flag is set if the stream is wide oriented. | |
2990 | ||
54d79e99 | 2991 | @item wchar_t pad |
28f540f4 RM |
2992 | This is the character to use for padding the output to the minimum field |
2993 | width. The value is @code{'0'} if the @samp{0} flag was specified, and | |
2994 | @code{' '} otherwise. | |
2995 | @end table | |
2996 | @end deftp | |
2997 | ||
2998 | ||
2999 | @node Defining the Output Handler | |
3000 | @subsection Defining the Output Handler | |
3001 | ||
3002 | Now let's look at how to define the handler and arginfo functions | |
3003 | which are passed as arguments to @code{register_printf_function}. | |
3004 | ||
04b9968b | 3005 | @strong{Compatibility Note:} The interface changed in GNU libc |
54d79e99 UD |
3006 | version 2.0. Previously the third argument was of type |
3007 | @code{va_list *}. | |
3008 | ||
28f540f4 RM |
3009 | You should define your handler functions with a prototype like: |
3010 | ||
3011 | @smallexample | |
3012 | int @var{function} (FILE *stream, const struct printf_info *info, | |
54d79e99 | 3013 | const void *const *args) |
28f540f4 RM |
3014 | @end smallexample |
3015 | ||
54d79e99 | 3016 | The @var{stream} argument passed to the handler function is the stream to |
28f540f4 RM |
3017 | which it should write output. |
3018 | ||
54d79e99 | 3019 | The @var{info} argument is a pointer to a structure that contains |
28f540f4 RM |
3020 | information about the various options that were included with the |
3021 | conversion in the template string. You should not modify this structure | |
3022 | inside your handler function. @xref{Conversion Specifier Options}, for | |
3023 | a description of this data structure. | |
3024 | ||
54d79e99 UD |
3025 | @c The following changes some time back. --drepper@gnu, 1996/11/14 |
3026 | @c | |
3027 | @c The @code{ap_pointer} argument is used to pass the tail of the variable | |
3028 | @c argument list containing the values to be printed to your handler. | |
3029 | @c Unlike most other functions that can be passed an explicit variable | |
3030 | @c argument list, this is a @emph{pointer} to a @code{va_list}, rather than | |
3031 | @c the @code{va_list} itself. Thus, you should fetch arguments by | |
3032 | @c means of @code{va_arg (*ap_pointer, @var{type})}. | |
3033 | @c | |
3034 | @c (Passing a pointer here allows the function that calls your handler | |
3035 | @c function to update its own @code{va_list} variable to account for the | |
3036 | @c arguments that your handler processes. @xref{Variadic Functions}.) | |
3037 | ||
3038 | The @var{args} is a vector of pointers to the arguments data. | |
04b9968b | 3039 | The number of arguments was determined by calling the argument |
54d79e99 | 3040 | information function provided by the user. |
28f540f4 RM |
3041 | |
3042 | Your handler function should return a value just like @code{printf} | |
3043 | does: it should return the number of characters it has written, or a | |
3044 | negative value to indicate an error. | |
3045 | ||
3046 | @comment printf.h | |
3047 | @comment GNU | |
3048 | @deftp {Data Type} printf_function | |
3049 | This is the data type that a handler function should have. | |
3050 | @end deftp | |
3051 | ||
3052 | If you are going to use @w{@code{parse_printf_format}} in your | |
54d79e99 | 3053 | application, you must also define a function to pass as the |
28f540f4 | 3054 | @var{arginfo-function} argument for each new conversion you install with |
19c3f208 | 3055 | @code{register_printf_function}. |
28f540f4 | 3056 | |
54d79e99 | 3057 | You have to define these functions with a prototype like: |
28f540f4 RM |
3058 | |
3059 | @smallexample | |
3060 | int @var{function} (const struct printf_info *info, | |
3061 | size_t n, int *argtypes) | |
3062 | @end smallexample | |
3063 | ||
3064 | The return value from the function should be the number of arguments the | |
3065 | conversion expects. The function should also fill in no more than | |
3066 | @var{n} elements of the @var{argtypes} array with information about the | |
3067 | types of each of these arguments. This information is encoded using the | |
3068 | various @samp{PA_} macros. (You will notice that this is the same | |
3069 | calling convention @code{parse_printf_format} itself uses.) | |
3070 | ||
3071 | @comment printf.h | |
3072 | @comment GNU | |
3073 | @deftp {Data Type} printf_arginfo_function | |
3074 | This type is used to describe functions that return information about | |
3075 | the number and type of arguments used by a conversion specifier. | |
3076 | @end deftp | |
3077 | ||
3078 | @node Printf Extension Example | |
3079 | @subsection @code{printf} Extension Example | |
3080 | ||
3081 | Here is an example showing how to define a @code{printf} handler function. | |
19c3f208 | 3082 | This program defines a data structure called a @code{Widget} and |
28f540f4 RM |
3083 | defines the @samp{%W} conversion to print information about @w{@code{Widget *}} |
3084 | arguments, including the pointer value and the name stored in the data | |
3085 | structure. The @samp{%W} conversion supports the minimum field width and | |
3086 | left-justification options, but ignores everything else. | |
3087 | ||
3088 | @smallexample | |
3089 | @include rprintf.c.texi | |
3090 | @end smallexample | |
3091 | ||
3092 | The output produced by this program looks like: | |
3093 | ||
3094 | @smallexample | |
3095 | |<Widget 0xffeffb7c: mywidget>| | |
3096 | | <Widget 0xffeffb7c: mywidget>| | |
3097 | |<Widget 0xffeffb7c: mywidget> | | |
3098 | @end smallexample | |
3099 | ||
29bb8719 UD |
3100 | @node Predefined Printf Handlers |
3101 | @subsection Predefined @code{printf} Handlers | |
3102 | ||
3103 | The GNU libc also contains a concrete and useful application of the | |
3104 | @code{printf} handler extension. There are two functions available | |
3105 | which implement a special way to print floating-point numbers. | |
3106 | ||
3107 | @comment printf.h | |
3108 | @comment GNU | |
3109 | @deftypefun int printf_size (FILE *@var{fp}, const struct printf_info *@var{info}, const void *const *@var{args}) | |
3110 | Print a given floating point number as for the format @code{%f} except | |
3111 | that there is a postfix character indicating the divisor for the | |
3112 | number to make this less than 1000. There are two possible divisors: | |
04b9968b | 3113 | powers of 1024 or powers of 1000. Which one is used depends on the |
29bb8719 UD |
3114 | format character specified while registered this handler. If the |
3115 | character is of lower case, 1024 is used. For upper case characters, | |
3116 | 1000 is used. | |
3117 | ||
3118 | The postfix tag corresponds to bytes, kilobytes, megabytes, gigabytes, | |
3119 | etc. The full table is: | |
3120 | ||
779ae82e UD |
3121 | @ifinfo |
3122 | @multitable @hsep @vsep {' '} {2^10 (1024)} {zetta} {Upper} {10^24 (1000)} | |
29bb8719 UD |
3123 | @item low @tab Multiplier @tab From @tab Upper @tab Multiplier |
3124 | @item ' ' @tab 1 @tab @tab ' ' @tab 1 | |
3125 | @item k @tab 2^10 (1024) @tab kilo @tab K @tab 10^3 (1000) | |
3126 | @item m @tab 2^20 @tab mega @tab M @tab 10^6 | |
3127 | @item g @tab 2^30 @tab giga @tab G @tab 10^9 | |
3128 | @item t @tab 2^40 @tab tera @tab T @tab 10^12 | |
3129 | @item p @tab 2^50 @tab peta @tab P @tab 10^15 | |
3130 | @item e @tab 2^60 @tab exa @tab E @tab 10^18 | |
3131 | @item z @tab 2^70 @tab zetta @tab Z @tab 10^21 | |
3132 | @item y @tab 2^80 @tab yotta @tab Y @tab 10^24 | |
3133 | @end multitable | |
779ae82e UD |
3134 | @end ifinfo |
3135 | @iftex | |
3136 | @tex | |
3137 | \hbox to\hsize{\hfil\vbox{\offinterlineskip | |
3138 | \hrule | |
3139 | \halign{\strut#& \vrule#\tabskip=1em plus2em& {\tt#}\hfil& \vrule#& #\hfil& \vrule#& #\hfil& \vrule#& {\tt#}\hfil& \vrule#& #\hfil& \vrule#\tabskip=0pt\cr | |
3140 | \noalign{\hrule} | |
3141 | \omit&height2pt&\omit&&\omit&&\omit&&\omit&&\omit&\cr | |
3142 | && \omit low && Multiplier && From && \omit Upper && Multiplier &\cr | |
3143 | \omit&height2pt&\omit&&\omit&&\omit&&\omit&&\omit&\cr | |
3144 | \noalign{\hrule} | |
3145 | && {\tt\char32} && 1 && && {\tt\char32} && 1 &\cr | |
3146 | && k && $2^{10} = 1024$ && kilo && K && $10^3 = 1000$ &\cr | |
3147 | && m && $2^{20}$ && mega && M && $10^6$ &\cr | |
3148 | && g && $2^{30}$ && giga && G && $10^9$ &\cr | |
3149 | && t && $2^{40}$ && tera && T && $10^{12}$ &\cr | |
3150 | && p && $2^{50}$ && peta && P && $10^{15}$ &\cr | |
3151 | && e && $2^{60}$ && exa && E && $10^{18}$ &\cr | |
3152 | && z && $2^{70}$ && zetta && Z && $10^{21}$ &\cr | |
3153 | && y && $2^{80}$ && yotta && Y && $10^{24}$ &\cr | |
3154 | \noalign{\hrule}}}\hfil} | |
3155 | @end tex | |
3156 | @end iftex | |
29bb8719 UD |
3157 | |
3158 | The default precision is 3, i.e., 1024 is printed with a lower-case | |
3159 | format character as if it were @code{%.3fk} and will yield @code{1.000k}. | |
3160 | @end deftypefun | |
3161 | ||
3162 | Due to the requirements of @code{register_printf_function} we must also | |
04b9968b | 3163 | provide the function which returns information about the arguments. |
29bb8719 UD |
3164 | |
3165 | @comment printf.h | |
3166 | @comment GNU | |
3167 | @deftypefun int printf_size_info (const struct printf_info *@var{info}, size_t @var{n}, int *@var{argtypes}) | |
3168 | This function will return in @var{argtypes} the information about the | |
3169 | used parameters in the way the @code{vfprintf} implementation expects | |
3170 | it. The format always takes one argument. | |
3171 | @end deftypefun | |
3172 | ||
3173 | To use these functions both functions must be registered with a call like | |
3174 | ||
3175 | @smallexample | |
3176 | register_printf_function ('B', printf_size, printf_size_info); | |
3177 | @end smallexample | |
3178 | ||
3179 | Here we register the functions to print numbers as powers of 1000 since | |
f2ea0f5b | 3180 | the format character @code{'B'} is an upper-case character. If we |
29bb8719 UD |
3181 | would additionally use @code{'b'} in a line like |
3182 | ||
3183 | @smallexample | |
3184 | register_printf_function ('b', printf_size, printf_size_info); | |
3185 | @end smallexample | |
3186 | ||
3187 | @noindent | |
04b9968b UD |
3188 | we could also print using a power of 1024. Please note that all that is |
3189 | different in these two lines is the format specifier. The | |
3190 | @code{printf_size} function knows about the difference between lower and upper | |
29bb8719 UD |
3191 | case format specifiers. |
3192 | ||
3193 | The use of @code{'B'} and @code{'b'} is no coincidence. Rather it is | |
3194 | the preferred way to use this functionality since it is available on | |
04b9968b | 3195 | some other systems which also use format specifiers. |
29bb8719 | 3196 | |
28f540f4 RM |
3197 | @node Formatted Input |
3198 | @section Formatted Input | |
3199 | ||
3200 | @cindex formatted input from a stream | |
3201 | @cindex reading from a stream, formatted | |
3202 | @cindex format string, for @code{scanf} | |
3203 | @cindex template, for @code{scanf} | |
3204 | The functions described in this section (@code{scanf} and related | |
3205 | functions) provide facilities for formatted input analogous to the | |
3206 | formatted output facilities. These functions provide a mechanism for | |
3207 | reading arbitrary values under the control of a @dfn{format string} or | |
3208 | @dfn{template string}. | |
3209 | ||
3210 | @menu | |
3211 | * Formatted Input Basics:: Some basics to get you started. | |
3212 | * Input Conversion Syntax:: Syntax of conversion specifications. | |
3213 | * Table of Input Conversions:: Summary of input conversions and what they do. | |
3214 | * Numeric Input Conversions:: Details of conversions for reading numbers. | |
3215 | * String Input Conversions:: Details of conversions for reading strings. | |
3216 | * Dynamic String Input:: String conversions that @code{malloc} the buffer. | |
3217 | * Other Input Conversions:: Details of miscellaneous other conversions. | |
3218 | * Formatted Input Functions:: Descriptions of the actual functions. | |
3219 | * Variable Arguments Input:: @code{vscanf} and friends. | |
3220 | @end menu | |
3221 | ||
3222 | @node Formatted Input Basics | |
3223 | @subsection Formatted Input Basics | |
3224 | ||
3225 | Calls to @code{scanf} are superficially similar to calls to | |
3226 | @code{printf} in that arbitrary arguments are read under the control of | |
3227 | a template string. While the syntax of the conversion specifications in | |
3228 | the template is very similar to that for @code{printf}, the | |
3229 | interpretation of the template is oriented more towards free-format | |
3230 | input and simple pattern matching, rather than fixed-field formatting. | |
3231 | For example, most @code{scanf} conversions skip over any amount of | |
3232 | ``white space'' (including spaces, tabs, and newlines) in the input | |
3233 | file, and there is no concept of precision for the numeric input | |
3234 | conversions as there is for the corresponding output conversions. | |
3235 | Ordinarily, non-whitespace characters in the template are expected to | |
3236 | match characters in the input stream exactly, but a matching failure is | |
3237 | distinct from an input error on the stream. | |
3238 | @cindex conversion specifications (@code{scanf}) | |
3239 | ||
3240 | Another area of difference between @code{scanf} and @code{printf} is | |
3241 | that you must remember to supply pointers rather than immediate values | |
3242 | as the optional arguments to @code{scanf}; the values that are read are | |
3243 | stored in the objects that the pointers point to. Even experienced | |
3244 | programmers tend to forget this occasionally, so if your program is | |
3245 | getting strange errors that seem to be related to @code{scanf}, you | |
3246 | might want to double-check this. | |
3247 | ||
3248 | When a @dfn{matching failure} occurs, @code{scanf} returns immediately, | |
3249 | leaving the first non-matching character as the next character to be | |
3250 | read from the stream. The normal return value from @code{scanf} is the | |
3251 | number of values that were assigned, so you can use this to determine if | |
3252 | a matching error happened before all the expected values were read. | |
3253 | @cindex matching failure, in @code{scanf} | |
3254 | ||
3255 | The @code{scanf} function is typically used for things like reading in | |
3256 | the contents of tables. For example, here is a function that uses | |
3257 | @code{scanf} to initialize an array of @code{double}: | |
3258 | ||
3259 | @smallexample | |
3260 | void | |
3261 | readarray (double *array, int n) | |
3262 | @{ | |
3263 | int i; | |
3264 | for (i=0; i<n; i++) | |
3265 | if (scanf (" %lf", &(array[i])) != 1) | |
3266 | invalid_input_error (); | |
3267 | @} | |
3268 | @end smallexample | |
3269 | ||
3270 | The formatted input functions are not used as frequently as the | |
3271 | formatted output functions. Partly, this is because it takes some care | |
3272 | to use them properly. Another reason is that it is difficult to recover | |
3273 | from a matching error. | |
3274 | ||
3275 | If you are trying to read input that doesn't match a single, fixed | |
3276 | pattern, you may be better off using a tool such as Flex to generate a | |
3277 | lexical scanner, or Bison to generate a parser, rather than using | |
9f508673 UD |
3278 | @code{scanf}. For more information about these tools, see @ref{Top, , , |
3279 | flex.info, Flex: The Lexical Scanner Generator}, and @ref{Top, , , | |
28f540f4 RM |
3280 | bison.info, The Bison Reference Manual}. |
3281 | ||
3282 | @node Input Conversion Syntax | |
3283 | @subsection Input Conversion Syntax | |
3284 | ||
3285 | A @code{scanf} template string is a string that contains ordinary | |
3286 | multibyte characters interspersed with conversion specifications that | |
3287 | start with @samp{%}. | |
3288 | ||
3289 | Any whitespace character (as defined by the @code{isspace} function; | |
3290 | @pxref{Classification of Characters}) in the template causes any number | |
3291 | of whitespace characters in the input stream to be read and discarded. | |
3292 | The whitespace characters that are matched need not be exactly the same | |
3293 | whitespace characters that appear in the template string. For example, | |
3294 | write @samp{ , } in the template to recognize a comma with optional | |
3295 | whitespace before and after. | |
3296 | ||
3297 | Other characters in the template string that are not part of conversion | |
3298 | specifications must match characters in the input stream exactly; if | |
3299 | this is not the case, a matching failure occurs. | |
3300 | ||
3301 | The conversion specifications in a @code{scanf} template string | |
3302 | have the general form: | |
3303 | ||
3304 | @smallexample | |
3305 | % @var{flags} @var{width} @var{type} @var{conversion} | |
3306 | @end smallexample | |
3307 | ||
3308 | In more detail, an input conversion specification consists of an initial | |
3309 | @samp{%} character followed in sequence by: | |
3310 | ||
3311 | @itemize @bullet | |
3312 | @item | |
3313 | An optional @dfn{flag character} @samp{*}, which says to ignore the text | |
3314 | read for this specification. When @code{scanf} finds a conversion | |
3315 | specification that uses this flag, it reads input as directed by the | |
3316 | rest of the conversion specification, but it discards this input, does | |
3317 | not use a pointer argument, and does not increment the count of | |
3318 | successful assignments. | |
3319 | @cindex flag character (@code{scanf}) | |
3320 | ||
3321 | @item | |
3322 | An optional flag character @samp{a} (valid with string conversions only) | |
3323 | which requests allocation of a buffer long enough to store the string in. | |
3324 | (This is a GNU extension.) | |
3325 | @xref{Dynamic String Input}. | |
3326 | ||
3327 | @item | |
3328 | An optional decimal integer that specifies the @dfn{maximum field | |
3329 | width}. Reading of characters from the input stream stops either when | |
3330 | this maximum is reached or when a non-matching character is found, | |
3331 | whichever happens first. Most conversions discard initial whitespace | |
3332 | characters (those that don't are explicitly documented), and these | |
3333 | discarded characters don't count towards the maximum field width. | |
3334 | String input conversions store a null character to mark the end of the | |
3335 | input; the maximum field width does not include this terminator. | |
3336 | @cindex maximum field width (@code{scanf}) | |
3337 | ||
3338 | @item | |
3339 | An optional @dfn{type modifier character}. For example, you can | |
3340 | specify a type modifier of @samp{l} with integer conversions such as | |
3341 | @samp{%d} to specify that the argument is a pointer to a @code{long int} | |
3342 | rather than a pointer to an @code{int}. | |
3343 | @cindex type modifier character (@code{scanf}) | |
3344 | ||
3345 | @item | |
3346 | A character that specifies the conversion to be applied. | |
3347 | @end itemize | |
3348 | ||
19c3f208 | 3349 | The exact options that are permitted and how they are interpreted vary |
28f540f4 RM |
3350 | between the different conversion specifiers. See the descriptions of the |
3351 | individual conversions for information about the particular options that | |
3352 | they allow. | |
3353 | ||
3354 | With the @samp{-Wformat} option, the GNU C compiler checks calls to | |
3355 | @code{scanf} and related functions. It examines the format string and | |
3356 | verifies that the correct number and types of arguments are supplied. | |
3357 | There is also a GNU C syntax to tell the compiler that a function you | |
19c3f208 | 3358 | write uses a @code{scanf}-style format string. |
28f540f4 RM |
3359 | @xref{Function Attributes, , Declaring Attributes of Functions, |
3360 | gcc.info, Using GNU CC}, for more information. | |
3361 | ||
3362 | @node Table of Input Conversions | |
3363 | @subsection Table of Input Conversions | |
3364 | @cindex input conversions, for @code{scanf} | |
3365 | ||
3366 | Here is a table that summarizes the various conversion specifications: | |
3367 | ||
3368 | @table @asis | |
3369 | @item @samp{%d} | |
3370 | Matches an optionally signed integer written in decimal. @xref{Numeric | |
3371 | Input Conversions}. | |
3372 | ||
3373 | @item @samp{%i} | |
3374 | Matches an optionally signed integer in any of the formats that the C | |
3375 | language defines for specifying an integer constant. @xref{Numeric | |
3376 | Input Conversions}. | |
3377 | ||
3378 | @item @samp{%o} | |
3379 | Matches an unsigned integer written in octal radix. | |
3380 | @xref{Numeric Input Conversions}. | |
3381 | ||
3382 | @item @samp{%u} | |
3383 | Matches an unsigned integer written in decimal radix. | |
3384 | @xref{Numeric Input Conversions}. | |
3385 | ||
3386 | @item @samp{%x}, @samp{%X} | |
3387 | Matches an unsigned integer written in hexadecimal radix. | |
3388 | @xref{Numeric Input Conversions}. | |
3389 | ||
3390 | @item @samp{%e}, @samp{%f}, @samp{%g}, @samp{%E}, @samp{%G} | |
3391 | Matches an optionally signed floating-point number. @xref{Numeric Input | |
3392 | Conversions}. | |
3393 | ||
3394 | @item @samp{%s} | |
b5e73f56 | 3395 | |
28f540f4 | 3396 | Matches a string containing only non-whitespace characters. |
b5e73f56 UD |
3397 | @xref{String Input Conversions}. The presence of the @samp{l} modifier |
3398 | determines whether the output is stored as a wide character string or a | |
3399 | multibyte string. If @samp{%s} is used in a wide character function the | |
3400 | string is converted as with multiple calls to @code{wcrtomb} into a | |
3401 | multibyte string. This means that the buffer must provide room for | |
3402 | @code{MB_CUR_MAX} bytes for each wide character read. In case | |
3403 | @samp{%ls} is used in a multibyte function the result is converted into | |
3404 | wide characters as with multiple calls of @code{mbrtowc} before being | |
3405 | stored in the user provided buffer. | |
3406 | ||
3407 | @item @samp{%S} | |
3408 | This is an alias for @samp{%ls} which is supported for compatibility | |
3409 | with the Unix standard. | |
28f540f4 RM |
3410 | |
3411 | @item @samp{%[} | |
3412 | Matches a string of characters that belong to a specified set. | |
b5e73f56 UD |
3413 | @xref{String Input Conversions}. The presence of the @samp{l} modifier |
3414 | determines whether the output is stored as a wide character string or a | |
3415 | multibyte string. If @samp{%[} is used in a wide character function the | |
3416 | string is converted as with multiple calls to @code{wcrtomb} into a | |
3417 | multibyte string. This means that the buffer must provide room for | |
3418 | @code{MB_CUR_MAX} bytes for each wide character read. In case | |
3419 | @samp{%l[} is used in a multibyte function the result is converted into | |
3420 | wide characters as with multiple calls of @code{mbrtowc} before being | |
3421 | stored in the user provided buffer. | |
28f540f4 RM |
3422 | |
3423 | @item @samp{%c} | |
3424 | Matches a string of one or more characters; the number of characters | |
3425 | read is controlled by the maximum field width given for the conversion. | |
3426 | @xref{String Input Conversions}. | |
3427 | ||
b5e73f56 UD |
3428 | If the @samp{%c} is used in a wide stream function the read value is |
3429 | converted from a wide character to the corresponding multibyte character | |
3430 | before storing it. Note that this conversion can produce more than one | |
3431 | byte of output and therefore the provided buffer be large enough for up | |
3432 | to @code{MB_CUR_MAX} bytes for each character. If @samp{%lc} is used in | |
3433 | a multibyte function the input is treated as a multibyte sequence (and | |
3434 | not bytes) and the result is converted as with calls to @code{mbrtowc}. | |
3435 | ||
3436 | @item @samp{%C} | |
3437 | This is an alias for @samp{%lc} which is supported for compatibility | |
3438 | with the Unix standard. | |
3439 | ||
28f540f4 RM |
3440 | @item @samp{%p} |
3441 | Matches a pointer value in the same implementation-defined format used | |
3442 | by the @samp{%p} output conversion for @code{printf}. @xref{Other Input | |
3443 | Conversions}. | |
3444 | ||
3445 | @item @samp{%n} | |
3446 | This conversion doesn't read any characters; it records the number of | |
3447 | characters read so far by this call. @xref{Other Input Conversions}. | |
3448 | ||
3449 | @item @samp{%%} | |
3450 | This matches a literal @samp{%} character in the input stream. No | |
3451 | corresponding argument is used. @xref{Other Input Conversions}. | |
3452 | @end table | |
3453 | ||
3454 | If the syntax of a conversion specification is invalid, the behavior is | |
3455 | undefined. If there aren't enough function arguments provided to supply | |
3456 | addresses for all the conversion specifications in the template strings | |
3457 | that perform assignments, or if the arguments are not of the correct | |
3458 | types, the behavior is also undefined. On the other hand, extra | |
3459 | arguments are simply ignored. | |
3460 | ||
3461 | @node Numeric Input Conversions | |
3462 | @subsection Numeric Input Conversions | |
3463 | ||
3464 | This section describes the @code{scanf} conversions for reading numeric | |
3465 | values. | |
3466 | ||
3467 | The @samp{%d} conversion matches an optionally signed integer in decimal | |
3468 | radix. The syntax that is recognized is the same as that for the | |
3469 | @code{strtol} function (@pxref{Parsing of Integers}) with the value | |
3470 | @code{10} for the @var{base} argument. | |
3471 | ||
3472 | The @samp{%i} conversion matches an optionally signed integer in any of | |
3473 | the formats that the C language defines for specifying an integer | |
3474 | constant. The syntax that is recognized is the same as that for the | |
3475 | @code{strtol} function (@pxref{Parsing of Integers}) with the value | |
3476 | @code{0} for the @var{base} argument. (You can print integers in this | |
3477 | syntax with @code{printf} by using the @samp{#} flag character with the | |
3478 | @samp{%x}, @samp{%o}, or @samp{%d} conversion. @xref{Integer Conversions}.) | |
3479 | ||
3480 | For example, any of the strings @samp{10}, @samp{0xa}, or @samp{012} | |
3481 | could be read in as integers under the @samp{%i} conversion. Each of | |
3482 | these specifies a number with decimal value @code{10}. | |
3483 | ||
3484 | The @samp{%o}, @samp{%u}, and @samp{%x} conversions match unsigned | |
3485 | integers in octal, decimal, and hexadecimal radices, respectively. The | |
3486 | syntax that is recognized is the same as that for the @code{strtoul} | |
3487 | function (@pxref{Parsing of Integers}) with the appropriate value | |
3488 | (@code{8}, @code{10}, or @code{16}) for the @var{base} argument. | |
3489 | ||
3490 | The @samp{%X} conversion is identical to the @samp{%x} conversion. They | |
3491 | both permit either uppercase or lowercase letters to be used as digits. | |
3492 | ||
3493 | The default type of the corresponding argument for the @code{%d} and | |
3494 | @code{%i} conversions is @code{int *}, and @code{unsigned int *} for the | |
3495 | other integer conversions. You can use the following type modifiers to | |
3496 | specify other sizes of integer: | |
3497 | ||
3498 | @table @samp | |
cc3fa755 UD |
3499 | @item hh |
3500 | Specifies that the argument is a @code{signed char *} or @code{unsigned | |
3501 | char *}. | |
3502 | ||
ec751a23 | 3503 | This modifier was introduced in @w{ISO C99}. |
e852e889 | 3504 | |
28f540f4 RM |
3505 | @item h |
3506 | Specifies that the argument is a @code{short int *} or @code{unsigned | |
3507 | short int *}. | |
3508 | ||
e852e889 UD |
3509 | @item j |
3510 | Specifies that the argument is a @code{intmax_t *} or @code{uintmax_t *}. | |
3511 | ||
ec751a23 | 3512 | This modifier was introduced in @w{ISO C99}. |
e852e889 | 3513 | |
28f540f4 RM |
3514 | @item l |
3515 | Specifies that the argument is a @code{long int *} or @code{unsigned | |
3516 | long int *}. Two @samp{l} characters is like the @samp{L} modifier, below. | |
3517 | ||
b5e73f56 UD |
3518 | If used with @samp{%c} or @samp{%s} the corresponding parameter is |
3519 | considered as a pointer to a wide character or wide character string | |
3520 | respectively. This use of @samp{l} was introduced in @w{Amendment 1} to | |
3521 | @w{ISO C90}. | |
3522 | ||
28f540f4 RM |
3523 | @need 100 |
3524 | @item ll | |
3525 | @itemx L | |
3526 | @itemx q | |
3527 | Specifies that the argument is a @code{long long int *} or @code{unsigned long long int *}. (The @code{long long} type is an extension supported by the | |
3528 | GNU C compiler. For systems that don't provide extra-long integers, this | |
3529 | is the same as @code{long int}.) | |
3530 | ||
3531 | The @samp{q} modifier is another name for the same thing, which comes | |
3532 | from 4.4 BSD; a @w{@code{long long int}} is sometimes called a ``quad'' | |
3533 | @code{int}. | |
e852e889 UD |
3534 | |
3535 | @item t | |
3536 | Specifies that the argument is a @code{ptrdiff_t *}. | |
3537 | ||
ec751a23 | 3538 | This modifier was introduced in @w{ISO C99}. |
e852e889 UD |
3539 | |
3540 | @item z | |
3541 | Specifies that the argument is a @code{size_t *}. | |
3542 | ||
ec751a23 | 3543 | This modifier was introduced in @w{ISO C99}. |
28f540f4 RM |
3544 | @end table |
3545 | ||
3546 | All of the @samp{%e}, @samp{%f}, @samp{%g}, @samp{%E}, and @samp{%G} | |
3547 | input conversions are interchangeable. They all match an optionally | |
3548 | signed floating point number, in the same syntax as for the | |
3549 | @code{strtod} function (@pxref{Parsing of Floats}). | |
3550 | ||
3551 | For the floating-point input conversions, the default argument type is | |
3552 | @code{float *}. (This is different from the corresponding output | |
3553 | conversions, where the default type is @code{double}; remember that | |
3554 | @code{float} arguments to @code{printf} are converted to @code{double} | |
3555 | by the default argument promotions, but @code{float *} arguments are | |
3556 | not promoted to @code{double *}.) You can specify other sizes of float | |
3557 | using these type modifiers: | |
3558 | ||
3559 | @table @samp | |
3560 | @item l | |
3561 | Specifies that the argument is of type @code{double *}. | |
3562 | ||
3563 | @item L | |
3564 | Specifies that the argument is of type @code{long double *}. | |
3565 | @end table | |
3566 | ||
2c6fe0bd UD |
3567 | For all the above number parsing formats there is an additional optional |
3568 | flag @samp{'}. When this flag is given the @code{scanf} function | |
3569 | expects the number represented in the input string to be formatted | |
3570 | according to the grouping rules of the currently selected locale | |
3571 | (@pxref{General Numeric}). | |
3572 | ||
3573 | If the @code{"C"} or @code{"POSIX"} locale is selected there is no | |
3574 | difference. But for a locale which specifies values for the appropriate | |
3575 | fields in the locale the input must have the correct form in the input. | |
3576 | Otherwise the longest prefix with a correct form is processed. | |
3577 | ||
28f540f4 RM |
3578 | @node String Input Conversions |
3579 | @subsection String Input Conversions | |
3580 | ||
3581 | This section describes the @code{scanf} input conversions for reading | |
b5e73f56 UD |
3582 | string and character values: @samp{%s}, @samp{%S}, @samp{%[}, @samp{%c}, |
3583 | and @samp{%C}. | |
28f540f4 RM |
3584 | |
3585 | You have two options for how to receive the input from these | |
3586 | conversions: | |
3587 | ||
3588 | @itemize @bullet | |
3589 | @item | |
b5e73f56 UD |
3590 | Provide a buffer to store it in. This is the default. You should |
3591 | provide an argument of type @code{char *} or @code{wchar_t *} (the | |
3592 | latter of the @samp{l} modifier is present). | |
28f540f4 RM |
3593 | |
3594 | @strong{Warning:} To make a robust program, you must make sure that the | |
3595 | input (plus its terminating null) cannot possibly exceed the size of the | |
3596 | buffer you provide. In general, the only way to do this is to specify a | |
3597 | maximum field width one less than the buffer size. @strong{If you | |
3598 | provide the buffer, always specify a maximum field width to prevent | |
3599 | overflow.} | |
3600 | ||
3601 | @item | |
3602 | Ask @code{scanf} to allocate a big enough buffer, by specifying the | |
3603 | @samp{a} flag character. This is a GNU extension. You should provide | |
3604 | an argument of type @code{char **} for the buffer address to be stored | |
3605 | in. @xref{Dynamic String Input}. | |
3606 | @end itemize | |
3607 | ||
3608 | The @samp{%c} conversion is the simplest: it matches a fixed number of | |
04b9968b | 3609 | characters, always. The maximum field width says how many characters to |
28f540f4 RM |
3610 | read; if you don't specify the maximum, the default is 1. This |
3611 | conversion doesn't append a null character to the end of the text it | |
3612 | reads. It also does not skip over initial whitespace characters. It | |
3613 | reads precisely the next @var{n} characters, and fails if it cannot get | |
3614 | that many. Since there is always a maximum field width with @samp{%c} | |
3615 | (whether specified, or 1 by default), you can always prevent overflow by | |
3616 | making the buffer long enough. | |
b5e73f56 UD |
3617 | @comment Is character == byte here??? --drepper |
3618 | ||
3619 | If the format is @samp{%lc} or @samp{%C} the function stores wide | |
3620 | characters which are converted using the conversion determined at the | |
3621 | time the stream was opened from the external byte stream. The number of | |
3622 | bytes read from the medium is limited by @code{MB_CUR_LEN * @var{n}} but | |
3623 | at most @var{n} wide character get stored in the output string. | |
28f540f4 RM |
3624 | |
3625 | The @samp{%s} conversion matches a string of non-whitespace characters. | |
3626 | It skips and discards initial whitespace, but stops when it encounters | |
3627 | more whitespace after having read something. It stores a null character | |
3628 | at the end of the text that it reads. | |
3629 | ||
3630 | For example, reading the input: | |
3631 | ||
3632 | @smallexample | |
3633 | hello, world | |
3634 | @end smallexample | |
3635 | ||
3636 | @noindent | |
3637 | with the conversion @samp{%10c} produces @code{" hello, wo"}, but | |
3638 | reading the same input with the conversion @samp{%10s} produces | |
3639 | @code{"hello,"}. | |
3640 | ||
3641 | @strong{Warning:} If you do not specify a field width for @samp{%s}, | |
3642 | then the number of characters read is limited only by where the next | |
3643 | whitespace character appears. This almost certainly means that invalid | |
3644 | input can make your program crash---which is a bug. | |
3645 | ||
b5e73f56 UD |
3646 | The @samp{%ls} and @samp{%S} format are handled just like @samp{%s} |
3647 | except that the external byte sequence is converted using the conversion | |
3648 | associated with the stream to wide characters with their own encoding. | |
3649 | A width or precision specified with the format do not directly determine | |
3650 | how many bytes are read from the stream since they measure wide | |
3651 | characters. But an upper limit can be computed by multiplying the value | |
3652 | of the width or precision by @code{MB_CUR_MAX}. | |
3653 | ||
28f540f4 RM |
3654 | To read in characters that belong to an arbitrary set of your choice, |
3655 | use the @samp{%[} conversion. You specify the set between the @samp{[} | |
3656 | character and a following @samp{]} character, using the same syntax used | |
3657 | in regular expressions. As special cases: | |
3658 | ||
3659 | @itemize @bullet | |
19c3f208 | 3660 | @item |
28f540f4 RM |
3661 | A literal @samp{]} character can be specified as the first character |
3662 | of the set. | |
3663 | ||
19c3f208 | 3664 | @item |
28f540f4 RM |
3665 | An embedded @samp{-} character (that is, one that is not the first or |
3666 | last character of the set) is used to specify a range of characters. | |
3667 | ||
19c3f208 | 3668 | @item |
28f540f4 RM |
3669 | If a caret character @samp{^} immediately follows the initial @samp{[}, |
3670 | then the set of allowed input characters is the everything @emph{except} | |
3671 | the characters listed. | |
3672 | @end itemize | |
3673 | ||
3674 | The @samp{%[} conversion does not skip over initial whitespace | |
3675 | characters. | |
3676 | ||
3677 | Here are some examples of @samp{%[} conversions and what they mean: | |
3678 | ||
3679 | @table @samp | |
3680 | @item %25[1234567890] | |
3681 | Matches a string of up to 25 digits. | |
3682 | ||
3683 | @item %25[][] | |
3684 | Matches a string of up to 25 square brackets. | |
3685 | ||
3686 | @item %25[^ \f\n\r\t\v] | |
3687 | Matches a string up to 25 characters long that doesn't contain any of | |
3688 | the standard whitespace characters. This is slightly different from | |
3689 | @samp{%s}, because if the input begins with a whitespace character, | |
3690 | @samp{%[} reports a matching failure while @samp{%s} simply discards the | |
3691 | initial whitespace. | |
3692 | ||
19c3f208 | 3693 | @item %25[a-z] |
28f540f4 RM |
3694 | Matches up to 25 lowercase characters. |
3695 | @end table | |
3696 | ||
b5e73f56 UD |
3697 | As for @samp{%c} and @samp{%s} the @samp{%[} format is also modified to |
3698 | produce wide characters if the @samp{l} modifier is present. All what | |
3699 | is said about @samp{%ls} above is true for @samp{%l[}. | |
3700 | ||
28f540f4 RM |
3701 | One more reminder: the @samp{%s} and @samp{%[} conversions are |
3702 | @strong{dangerous} if you don't specify a maximum width or use the | |
3703 | @samp{a} flag, because input too long would overflow whatever buffer you | |
3704 | have provided for it. No matter how long your buffer is, a user could | |
3705 | supply input that is longer. A well-written program reports invalid | |
3706 | input with a comprehensible error message, not with a crash. | |
3707 | ||
3708 | @node Dynamic String Input | |
3709 | @subsection Dynamically Allocating String Conversions | |
3710 | ||
3711 | A GNU extension to formatted input lets you safely read a string with no | |
3712 | maximum size. Using this feature, you don't supply a buffer; instead, | |
3713 | @code{scanf} allocates a buffer big enough to hold the data and gives | |
3714 | you its address. To use this feature, write @samp{a} as a flag | |
3715 | character, as in @samp{%as} or @samp{%a[0-9a-z]}. | |
3716 | ||
3717 | The pointer argument you supply for where to store the input should have | |
3718 | type @code{char **}. The @code{scanf} function allocates a buffer and | |
3719 | stores its address in the word that the argument points to. You should | |
3720 | free the buffer with @code{free} when you no longer need it. | |
3721 | ||
3722 | Here is an example of using the @samp{a} flag with the @samp{%[@dots{}]} | |
3723 | conversion specification to read a ``variable assignment'' of the form | |
3724 | @samp{@var{variable} = @var{value}}. | |
3725 | ||
3726 | @smallexample | |
3727 | @{ | |
3728 | char *variable, *value; | |
3729 | ||
3730 | if (2 > scanf ("%a[a-zA-Z0-9] = %a[^\n]\n", | |
3731 | &variable, &value)) | |
3732 | @{ | |
3733 | invalid_input_error (); | |
3734 | return 0; | |
3735 | @} | |
3736 | ||
3737 | @dots{} | |
3738 | @} | |
3739 | @end smallexample | |
3740 | ||
3741 | @node Other Input Conversions | |
3742 | @subsection Other Input Conversions | |
3743 | ||
3744 | This section describes the miscellaneous input conversions. | |
3745 | ||
3746 | The @samp{%p} conversion is used to read a pointer value. It recognizes | |
04b9968b | 3747 | the same syntax used by the @samp{%p} output conversion for |
28f540f4 RM |
3748 | @code{printf} (@pxref{Other Output Conversions}); that is, a hexadecimal |
3749 | number just as the @samp{%x} conversion accepts. The corresponding | |
3750 | argument should be of type @code{void **}; that is, the address of a | |
3751 | place to store a pointer. | |
3752 | ||
3753 | The resulting pointer value is not guaranteed to be valid if it was not | |
3754 | originally written during the same program execution that reads it in. | |
3755 | ||
3756 | The @samp{%n} conversion produces the number of characters read so far | |
3757 | by this call. The corresponding argument should be of type @code{int *}. | |
3758 | This conversion works in the same way as the @samp{%n} conversion for | |
3759 | @code{printf}; see @ref{Other Output Conversions}, for an example. | |
3760 | ||
3761 | The @samp{%n} conversion is the only mechanism for determining the | |
3762 | success of literal matches or conversions with suppressed assignments. | |
3763 | If the @samp{%n} follows the locus of a matching failure, then no value | |
3764 | is stored for it since @code{scanf} returns before processing the | |
3765 | @samp{%n}. If you store @code{-1} in that argument slot before calling | |
3766 | @code{scanf}, the presence of @code{-1} after @code{scanf} indicates an | |
3767 | error occurred before the @samp{%n} was reached. | |
3768 | ||
3769 | Finally, the @samp{%%} conversion matches a literal @samp{%} character | |
3770 | in the input stream, without using an argument. This conversion does | |
3771 | not permit any flags, field width, or type modifier to be specified. | |
3772 | ||
3773 | @node Formatted Input Functions | |
3774 | @subsection Formatted Input Functions | |
3775 | ||
3776 | Here are the descriptions of the functions for performing formatted | |
3777 | input. | |
3778 | Prototypes for these functions are in the header file @file{stdio.h}. | |
3779 | @pindex stdio.h | |
3780 | ||
3781 | @comment stdio.h | |
f65fd747 | 3782 | @comment ISO |
28f540f4 RM |
3783 | @deftypefun int scanf (const char *@var{template}, @dots{}) |
3784 | The @code{scanf} function reads formatted input from the stream | |
3785 | @code{stdin} under the control of the template string @var{template}. | |
3786 | The optional arguments are pointers to the places which receive the | |
3787 | resulting values. | |
3788 | ||
3789 | The return value is normally the number of successful assignments. If | |
04b9968b UD |
3790 | an end-of-file condition is detected before any matches are performed, |
3791 | including matches against whitespace and literal characters in the | |
3792 | template, then @code{EOF} is returned. | |
28f540f4 RM |
3793 | @end deftypefun |
3794 | ||
b5e73f56 UD |
3795 | @comment wchar.h |
3796 | @comment ISO | |
3797 | @deftypefun int wscanf (const wchar_t *@var{template}, @dots{}) | |
3798 | The @code{wscanf} function reads formatted input from the stream | |
3799 | @code{stdin} under the control of the template string @var{template}. | |
3800 | The optional arguments are pointers to the places which receive the | |
3801 | resulting values. | |
3802 | ||
3803 | The return value is normally the number of successful assignments. If | |
3804 | an end-of-file condition is detected before any matches are performed, | |
3805 | including matches against whitespace and literal characters in the | |
3806 | template, then @code{WEOF} is returned. | |
3807 | @end deftypefun | |
3808 | ||
28f540f4 | 3809 | @comment stdio.h |
f65fd747 | 3810 | @comment ISO |
28f540f4 RM |
3811 | @deftypefun int fscanf (FILE *@var{stream}, const char *@var{template}, @dots{}) |
3812 | This function is just like @code{scanf}, except that the input is read | |
3813 | from the stream @var{stream} instead of @code{stdin}. | |
3814 | @end deftypefun | |
3815 | ||
b5e73f56 UD |
3816 | @comment wchar.h |
3817 | @comment ISO | |
3818 | @deftypefun int fwscanf (FILE *@var{stream}, const wchar_t *@var{template}, @dots{}) | |
3819 | This function is just like @code{wscanf}, except that the input is read | |
3820 | from the stream @var{stream} instead of @code{stdin}. | |
3821 | @end deftypefun | |
3822 | ||
28f540f4 | 3823 | @comment stdio.h |
f65fd747 | 3824 | @comment ISO |
28f540f4 RM |
3825 | @deftypefun int sscanf (const char *@var{s}, const char *@var{template}, @dots{}) |
3826 | This is like @code{scanf}, except that the characters are taken from the | |
3827 | null-terminated string @var{s} instead of from a stream. Reaching the | |
3828 | end of the string is treated as an end-of-file condition. | |
3829 | ||
3830 | The behavior of this function is undefined if copying takes place | |
3831 | between objects that overlap---for example, if @var{s} is also given | |
b5e73f56 UD |
3832 | as an argument to receive a string read under control of the @samp{%s}, |
3833 | @samp{%S}, or @samp{%[} conversion. | |
3834 | @end deftypefun | |
3835 | ||
3836 | @comment wchar.h | |
3837 | @comment ISO | |
3838 | @deftypefun int swscanf (const wchar_t *@var{ws}, const char *@var{template}, @dots{}) | |
3839 | This is like @code{wscanf}, except that the characters are taken from the | |
3840 | null-terminated string @var{ws} instead of from a stream. Reaching the | |
3841 | end of the string is treated as an end-of-file condition. | |
3842 | ||
3843 | The behavior of this function is undefined if copying takes place | |
3844 | between objects that overlap---for example, if @var{ws} is also given as | |
3845 | an argument to receive a string read under control of the @samp{%s}, | |
3846 | @samp{%S}, or @samp{%[} conversion. | |
28f540f4 RM |
3847 | @end deftypefun |
3848 | ||
3849 | @node Variable Arguments Input | |
3850 | @subsection Variable Arguments Input Functions | |
3851 | ||
3852 | The functions @code{vscanf} and friends are provided so that you can | |
3853 | define your own variadic @code{scanf}-like functions that make use of | |
3854 | the same internals as the built-in formatted output functions. | |
3855 | These functions are analogous to the @code{vprintf} series of output | |
3856 | functions. @xref{Variable Arguments Output}, for important | |
3857 | information on how to use them. | |
3858 | ||
b5e73f56 UD |
3859 | @strong{Portability Note:} The functions listed in this section were |
3860 | introduced in @w{ISO C99} and were before available as GNU extensions. | |
28f540f4 RM |
3861 | |
3862 | @comment stdio.h | |
b5e73f56 | 3863 | @comment ISO |
28f540f4 | 3864 | @deftypefun int vscanf (const char *@var{template}, va_list @var{ap}) |
04b9968b | 3865 | This function is similar to @code{scanf}, but instead of taking |
28f540f4 RM |
3866 | a variable number of arguments directly, it takes an argument list |
3867 | pointer @var{ap} of type @code{va_list} (@pxref{Variadic Functions}). | |
3868 | @end deftypefun | |
3869 | ||
b5e73f56 UD |
3870 | @comment wchar.h |
3871 | @comment ISO | |
3872 | @deftypefun int vwscanf (const wchar_t *@var{template}, va_list @var{ap}) | |
3873 | This function is similar to @code{wscanf}, but instead of taking | |
3874 | a variable number of arguments directly, it takes an argument list | |
3875 | pointer @var{ap} of type @code{va_list} (@pxref{Variadic Functions}). | |
3876 | @end deftypefun | |
3877 | ||
28f540f4 | 3878 | @comment stdio.h |
b5e73f56 | 3879 | @comment ISO |
28f540f4 RM |
3880 | @deftypefun int vfscanf (FILE *@var{stream}, const char *@var{template}, va_list @var{ap}) |
3881 | This is the equivalent of @code{fscanf} with the variable argument list | |
3882 | specified directly as for @code{vscanf}. | |
3883 | @end deftypefun | |
3884 | ||
b5e73f56 UD |
3885 | @comment wchar.h |
3886 | @comment ISO | |
3887 | @deftypefun int vfwscanf (FILE *@var{stream}, const wchar_t *@var{template}, va_list @var{ap}) | |
3888 | This is the equivalent of @code{fwscanf} with the variable argument list | |
3889 | specified directly as for @code{vwscanf}. | |
3890 | @end deftypefun | |
3891 | ||
28f540f4 | 3892 | @comment stdio.h |
b5e73f56 | 3893 | @comment ISO |
28f540f4 RM |
3894 | @deftypefun int vsscanf (const char *@var{s}, const char *@var{template}, va_list @var{ap}) |
3895 | This is the equivalent of @code{sscanf} with the variable argument list | |
3896 | specified directly as for @code{vscanf}. | |
3897 | @end deftypefun | |
3898 | ||
b5e73f56 UD |
3899 | @comment wchar.h |
3900 | @comment ISO | |
3901 | @deftypefun int vswscanf (const wchar_t *@var{s}, const wchar_t *@var{template}, va_list @var{ap}) | |
3902 | This is the equivalent of @code{swscanf} with the variable argument list | |
3903 | specified directly as for @code{vwscanf}. | |
3904 | @end deftypefun | |
3905 | ||
28f540f4 RM |
3906 | In GNU C, there is a special construct you can use to let the compiler |
3907 | know that a function uses a @code{scanf}-style format string. Then it | |
3908 | can check the number and types of arguments in each call to the | |
3909 | function, and warn you when they do not match the format string. | |
04b9968b UD |
3910 | For details, @xref{Function Attributes, , Declaring Attributes of Functions, |
3911 | gcc.info, Using GNU CC}. | |
28f540f4 RM |
3912 | |
3913 | @node EOF and Errors | |
3914 | @section End-Of-File and Errors | |
3915 | ||
3916 | @cindex end of file, on a stream | |
3917 | Many of the functions described in this chapter return the value of the | |
3918 | macro @code{EOF} to indicate unsuccessful completion of the operation. | |
3919 | Since @code{EOF} is used to report both end of file and random errors, | |
3920 | it's often better to use the @code{feof} function to check explicitly | |
3921 | for end of file and @code{ferror} to check for errors. These functions | |
3922 | check indicators that are part of the internal state of the stream | |
3923 | object, indicators set if the appropriate condition was detected by a | |
3924 | previous I/O operation on that stream. | |
3925 | ||
28f540f4 | 3926 | @comment stdio.h |
f65fd747 | 3927 | @comment ISO |
28f540f4 | 3928 | @deftypevr Macro int EOF |
b5e73f56 UD |
3929 | This macro is an integer value that is returned by a number of narrow |
3930 | stream functions to indicate an end-of-file condition, or some other | |
3931 | error situation. With the GNU library, @code{EOF} is @code{-1}. In | |
3932 | other libraries, its value may be some other negative number. | |
3933 | ||
3934 | This symbol is declared in @file{stdio.h}. | |
3935 | @end deftypevr | |
3936 | ||
3937 | @comment wchar.h | |
3938 | @comment ISO | |
3939 | @deftypevr Macro int WEOF | |
3940 | This macro is an integer value that is returned by a number of wide | |
3941 | stream functions to indicate an end-of-file condition, or some other | |
3942 | error situation. With the GNU library, @code{WEOF} is @code{-1}. In | |
3943 | other libraries, its value may be some other negative number. | |
3944 | ||
3945 | This symbol is declared in @file{wchar.h}. | |
28f540f4 RM |
3946 | @end deftypevr |
3947 | ||
28f540f4 | 3948 | @comment stdio.h |
f65fd747 | 3949 | @comment ISO |
28f540f4 RM |
3950 | @deftypefun int feof (FILE *@var{stream}) |
3951 | The @code{feof} function returns nonzero if and only if the end-of-file | |
3952 | indicator for the stream @var{stream} is set. | |
b5e73f56 UD |
3953 | |
3954 | This symbol is declared in @file{stdio.h}. | |
28f540f4 RM |
3955 | @end deftypefun |
3956 | ||
7b4161bb UD |
3957 | @comment stdio.h |
3958 | @comment GNU | |
3959 | @deftypefun int feof_unlocked (FILE *@var{stream}) | |
3960 | The @code{feof_unlocked} function is equivalent to the @code{feof} | |
1dc843f7 | 3961 | function except that it does not implicitly lock the stream. |
7b4161bb UD |
3962 | |
3963 | This function is a GNU extension. | |
b5e73f56 UD |
3964 | |
3965 | This symbol is declared in @file{stdio.h}. | |
7b4161bb UD |
3966 | @end deftypefun |
3967 | ||
28f540f4 | 3968 | @comment stdio.h |
f65fd747 | 3969 | @comment ISO |
28f540f4 RM |
3970 | @deftypefun int ferror (FILE *@var{stream}) |
3971 | The @code{ferror} function returns nonzero if and only if the error | |
3972 | indicator for the stream @var{stream} is set, indicating that an error | |
3973 | has occurred on a previous operation on the stream. | |
b5e73f56 UD |
3974 | |
3975 | This symbol is declared in @file{stdio.h}. | |
28f540f4 RM |
3976 | @end deftypefun |
3977 | ||
7b4161bb UD |
3978 | @comment stdio.h |
3979 | @comment GNU | |
3980 | @deftypefun int ferror_unlocked (FILE *@var{stream}) | |
3981 | The @code{ferror_unlocked} function is equivalent to the @code{ferror} | |
1dc843f7 | 3982 | function except that it does not implicitly lock the stream. |
7b4161bb UD |
3983 | |
3984 | This function is a GNU extension. | |
b5e73f56 UD |
3985 | |
3986 | This symbol is declared in @file{stdio.h}. | |
7b4161bb UD |
3987 | @end deftypefun |
3988 | ||
28f540f4 RM |
3989 | In addition to setting the error indicator associated with the stream, |
3990 | the functions that operate on streams also set @code{errno} in the same | |
3991 | way as the corresponding low-level functions that operate on file | |
3992 | descriptors. For example, all of the functions that perform output to a | |
3993 | stream---such as @code{fputc}, @code{printf}, and @code{fflush}---are | |
3994 | implemented in terms of @code{write}, and all of the @code{errno} error | |
3995 | conditions defined for @code{write} are meaningful for these functions. | |
3996 | For more information about the descriptor-level I/O functions, see | |
3997 | @ref{Low-Level I/O}. | |
3998 | ||
460e040a UD |
3999 | @node Error Recovery |
4000 | @section Recovering from errors | |
4001 | ||
7ba73c63 | 4002 | You may explicitly clear the error and EOF flags with the @code{clearerr} |
460e040a UD |
4003 | function. |
4004 | ||
4005 | @comment stdio.h | |
4006 | @comment ISO | |
4007 | @deftypefun void clearerr (FILE *@var{stream}) | |
4008 | This function clears the end-of-file and error indicators for the | |
4009 | stream @var{stream}. | |
4010 | ||
4011 | The file positioning functions (@pxref{File Positioning}) also clear the | |
4012 | end-of-file indicator for the stream. | |
4013 | @end deftypefun | |
4014 | ||
7b4161bb UD |
4015 | @comment stdio.h |
4016 | @comment GNU | |
4017 | @deftypefun void clearerr_unlocked (FILE *@var{stream}) | |
4018 | The @code{clearerr_unlocked} function is equivalent to the @code{clearerr} | |
1dc843f7 | 4019 | function except that it does not implicitly lock the stream. |
7b4161bb UD |
4020 | |
4021 | This function is a GNU extension. | |
4022 | @end deftypefun | |
4023 | ||
460e040a UD |
4024 | Note that it is @emph{not} correct to just clear the error flag and retry |
4025 | a failed stream operation. After a failed write, any number of | |
4026 | characters since the last buffer flush may have been committed to the | |
4027 | file, while some buffered data may have been discarded. Merely retrying | |
4028 | can thus cause lost or repeated data. | |
4029 | ||
4030 | A failed read may leave the file pointer in an inappropriate position for | |
4031 | a second try. In both cases, you should seek to a known position before | |
4032 | retrying. | |
4033 | ||
4034 | Most errors that can happen are not recoverable --- a second try will | |
4035 | always fail again in the same way. So usually it is best to give up and | |
4036 | report the error to the user, rather than install complicated recovery | |
4037 | logic. | |
4038 | ||
7ba73c63 | 4039 | One important exception is @code{EINTR} (@pxref{Interrupted Primitives}). |
460e040a UD |
4040 | Many stream I/O implementations will treat it as an ordinary error, which |
4041 | can be quite inconvenient. You can avoid this hassle by installing all | |
4042 | signals with the @code{SA_RESTART} flag. | |
4043 | ||
4044 | For similar reasons, setting nonblocking I/O on a stream's file | |
4045 | descriptor is not usually advisable. | |
4046 | ||
28f540f4 RM |
4047 | @node Binary Streams |
4048 | @section Text and Binary Streams | |
4049 | ||
4050 | The GNU system and other POSIX-compatible operating systems organize all | |
4051 | files as uniform sequences of characters. However, some other systems | |
4052 | make a distinction between files containing text and files containing | |
f65fd747 | 4053 | binary data, and the input and output facilities of @w{ISO C} provide for |
28f540f4 RM |
4054 | this distinction. This section tells you how to write programs portable |
4055 | to such systems. | |
4056 | ||
4057 | @cindex text stream | |
4058 | @cindex binary stream | |
4059 | When you open a stream, you can specify either a @dfn{text stream} or a | |
4060 | @dfn{binary stream}. You indicate that you want a binary stream by | |
4061 | specifying the @samp{b} modifier in the @var{opentype} argument to | |
4062 | @code{fopen}; see @ref{Opening Streams}. Without this | |
4063 | option, @code{fopen} opens the file as a text stream. | |
4064 | ||
4065 | Text and binary streams differ in several ways: | |
4066 | ||
4067 | @itemize @bullet | |
4068 | @item | |
4069 | The data read from a text stream is divided into @dfn{lines} which are | |
4070 | terminated by newline (@code{'\n'}) characters, while a binary stream is | |
4071 | simply a long series of characters. A text stream might on some systems | |
4072 | fail to handle lines more than 254 characters long (including the | |
4073 | terminating newline character). | |
4074 | @cindex lines (in a text file) | |
4075 | ||
4076 | @item | |
4077 | On some systems, text files can contain only printing characters, | |
4078 | horizontal tab characters, and newlines, and so text streams may not | |
4079 | support other characters. However, binary streams can handle any | |
4080 | character value. | |
4081 | ||
4082 | @item | |
4083 | Space characters that are written immediately preceding a newline | |
4084 | character in a text stream may disappear when the file is read in again. | |
4085 | ||
4086 | @item | |
4087 | More generally, there need not be a one-to-one mapping between | |
4088 | characters that are read from or written to a text stream, and the | |
4089 | characters in the actual file. | |
4090 | @end itemize | |
4091 | ||
4092 | Since a binary stream is always more capable and more predictable than a | |
4093 | text stream, you might wonder what purpose text streams serve. Why not | |
4094 | simply always use binary streams? The answer is that on these operating | |
4095 | systems, text and binary streams use different file formats, and the | |
4096 | only way to read or write ``an ordinary file of text'' that can work | |
4097 | with other text-oriented programs is through a text stream. | |
4098 | ||
4099 | In the GNU library, and on all POSIX systems, there is no difference | |
4100 | between text streams and binary streams. When you open a stream, you | |
4101 | get the same kind of stream regardless of whether you ask for binary. | |
4102 | This stream can handle any file content, and has none of the | |
4103 | restrictions that text streams sometimes have. | |
4104 | ||
4105 | @node File Positioning | |
4106 | @section File Positioning | |
4107 | @cindex file positioning on a stream | |
4108 | @cindex positioning a stream | |
4109 | @cindex seeking on a stream | |
4110 | ||
4111 | The @dfn{file position} of a stream describes where in the file the | |
4112 | stream is currently reading or writing. I/O on the stream advances the | |
4113 | file position through the file. In the GNU system, the file position is | |
4114 | represented as an integer, which counts the number of bytes from the | |
4115 | beginning of the file. @xref{File Position}. | |
4116 | ||
4117 | During I/O to an ordinary disk file, you can change the file position | |
4118 | whenever you wish, so as to read or write any portion of the file. Some | |
4119 | other kinds of files may also permit this. Files which support changing | |
4120 | the file position are sometimes referred to as @dfn{random-access} | |
4121 | files. | |
4122 | ||
4123 | You can use the functions in this section to examine or modify the file | |
4124 | position indicator associated with a stream. The symbols listed below | |
4125 | are declared in the header file @file{stdio.h}. | |
4126 | @pindex stdio.h | |
4127 | ||
4128 | @comment stdio.h | |
f65fd747 | 4129 | @comment ISO |
28f540f4 RM |
4130 | @deftypefun {long int} ftell (FILE *@var{stream}) |
4131 | This function returns the current file position of the stream | |
4132 | @var{stream}. | |
4133 | ||
4134 | This function can fail if the stream doesn't support file positioning, | |
4135 | or if the file position can't be represented in a @code{long int}, and | |
4136 | possibly for other reasons as well. If a failure occurs, a value of | |
4137 | @code{-1} is returned. | |
4138 | @end deftypefun | |
4139 | ||
a5a0310d UD |
4140 | @comment stdio.h |
4141 | @comment Unix98 | |
4142 | @deftypefun off_t ftello (FILE *@var{stream}) | |
04b9968b UD |
4143 | The @code{ftello} function is similar to @code{ftell}, except that it |
4144 | returns a value of type @code{off_t}. Systems which support this type | |
4145 | use it to describe all file positions, unlike the POSIX specification | |
4146 | which uses a long int. The two are not necessarily the same size. | |
4147 | Therefore, using ftell can lead to problems if the implementation is | |
4148 | written on top of a POSIX compliant low-level I/O implementation, and using | |
4149 | @code{ftello} is preferable whenever it is available. | |
4150 | ||
4151 | If this function fails it returns @code{(off_t) -1}. This can happen due | |
a5a0310d UD |
4152 | to missing support for file positioning or internal errors. Otherwise |
4153 | the return value is the current file position. | |
4154 | ||
4155 | The function is an extension defined in the Unix Single Specification | |
4156 | version 2. | |
a3a4a74e UD |
4157 | |
4158 | When the sources are compiled with @code{_FILE_OFFSET_BITS == 64} on a | |
0be8752b | 4159 | 32 bit system this function is in fact @code{ftello64}. I.e., the |
a3a4a74e UD |
4160 | LFS interface transparently replaces the old interface. |
4161 | @end deftypefun | |
4162 | ||
4163 | @comment stdio.h | |
4164 | @comment Unix98 | |
4165 | @deftypefun off64_t ftello64 (FILE *@var{stream}) | |
4166 | This function is similar to @code{ftello} with the only difference that | |
4167 | the return value is of type @code{off64_t}. This also requires that the | |
4168 | stream @var{stream} was opened using either @code{fopen64}, | |
4169 | @code{freopen64}, or @code{tmpfile64} since otherwise the underlying | |
4170 | file operations to position the file pointer beyond the @math{2^31} | |
4171 | bytes limit might fail. | |
4172 | ||
4173 | If the sources are compiled with @code{_FILE_OFFSET_BITS == 64} on a 32 | |
4174 | bits machine this function is available under the name @code{ftello} | |
4175 | and so transparently replaces the old interface. | |
a5a0310d UD |
4176 | @end deftypefun |
4177 | ||
28f540f4 | 4178 | @comment stdio.h |
f65fd747 | 4179 | @comment ISO |
28f540f4 RM |
4180 | @deftypefun int fseek (FILE *@var{stream}, long int @var{offset}, int @var{whence}) |
4181 | The @code{fseek} function is used to change the file position of the | |
4182 | stream @var{stream}. The value of @var{whence} must be one of the | |
4183 | constants @code{SEEK_SET}, @code{SEEK_CUR}, or @code{SEEK_END}, to | |
4184 | indicate whether the @var{offset} is relative to the beginning of the | |
4185 | file, the current file position, or the end of the file, respectively. | |
4186 | ||
4187 | This function returns a value of zero if the operation was successful, | |
4188 | and a nonzero value to indicate failure. A successful call also clears | |
4189 | the end-of-file indicator of @var{stream} and discards any characters | |
4190 | that were ``pushed back'' by the use of @code{ungetc}. | |
4191 | ||
4192 | @code{fseek} either flushes any buffered output before setting the file | |
4193 | position or else remembers it so it will be written later in its proper | |
4194 | place in the file. | |
4195 | @end deftypefun | |
4196 | ||
a5a0310d UD |
4197 | @comment stdio.h |
4198 | @comment Unix98 | |
4199 | @deftypefun int fseeko (FILE *@var{stream}, off_t @var{offset}, int @var{whence}) | |
4200 | This function is similar to @code{fseek} but it corrects a problem with | |
4201 | @code{fseek} in a system with POSIX types. Using a value of type | |
4202 | @code{long int} for the offset is not compatible with POSIX. | |
4203 | @code{fseeko} uses the correct type @code{off_t} for the @var{offset} | |
4204 | parameter. | |
4205 | ||
f2ea0f5b | 4206 | For this reason it is a good idea to prefer @code{ftello} whenever it is |
a5a0310d UD |
4207 | available since its functionality is (if different at all) closer the |
4208 | underlying definition. | |
4209 | ||
4210 | The functionality and return value is the same as for @code{fseek}. | |
4211 | ||
4212 | The function is an extension defined in the Unix Single Specification | |
4213 | version 2. | |
a3a4a74e UD |
4214 | |
4215 | When the sources are compiled with @code{_FILE_OFFSET_BITS == 64} on a | |
0be8752b | 4216 | 32 bit system this function is in fact @code{fseeko64}. I.e., the |
a3a4a74e UD |
4217 | LFS interface transparently replaces the old interface. |
4218 | @end deftypefun | |
4219 | ||
4220 | @comment stdio.h | |
4221 | @comment Unix98 | |
4222 | @deftypefun int fseeko64 (FILE *@var{stream}, off64_t @var{offset}, int @var{whence}) | |
4223 | This function is similar to @code{fseeko} with the only difference that | |
4224 | the @var{offset} parameter is of type @code{off64_t}. This also | |
4225 | requires that the stream @var{stream} was opened using either | |
4226 | @code{fopen64}, @code{freopen64}, or @code{tmpfile64} since otherwise | |
4227 | the underlying file operations to position the file pointer beyond the | |
4228 | @math{2^31} bytes limit might fail. | |
4229 | ||
4230 | If the sources are compiled with @code{_FILE_OFFSET_BITS == 64} on a 32 | |
4231 | bits machine this function is available under the name @code{fseeko} | |
4232 | and so transparently replaces the old interface. | |
a5a0310d UD |
4233 | @end deftypefun |
4234 | ||
4235 | @strong{Portability Note:} In non-POSIX systems, @code{ftell}, | |
4236 | @code{ftello}, @code{fseek} and @code{fseeko} might work reliably only | |
4237 | on binary streams. @xref{Binary Streams}. | |
28f540f4 RM |
4238 | |
4239 | The following symbolic constants are defined for use as the @var{whence} | |
4240 | argument to @code{fseek}. They are also used with the @code{lseek} | |
4241 | function (@pxref{I/O Primitives}) and to specify offsets for file locks | |
4242 | (@pxref{Control Operations}). | |
4243 | ||
4244 | @comment stdio.h | |
f65fd747 | 4245 | @comment ISO |
28f540f4 RM |
4246 | @deftypevr Macro int SEEK_SET |
4247 | This is an integer constant which, when used as the @var{whence} | |
a5a0310d UD |
4248 | argument to the @code{fseek} or @code{fseeko} function, specifies that |
4249 | the offset provided is relative to the beginning of the file. | |
28f540f4 RM |
4250 | @end deftypevr |
4251 | ||
4252 | @comment stdio.h | |
f65fd747 | 4253 | @comment ISO |
28f540f4 RM |
4254 | @deftypevr Macro int SEEK_CUR |
4255 | This is an integer constant which, when used as the @var{whence} | |
a5a0310d UD |
4256 | argument to the @code{fseek} or @code{fseeko} function, specifies that |
4257 | the offset provided is relative to the current file position. | |
28f540f4 RM |
4258 | @end deftypevr |
4259 | ||
4260 | @comment stdio.h | |
f65fd747 | 4261 | @comment ISO |
28f540f4 RM |
4262 | @deftypevr Macro int SEEK_END |
4263 | This is an integer constant which, when used as the @var{whence} | |
a5a0310d UD |
4264 | argument to the @code{fseek} or @code{fseeko} function, specifies that |
4265 | the offset provided is relative to the end of the file. | |
28f540f4 RM |
4266 | @end deftypevr |
4267 | ||
4268 | @comment stdio.h | |
f65fd747 | 4269 | @comment ISO |
28f540f4 RM |
4270 | @deftypefun void rewind (FILE *@var{stream}) |
4271 | The @code{rewind} function positions the stream @var{stream} at the | |
f2ea0f5b | 4272 | beginning of the file. It is equivalent to calling @code{fseek} or |
a5a0310d UD |
4273 | @code{fseeko} on the @var{stream} with an @var{offset} argument of |
4274 | @code{0L} and a @var{whence} argument of @code{SEEK_SET}, except that | |
4275 | the return value is discarded and the error indicator for the stream is | |
4276 | reset. | |
28f540f4 RM |
4277 | @end deftypefun |
4278 | ||
4279 | These three aliases for the @samp{SEEK_@dots{}} constants exist for the | |
4280 | sake of compatibility with older BSD systems. They are defined in two | |
4281 | different header files: @file{fcntl.h} and @file{sys/file.h}. | |
4282 | ||
4283 | @table @code | |
4284 | @comment sys/file.h | |
4285 | @comment BSD | |
4286 | @item L_SET | |
4287 | @vindex L_SET | |
4288 | An alias for @code{SEEK_SET}. | |
4289 | ||
4290 | @comment sys/file.h | |
4291 | @comment BSD | |
4292 | @item L_INCR | |
4293 | @vindex L_INCR | |
4294 | An alias for @code{SEEK_CUR}. | |
4295 | ||
4296 | @comment sys/file.h | |
4297 | @comment BSD | |
4298 | @item L_XTND | |
4299 | @vindex L_XTND | |
4300 | An alias for @code{SEEK_END}. | |
4301 | @end table | |
4302 | ||
4303 | @node Portable Positioning | |
4304 | @section Portable File-Position Functions | |
4305 | ||
4306 | On the GNU system, the file position is truly a character count. You | |
a5a0310d UD |
4307 | can specify any character count value as an argument to @code{fseek} or |
4308 | @code{fseeko} and get reliable results for any random access file. | |
4309 | However, some @w{ISO C} systems do not represent file positions in this | |
4310 | way. | |
28f540f4 RM |
4311 | |
4312 | On some systems where text streams truly differ from binary streams, it | |
4313 | is impossible to represent the file position of a text stream as a count | |
4314 | of characters from the beginning of the file. For example, the file | |
4315 | position on some systems must encode both a record offset within the | |
4316 | file, and a character offset within the record. | |
4317 | ||
4318 | As a consequence, if you want your programs to be portable to these | |
4319 | systems, you must observe certain rules: | |
4320 | ||
4321 | @itemize @bullet | |
4322 | @item | |
4323 | The value returned from @code{ftell} on a text stream has no predictable | |
4324 | relationship to the number of characters you have read so far. The only | |
4325 | thing you can rely on is that you can use it subsequently as the | |
a5a0310d UD |
4326 | @var{offset} argument to @code{fseek} or @code{fseeko} to move back to |
4327 | the same file position. | |
28f540f4 | 4328 | |
19c3f208 | 4329 | @item |
a5a0310d | 4330 | In a call to @code{fseek} or @code{fseeko} on a text stream, either the |
04b9968b UD |
4331 | @var{offset} must be zero, or @var{whence} must be @code{SEEK_SET} and |
4332 | and the @var{offset} must be the result of an earlier call to @code{ftell} | |
4333 | on the same stream. | |
28f540f4 RM |
4334 | |
4335 | @item | |
4336 | The value of the file position indicator of a text stream is undefined | |
4337 | while there are characters that have been pushed back with @code{ungetc} | |
4338 | that haven't been read or discarded. @xref{Unreading}. | |
4339 | @end itemize | |
4340 | ||
4341 | But even if you observe these rules, you may still have trouble for long | |
4342 | files, because @code{ftell} and @code{fseek} use a @code{long int} value | |
4343 | to represent the file position. This type may not have room to encode | |
a5a0310d UD |
4344 | all the file positions in a large file. Using the @code{ftello} and |
4345 | @code{fseeko} functions might help here since the @code{off_t} type is | |
4346 | expected to be able to hold all file position values but this still does | |
4347 | not help to handle additional information which must be associated with | |
4348 | a file position. | |
28f540f4 RM |
4349 | |
4350 | So if you do want to support systems with peculiar encodings for the | |
4351 | file positions, it is better to use the functions @code{fgetpos} and | |
4352 | @code{fsetpos} instead. These functions represent the file position | |
4353 | using the data type @code{fpos_t}, whose internal representation varies | |
4354 | from system to system. | |
4355 | ||
4356 | These symbols are declared in the header file @file{stdio.h}. | |
4357 | @pindex stdio.h | |
4358 | ||
4359 | @comment stdio.h | |
f65fd747 | 4360 | @comment ISO |
28f540f4 RM |
4361 | @deftp {Data Type} fpos_t |
4362 | This is the type of an object that can encode information about the | |
4363 | file position of a stream, for use by the functions @code{fgetpos} and | |
4364 | @code{fsetpos}. | |
4365 | ||
5988b69d AJ |
4366 | In the GNU system, @code{fpos_t} is an opaque data structure that |
4367 | contains internal data to represent file offset and conversion state | |
4368 | information. In other systems, it might have a different internal | |
28f540f4 | 4369 | representation. |
a3a4a74e | 4370 | |
0be8752b | 4371 | When compiling with @code{_FILE_OFFSET_BITS == 64} on a 32 bit machine |
5988b69d AJ |
4372 | this type is in fact equivalent to @code{fpos64_t} since the LFS |
4373 | interface transparently replaces the old interface. | |
a3a4a74e UD |
4374 | @end deftp |
4375 | ||
4376 | @comment stdio.h | |
4377 | @comment Unix98 | |
4378 | @deftp {Data Type} fpos64_t | |
4379 | This is the type of an object that can encode information about the | |
4380 | file position of a stream, for use by the functions @code{fgetpos64} and | |
4381 | @code{fsetpos64}. | |
4382 | ||
5988b69d AJ |
4383 | In the GNU system, @code{fpos64_t} is an opaque data structure that |
4384 | contains internal data to represent file offset and conversion state | |
4385 | information. In other systems, it might have a different internal | |
a3a4a74e | 4386 | representation. |
28f540f4 RM |
4387 | @end deftp |
4388 | ||
4389 | @comment stdio.h | |
f65fd747 | 4390 | @comment ISO |
28f540f4 RM |
4391 | @deftypefun int fgetpos (FILE *@var{stream}, fpos_t *@var{position}) |
4392 | This function stores the value of the file position indicator for the | |
4393 | stream @var{stream} in the @code{fpos_t} object pointed to by | |
4394 | @var{position}. If successful, @code{fgetpos} returns zero; otherwise | |
4395 | it returns a nonzero value and stores an implementation-defined positive | |
4396 | value in @code{errno}. | |
a3a4a74e UD |
4397 | |
4398 | When the sources are compiled with @code{_FILE_OFFSET_BITS == 64} on a | |
0be8752b | 4399 | 32 bit system the function is in fact @code{fgetpos64}. I.e., the LFS |
5988b69d | 4400 | interface transparently replaces the old interface. |
a3a4a74e UD |
4401 | @end deftypefun |
4402 | ||
4403 | @comment stdio.h | |
4404 | @comment Unix98 | |
4405 | @deftypefun int fgetpos64 (FILE *@var{stream}, fpos64_t *@var{position}) | |
4406 | This function is similar to @code{fgetpos} but the file position is | |
4407 | returned in a variable of type @code{fpos64_t} to which @var{position} | |
4408 | points. | |
4409 | ||
4410 | If the sources are compiled with @code{_FILE_OFFSET_BITS == 64} on a 32 | |
4411 | bits machine this function is available under the name @code{fgetpos} | |
4412 | and so transparently replaces the old interface. | |
28f540f4 RM |
4413 | @end deftypefun |
4414 | ||
4415 | @comment stdio.h | |
f65fd747 | 4416 | @comment ISO |
a3a4a74e | 4417 | @deftypefun int fsetpos (FILE *@var{stream}, const fpos_t *@var{position}) |
28f540f4 RM |
4418 | This function sets the file position indicator for the stream @var{stream} |
4419 | to the position @var{position}, which must have been set by a previous | |
4420 | call to @code{fgetpos} on the same stream. If successful, @code{fsetpos} | |
4421 | clears the end-of-file indicator on the stream, discards any characters | |
4422 | that were ``pushed back'' by the use of @code{ungetc}, and returns a value | |
4423 | of zero. Otherwise, @code{fsetpos} returns a nonzero value and stores | |
4424 | an implementation-defined positive value in @code{errno}. | |
a3a4a74e UD |
4425 | |
4426 | When the sources are compiled with @code{_FILE_OFFSET_BITS == 64} on a | |
0be8752b | 4427 | 32 bit system the function is in fact @code{fsetpos64}. I.e., the LFS |
5988b69d | 4428 | interface transparently replaces the old interface. |
a3a4a74e UD |
4429 | @end deftypefun |
4430 | ||
4431 | @comment stdio.h | |
4432 | @comment Unix98 | |
4433 | @deftypefun int fsetpos64 (FILE *@var{stream}, const fpos64_t *@var{position}) | |
4434 | This function is similar to @code{fsetpos} but the file position used | |
4435 | for positioning is provided in a variable of type @code{fpos64_t} to | |
4436 | which @var{position} points. | |
4437 | ||
4438 | If the sources are compiled with @code{_FILE_OFFSET_BITS == 64} on a 32 | |
4439 | bits machine this function is available under the name @code{fsetpos} | |
4440 | and so transparently replaces the old interface. | |
28f540f4 RM |
4441 | @end deftypefun |
4442 | ||
4443 | @node Stream Buffering | |
4444 | @section Stream Buffering | |
4445 | ||
4446 | @cindex buffering of streams | |
4447 | Characters that are written to a stream are normally accumulated and | |
4448 | transmitted asynchronously to the file in a block, instead of appearing | |
4449 | as soon as they are output by the application program. Similarly, | |
4450 | streams often retrieve input from the host environment in blocks rather | |
4451 | than on a character-by-character basis. This is called @dfn{buffering}. | |
4452 | ||
4453 | If you are writing programs that do interactive input and output using | |
4454 | streams, you need to understand how buffering works when you design the | |
4455 | user interface to your program. Otherwise, you might find that output | |
4456 | (such as progress or prompt messages) doesn't appear when you intended | |
04b9968b | 4457 | it to, or displays some other unexpected behavior. |
28f540f4 RM |
4458 | |
4459 | This section deals only with controlling when characters are transmitted | |
4460 | between the stream and the file or device, and @emph{not} with how | |
4461 | things like echoing, flow control, and the like are handled on specific | |
4462 | classes of devices. For information on common control operations on | |
4463 | terminal devices, see @ref{Low-Level Terminal Interface}. | |
4464 | ||
4465 | You can bypass the stream buffering facilities altogether by using the | |
4466 | low-level input and output functions that operate on file descriptors | |
4467 | instead. @xref{Low-Level I/O}. | |
4468 | ||
4469 | @menu | |
4470 | * Buffering Concepts:: Terminology is defined here. | |
4471 | * Flushing Buffers:: How to ensure that output buffers are flushed. | |
4472 | * Controlling Buffering:: How to specify what kind of buffering to use. | |
4473 | @end menu | |
4474 | ||
4475 | @node Buffering Concepts | |
4476 | @subsection Buffering Concepts | |
4477 | ||
4478 | There are three different kinds of buffering strategies: | |
4479 | ||
4480 | @itemize @bullet | |
4481 | @item | |
4482 | Characters written to or read from an @dfn{unbuffered} stream are | |
4483 | transmitted individually to or from the file as soon as possible. | |
4484 | @cindex unbuffered stream | |
4485 | ||
4486 | @item | |
4487 | Characters written to a @dfn{line buffered} stream are transmitted to | |
4488 | the file in blocks when a newline character is encountered. | |
4489 | @cindex line buffered stream | |
4490 | ||
4491 | @item | |
4492 | Characters written to or read from a @dfn{fully buffered} stream are | |
4493 | transmitted to or from the file in blocks of arbitrary size. | |
4494 | @cindex fully buffered stream | |
4495 | @end itemize | |
4496 | ||
4497 | Newly opened streams are normally fully buffered, with one exception: a | |
4498 | stream connected to an interactive device such as a terminal is | |
4499 | initially line buffered. @xref{Controlling Buffering}, for information | |
4500 | on how to select a different kind of buffering. Usually the automatic | |
4501 | selection gives you the most convenient kind of buffering for the file | |
4502 | or device you open. | |
4503 | ||
4504 | The use of line buffering for interactive devices implies that output | |
4505 | messages ending in a newline will appear immediately---which is usually | |
4506 | what you want. Output that doesn't end in a newline might or might not | |
4507 | show up immediately, so if you want them to appear immediately, you | |
4508 | should flush buffered output explicitly with @code{fflush}, as described | |
4509 | in @ref{Flushing Buffers}. | |
4510 | ||
4511 | @node Flushing Buffers | |
4512 | @subsection Flushing Buffers | |
4513 | ||
4514 | @cindex flushing a stream | |
4515 | @dfn{Flushing} output on a buffered stream means transmitting all | |
4516 | accumulated characters to the file. There are many circumstances when | |
4517 | buffered output on a stream is flushed automatically: | |
4518 | ||
4519 | @itemize @bullet | |
4520 | @item | |
4521 | When you try to do output and the output buffer is full. | |
4522 | ||
4523 | @item | |
4524 | When the stream is closed. @xref{Closing Streams}. | |
4525 | ||
19c3f208 | 4526 | @item |
28f540f4 RM |
4527 | When the program terminates by calling @code{exit}. |
4528 | @xref{Normal Termination}. | |
4529 | ||
4530 | @item | |
4531 | When a newline is written, if the stream is line buffered. | |
4532 | ||
4533 | @item | |
4534 | Whenever an input operation on @emph{any} stream actually reads data | |
4535 | from its file. | |
4536 | @end itemize | |
4537 | ||
4538 | If you want to flush the buffered output at another time, call | |
4539 | @code{fflush}, which is declared in the header file @file{stdio.h}. | |
4540 | @pindex stdio.h | |
4541 | ||
4542 | @comment stdio.h | |
f65fd747 | 4543 | @comment ISO |
28f540f4 RM |
4544 | @deftypefun int fflush (FILE *@var{stream}) |
4545 | This function causes any buffered output on @var{stream} to be delivered | |
4546 | to the file. If @var{stream} is a null pointer, then | |
4547 | @code{fflush} causes buffered output on @emph{all} open output streams | |
4548 | to be flushed. | |
4549 | ||
4550 | This function returns @code{EOF} if a write error occurs, or zero | |
4551 | otherwise. | |
4552 | @end deftypefun | |
4553 | ||
7b4161bb UD |
4554 | @comment stdio.h |
4555 | @comment POSIX | |
4556 | @deftypefun int fflush_unlocked (FILE *@var{stream}) | |
4557 | The @code{fflush_unlocked} function is equivalent to the @code{fflush} | |
1dc843f7 | 4558 | function except that it does not implicitly lock the stream. |
7b4161bb UD |
4559 | @end deftypefun |
4560 | ||
4561 | The @code{fflush} function can be used to flush all streams currently | |
4562 | opened. While this is useful in some situations it does often more than | |
4563 | necessary since it might be done in situations when terminal input is | |
4564 | required and the program wants to be sure that all output is visible on | |
4565 | the terminal. But this means that only line buffered streams have to be | |
4566 | flushed. Solaris introduced a function especially for this. It was | |
4567 | always available in the GNU C library in some form but never officially | |
4568 | exported. | |
4569 | ||
9deee4bb | 4570 | @comment stdio_ext.h |
7b4161bb UD |
4571 | @comment GNU |
4572 | @deftypefun void _flushlbf (void) | |
4573 | The @code{_flushlbf} function flushes all line buffered streams | |
4574 | currently opened. | |
4575 | ||
4576 | This function is declared in the @file{stdio_ext.h} header. | |
4577 | @end deftypefun | |
4578 | ||
28f540f4 RM |
4579 | @strong{Compatibility Note:} Some brain-damaged operating systems have |
4580 | been known to be so thoroughly fixated on line-oriented input and output | |
4581 | that flushing a line buffered stream causes a newline to be written! | |
4582 | Fortunately, this ``feature'' seems to be becoming less common. You do | |
4583 | not need to worry about this in the GNU system. | |
4584 | ||
7b4161bb UD |
4585 | In some situations it might be useful to not flush the output pending |
4586 | for a stream but instead simply forget it. If transmission is costly | |
4587 | and the output is not needed anymore this is valid reasoning. In this | |
4588 | situation a non-standard function introduced in Solaris and available in | |
4589 | the GNU C library can be used. | |
4590 | ||
4591 | @comment stdio_ext.h | |
4592 | @comment GNU | |
4593 | @deftypefun void __fpurge (FILE *@var{stream}) | |
4594 | The @code{__fpurge} function causes the buffer of the stream | |
4595 | @var{stream} to be emptied. If the stream is currently in read mode all | |
4596 | input in the buffer is lost. If the stream is in output mode the | |
4597 | buffered output is not written to the device (or whatever other | |
4598 | underlying storage) and the buffer the cleared. | |
4599 | ||
4600 | This function is declared in @file{stdio_ext.h}. | |
4601 | @end deftypefun | |
28f540f4 RM |
4602 | |
4603 | @node Controlling Buffering | |
4604 | @subsection Controlling Which Kind of Buffering | |
4605 | ||
4606 | After opening a stream (but before any other operations have been | |
4607 | performed on it), you can explicitly specify what kind of buffering you | |
4608 | want it to have using the @code{setvbuf} function. | |
4609 | @cindex buffering, controlling | |
4610 | ||
4611 | The facilities listed in this section are declared in the header | |
4612 | file @file{stdio.h}. | |
4613 | @pindex stdio.h | |
4614 | ||
4615 | @comment stdio.h | |
f65fd747 | 4616 | @comment ISO |
28f540f4 RM |
4617 | @deftypefun int setvbuf (FILE *@var{stream}, char *@var{buf}, int @var{mode}, size_t @var{size}) |
4618 | This function is used to specify that the stream @var{stream} should | |
4619 | have the buffering mode @var{mode}, which can be either @code{_IOFBF} | |
4620 | (for full buffering), @code{_IOLBF} (for line buffering), or | |
4621 | @code{_IONBF} (for unbuffered input/output). | |
4622 | ||
4623 | If you specify a null pointer as the @var{buf} argument, then @code{setvbuf} | |
4624 | allocates a buffer itself using @code{malloc}. This buffer will be freed | |
4625 | when you close the stream. | |
4626 | ||
4627 | Otherwise, @var{buf} should be a character array that can hold at least | |
4628 | @var{size} characters. You should not free the space for this array as | |
4629 | long as the stream remains open and this array remains its buffer. You | |
4630 | should usually either allocate it statically, or @code{malloc} | |
4631 | (@pxref{Unconstrained Allocation}) the buffer. Using an automatic array | |
4632 | is not a good idea unless you close the file before exiting the block | |
4633 | that declares the array. | |
4634 | ||
4635 | While the array remains a stream buffer, the stream I/O functions will | |
4636 | use the buffer for their internal purposes. You shouldn't try to access | |
4637 | the values in the array directly while the stream is using it for | |
4638 | buffering. | |
4639 | ||
4640 | The @code{setvbuf} function returns zero on success, or a nonzero value | |
4641 | if the value of @var{mode} is not valid or if the request could not | |
4642 | be honored. | |
4643 | @end deftypefun | |
4644 | ||
4645 | @comment stdio.h | |
f65fd747 | 4646 | @comment ISO |
28f540f4 RM |
4647 | @deftypevr Macro int _IOFBF |
4648 | The value of this macro is an integer constant expression that can be | |
4649 | used as the @var{mode} argument to the @code{setvbuf} function to | |
4650 | specify that the stream should be fully buffered. | |
4651 | @end deftypevr | |
4652 | ||
4653 | @comment stdio.h | |
f65fd747 | 4654 | @comment ISO |
28f540f4 RM |
4655 | @deftypevr Macro int _IOLBF |
4656 | The value of this macro is an integer constant expression that can be | |
4657 | used as the @var{mode} argument to the @code{setvbuf} function to | |
4658 | specify that the stream should be line buffered. | |
4659 | @end deftypevr | |
4660 | ||
4661 | @comment stdio.h | |
f65fd747 | 4662 | @comment ISO |
28f540f4 RM |
4663 | @deftypevr Macro int _IONBF |
4664 | The value of this macro is an integer constant expression that can be | |
4665 | used as the @var{mode} argument to the @code{setvbuf} function to | |
4666 | specify that the stream should be unbuffered. | |
4667 | @end deftypevr | |
4668 | ||
4669 | @comment stdio.h | |
f65fd747 | 4670 | @comment ISO |
28f540f4 RM |
4671 | @deftypevr Macro int BUFSIZ |
4672 | The value of this macro is an integer constant expression that is good | |
4673 | to use for the @var{size} argument to @code{setvbuf}. This value is | |
4674 | guaranteed to be at least @code{256}. | |
4675 | ||
4676 | The value of @code{BUFSIZ} is chosen on each system so as to make stream | |
19c3f208 | 4677 | I/O efficient. So it is a good idea to use @code{BUFSIZ} as the size |
28f540f4 RM |
4678 | for the buffer when you call @code{setvbuf}. |
4679 | ||
4680 | Actually, you can get an even better value to use for the buffer size | |
4681 | by means of the @code{fstat} system call: it is found in the | |
4682 | @code{st_blksize} field of the file attributes. @xref{Attribute Meanings}. | |
4683 | ||
4684 | Sometimes people also use @code{BUFSIZ} as the allocation size of | |
4685 | buffers used for related purposes, such as strings used to receive a | |
4686 | line of input with @code{fgets} (@pxref{Character Input}). There is no | |
4687 | particular reason to use @code{BUFSIZ} for this instead of any other | |
4688 | integer, except that it might lead to doing I/O in chunks of an | |
4689 | efficient size. | |
4690 | @end deftypevr | |
4691 | ||
4692 | @comment stdio.h | |
f65fd747 | 4693 | @comment ISO |
28f540f4 RM |
4694 | @deftypefun void setbuf (FILE *@var{stream}, char *@var{buf}) |
4695 | If @var{buf} is a null pointer, the effect of this function is | |
4696 | equivalent to calling @code{setvbuf} with a @var{mode} argument of | |
4697 | @code{_IONBF}. Otherwise, it is equivalent to calling @code{setvbuf} | |
4698 | with @var{buf}, and a @var{mode} of @code{_IOFBF} and a @var{size} | |
4699 | argument of @code{BUFSIZ}. | |
4700 | ||
4701 | The @code{setbuf} function is provided for compatibility with old code; | |
4702 | use @code{setvbuf} in all new programs. | |
4703 | @end deftypefun | |
4704 | ||
4705 | @comment stdio.h | |
4706 | @comment BSD | |
4707 | @deftypefun void setbuffer (FILE *@var{stream}, char *@var{buf}, size_t @var{size}) | |
4708 | If @var{buf} is a null pointer, this function makes @var{stream} unbuffered. | |
4709 | Otherwise, it makes @var{stream} fully buffered using @var{buf} as the | |
4710 | buffer. The @var{size} argument specifies the length of @var{buf}. | |
4711 | ||
4712 | This function is provided for compatibility with old BSD code. Use | |
4713 | @code{setvbuf} instead. | |
4714 | @end deftypefun | |
4715 | ||
4716 | @comment stdio.h | |
4717 | @comment BSD | |
4718 | @deftypefun void setlinebuf (FILE *@var{stream}) | |
4719 | This function makes @var{stream} be line buffered, and allocates the | |
4720 | buffer for you. | |
4721 | ||
4722 | This function is provided for compatibility with old BSD code. Use | |
4723 | @code{setvbuf} instead. | |
4724 | @end deftypefun | |
4725 | ||
7b4161bb UD |
4726 | It is possible to query whether a given stream is line buffered or not |
4727 | using a non-standard function introduced in Solaris and available in the | |
4728 | GNU C library. | |
4729 | ||
4730 | @comment stdio_ext.h | |
4731 | @comment GNU | |
4732 | @deftypefun int __flbf (FILE *@var{stream}) | |
4733 | The @code{__flbf} function will return a nonzero value in case the | |
4734 | stream @var{stream} is line buffered. Otherwise the return value is | |
4735 | zero. | |
4736 | ||
4737 | This function is declared in the @file{stdio_ext.h} header. | |
4738 | @end deftypefun | |
4739 | ||
4740 | Two more extensions allow to determine the size of the buffer and how | |
4741 | much of it is used. These functions were also introduced in Solaris. | |
4742 | ||
4743 | @comment stdio_ext.h | |
4744 | @comment GNU | |
4745 | @deftypefun size_t __fbufsize (FILE *@var{stream}) | |
4746 | The @code{__fbufsize} function return the size of the buffer in the | |
4747 | stream @var{stream}. This value can be used to optimize the use of the | |
4748 | stream. | |
4749 | ||
4750 | This function is declared in the @file{stdio_ext.h} header. | |
4751 | @end deftypefun | |
4752 | ||
4753 | @comment stdio_ext.h | |
4754 | @comment GNU | |
f126ef67 UD |
4755 | @deftypefun size_t __fpending (FILE *@var{stream}) The @code{__fpending} |
4756 | function returns the number of bytes currently in the output buffer. | |
4757 | For wide-oriented stream the measuring unit is wide characters. This | |
4758 | function should not be used on buffers in read mode or opened read-only. | |
7b4161bb UD |
4759 | |
4760 | This function is declared in the @file{stdio_ext.h} header. | |
4761 | @end deftypefun | |
4762 | ||
28f540f4 RM |
4763 | @node Other Kinds of Streams |
4764 | @section Other Kinds of Streams | |
4765 | ||
4766 | The GNU library provides ways for you to define additional kinds of | |
4767 | streams that do not necessarily correspond to an open file. | |
4768 | ||
4769 | One such type of stream takes input from or writes output to a string. | |
4770 | These kinds of streams are used internally to implement the | |
4771 | @code{sprintf} and @code{sscanf} functions. You can also create such a | |
4772 | stream explicitly, using the functions described in @ref{String Streams}. | |
4773 | ||
4774 | More generally, you can define streams that do input/output to arbitrary | |
4775 | objects using functions supplied by your program. This protocol is | |
4776 | discussed in @ref{Custom Streams}. | |
4777 | ||
4778 | @strong{Portability Note:} The facilities described in this section are | |
4779 | specific to GNU. Other systems or C implementations might or might not | |
4780 | provide equivalent functionality. | |
4781 | ||
4782 | @menu | |
19c3f208 | 4783 | * String Streams:: Streams that get data from or put data in |
28f540f4 RM |
4784 | a string or memory buffer. |
4785 | * Obstack Streams:: Streams that store data in an obstack. | |
4786 | * Custom Streams:: Defining your own streams with an arbitrary | |
4787 | input data source and/or output data sink. | |
4788 | @end menu | |
4789 | ||
4790 | @node String Streams | |
4791 | @subsection String Streams | |
4792 | ||
4793 | @cindex stream, for I/O to a string | |
4794 | @cindex string stream | |
4795 | The @code{fmemopen} and @code{open_memstream} functions allow you to do | |
4796 | I/O to a string or memory buffer. These facilities are declared in | |
4797 | @file{stdio.h}. | |
4798 | @pindex stdio.h | |
4799 | ||
4800 | @comment stdio.h | |
4801 | @comment GNU | |
4802 | @deftypefun {FILE *} fmemopen (void *@var{buf}, size_t @var{size}, const char *@var{opentype}) | |
4803 | This function opens a stream that allows the access specified by the | |
4804 | @var{opentype} argument, that reads from or writes to the buffer specified | |
4805 | by the argument @var{buf}. This array must be at least @var{size} bytes long. | |
4806 | ||
4807 | If you specify a null pointer as the @var{buf} argument, @code{fmemopen} | |
04b9968b UD |
4808 | dynamically allocates an array @var{size} bytes long (as with @code{malloc}; |
4809 | @pxref{Unconstrained Allocation}). This is really only useful | |
28f540f4 RM |
4810 | if you are going to write things to the buffer and then read them back |
4811 | in again, because you have no way of actually getting a pointer to the | |
4812 | buffer (for this, try @code{open_memstream}, below). The buffer is | |
417d6b34 | 4813 | freed when the stream is closed. |
28f540f4 RM |
4814 | |
4815 | The argument @var{opentype} is the same as in @code{fopen} | |
8b7fb588 | 4816 | (@pxref{Opening Streams}). If the @var{opentype} specifies |
28f540f4 RM |
4817 | append mode, then the initial file position is set to the first null |
4818 | character in the buffer. Otherwise the initial file position is at the | |
4819 | beginning of the buffer. | |
4820 | ||
4821 | When a stream open for writing is flushed or closed, a null character | |
4822 | (zero byte) is written at the end of the buffer if it fits. You | |
4823 | should add an extra byte to the @var{size} argument to account for this. | |
4824 | Attempts to write more than @var{size} bytes to the buffer result | |
4825 | in an error. | |
4826 | ||
4827 | For a stream open for reading, null characters (zero bytes) in the | |
4828 | buffer do not count as ``end of file''. Read operations indicate end of | |
4829 | file only when the file position advances past @var{size} bytes. So, if | |
4830 | you want to read characters from a null-terminated string, you should | |
4831 | supply the length of the string as the @var{size} argument. | |
4832 | @end deftypefun | |
4833 | ||
4834 | Here is an example of using @code{fmemopen} to create a stream for | |
4835 | reading from a string: | |
4836 | ||
4837 | @smallexample | |
4838 | @include memopen.c.texi | |
4839 | @end smallexample | |
4840 | ||
4841 | This program produces the following output: | |
4842 | ||
4843 | @smallexample | |
4844 | Got f | |
4845 | Got o | |
4846 | Got o | |
4847 | Got b | |
4848 | Got a | |
4849 | Got r | |
4850 | @end smallexample | |
4851 | ||
4852 | @comment stdio.h | |
4853 | @comment GNU | |
4854 | @deftypefun {FILE *} open_memstream (char **@var{ptr}, size_t *@var{sizeloc}) | |
4855 | This function opens a stream for writing to a buffer. The buffer is | |
29402b12 RM |
4856 | allocated dynamically and grown as necessary, using @code{malloc}. |
4857 | After you've closed the stream, this buffer is your responsibility to | |
4858 | clean up using @code{free} or @code{realloc}. @xref{Unconstrained Allocation}. | |
28f540f4 RM |
4859 | |
4860 | When the stream is closed with @code{fclose} or flushed with | |
4861 | @code{fflush}, the locations @var{ptr} and @var{sizeloc} are updated to | |
4862 | contain the pointer to the buffer and its size. The values thus stored | |
4863 | remain valid only as long as no further output on the stream takes | |
4864 | place. If you do more output, you must flush the stream again to store | |
4865 | new values before you use them again. | |
4866 | ||
4867 | A null character is written at the end of the buffer. This null character | |
4868 | is @emph{not} included in the size value stored at @var{sizeloc}. | |
4869 | ||
a5a0310d UD |
4870 | You can move the stream's file position with @code{fseek} or |
4871 | @code{fseeko} (@pxref{File Positioning}). Moving the file position past | |
4872 | the end of the data already written fills the intervening space with | |
4873 | zeroes. | |
28f540f4 RM |
4874 | @end deftypefun |
4875 | ||
4876 | Here is an example of using @code{open_memstream}: | |
4877 | ||
4878 | @smallexample | |
4879 | @include memstrm.c.texi | |
4880 | @end smallexample | |
4881 | ||
4882 | This program produces the following output: | |
4883 | ||
4884 | @smallexample | |
4885 | buf = `hello', size = 5 | |
4886 | buf = `hello, world', size = 12 | |
4887 | @end smallexample | |
4888 | ||
4889 | @c @group Invalid outside @example. | |
4890 | @node Obstack Streams | |
4891 | @subsection Obstack Streams | |
4892 | ||
4893 | You can open an output stream that puts it data in an obstack. | |
4894 | @xref{Obstacks}. | |
4895 | ||
4896 | @comment stdio.h | |
4897 | @comment GNU | |
4898 | @deftypefun {FILE *} open_obstack_stream (struct obstack *@var{obstack}) | |
4899 | This function opens a stream for writing data into the obstack @var{obstack}. | |
4900 | This starts an object in the obstack and makes it grow as data is | |
4901 | written (@pxref{Growing Objects}). | |
4902 | @c @end group Doubly invalid because not nested right. | |
4903 | ||
4904 | Calling @code{fflush} on this stream updates the current size of the | |
4905 | object to match the amount of data that has been written. After a call | |
4906 | to @code{fflush}, you can examine the object temporarily. | |
4907 | ||
a5a0310d UD |
4908 | You can move the file position of an obstack stream with @code{fseek} or |
4909 | @code{fseeko} (@pxref{File Positioning}). Moving the file position past | |
4910 | the end of the data written fills the intervening space with zeros. | |
28f540f4 RM |
4911 | |
4912 | To make the object permanent, update the obstack with @code{fflush}, and | |
4913 | then use @code{obstack_finish} to finalize the object and get its address. | |
4914 | The following write to the stream starts a new object in the obstack, | |
4915 | and later writes add to that object until you do another @code{fflush} | |
4916 | and @code{obstack_finish}. | |
4917 | ||
4918 | But how do you find out how long the object is? You can get the length | |
4919 | in bytes by calling @code{obstack_object_size} (@pxref{Status of an | |
4920 | Obstack}), or you can null-terminate the object like this: | |
4921 | ||
4922 | @smallexample | |
4923 | obstack_1grow (@var{obstack}, 0); | |
4924 | @end smallexample | |
4925 | ||
4926 | Whichever one you do, you must do it @emph{before} calling | |
4927 | @code{obstack_finish}. (You can do both if you wish.) | |
4928 | @end deftypefun | |
4929 | ||
4930 | Here is a sample function that uses @code{open_obstack_stream}: | |
4931 | ||
4932 | @smallexample | |
4933 | char * | |
4934 | make_message_string (const char *a, int b) | |
4935 | @{ | |
4936 | FILE *stream = open_obstack_stream (&message_obstack); | |
4937 | output_task (stream); | |
4938 | fprintf (stream, ": "); | |
4939 | fprintf (stream, a, b); | |
4940 | fprintf (stream, "\n"); | |
4941 | fclose (stream); | |
4942 | obstack_1grow (&message_obstack, 0); | |
4943 | return obstack_finish (&message_obstack); | |
4944 | @} | |
4945 | @end smallexample | |
4946 | ||
4947 | @node Custom Streams | |
4948 | @subsection Programming Your Own Custom Streams | |
4949 | @cindex custom streams | |
4950 | @cindex programming your own streams | |
4951 | ||
4952 | This section describes how you can make a stream that gets input from an | |
4953 | arbitrary data source or writes output to an arbitrary data sink | |
52cea457 UD |
4954 | programmed by you. We call these @dfn{custom streams}. The functions |
4955 | and types described here are all GNU extensions. | |
28f540f4 RM |
4956 | |
4957 | @c !!! this does not talk at all about the higher-level hooks | |
4958 | ||
4959 | @menu | |
4960 | * Streams and Cookies:: The @dfn{cookie} records where to fetch or | |
19c3f208 | 4961 | store data that is read or written. |
28f540f4 | 4962 | * Hook Functions:: How you should define the four @dfn{hook |
19c3f208 | 4963 | functions} that a custom stream needs. |
28f540f4 RM |
4964 | @end menu |
4965 | ||
4966 | @node Streams and Cookies | |
4967 | @subsubsection Custom Streams and Cookies | |
4968 | @cindex cookie, for custom stream | |
4969 | ||
4970 | Inside every custom stream is a special object called the @dfn{cookie}. | |
4971 | This is an object supplied by you which records where to fetch or store | |
4972 | the data read or written. It is up to you to define a data type to use | |
4973 | for the cookie. The stream functions in the library never refer | |
4974 | directly to its contents, and they don't even know what the type is; | |
4975 | they record its address with type @code{void *}. | |
4976 | ||
4977 | To implement a custom stream, you must specify @emph{how} to fetch or | |
4978 | store the data in the specified place. You do this by defining | |
4979 | @dfn{hook functions} to read, write, change ``file position'', and close | |
4980 | the stream. All four of these functions will be passed the stream's | |
4981 | cookie so they can tell where to fetch or store the data. The library | |
4982 | functions don't know what's inside the cookie, but your functions will | |
4983 | know. | |
4984 | ||
4985 | When you create a custom stream, you must specify the cookie pointer, | |
19c3f208 | 4986 | and also the four hook functions stored in a structure of type |
28f540f4 RM |
4987 | @code{cookie_io_functions_t}. |
4988 | ||
4989 | These facilities are declared in @file{stdio.h}. | |
4990 | @pindex stdio.h | |
4991 | ||
4992 | @comment stdio.h | |
4993 | @comment GNU | |
4994 | @deftp {Data Type} {cookie_io_functions_t} | |
19c3f208 | 4995 | This is a structure type that holds the functions that define the |
28f540f4 RM |
4996 | communications protocol between the stream and its cookie. It has |
4997 | the following members: | |
4998 | ||
4999 | @table @code | |
5000 | @item cookie_read_function_t *read | |
5001 | This is the function that reads data from the cookie. If the value is a | |
f2ea0f5b | 5002 | null pointer instead of a function, then read operations on this stream |
28f540f4 RM |
5003 | always return @code{EOF}. |
5004 | ||
5005 | @item cookie_write_function_t *write | |
5006 | This is the function that writes data to the cookie. If the value is a | |
5007 | null pointer instead of a function, then data written to the stream is | |
5008 | discarded. | |
5009 | ||
5010 | @item cookie_seek_function_t *seek | |
5011 | This is the function that performs the equivalent of file positioning on | |
5012 | the cookie. If the value is a null pointer instead of a function, calls | |
a5a0310d UD |
5013 | to @code{fseek} or @code{fseeko} on this stream can only seek to |
5014 | locations within the buffer; any attempt to seek outside the buffer will | |
5015 | return an @code{ESPIPE} error. | |
28f540f4 RM |
5016 | |
5017 | @item cookie_close_function_t *close | |
5018 | This function performs any appropriate cleanup on the cookie when | |
5019 | closing the stream. If the value is a null pointer instead of a | |
5020 | function, nothing special is done to close the cookie when the stream is | |
5021 | closed. | |
5022 | @end table | |
5023 | @end deftp | |
5024 | ||
5025 | @comment stdio.h | |
5026 | @comment GNU | |
5027 | @deftypefun {FILE *} fopencookie (void *@var{cookie}, const char *@var{opentype}, cookie_io_functions_t @var{io-functions}) | |
5028 | This function actually creates the stream for communicating with the | |
5029 | @var{cookie} using the functions in the @var{io-functions} argument. | |
5030 | The @var{opentype} argument is interpreted as for @code{fopen}; | |
5031 | see @ref{Opening Streams}. (But note that the ``truncate on | |
5032 | open'' option is ignored.) The new stream is fully buffered. | |
5033 | ||
5034 | The @code{fopencookie} function returns the newly created stream, or a null | |
5035 | pointer in case of an error. | |
5036 | @end deftypefun | |
5037 | ||
5038 | @node Hook Functions | |
5039 | @subsubsection Custom Stream Hook Functions | |
5040 | @cindex hook functions (of custom streams) | |
5041 | ||
5042 | Here are more details on how you should define the four hook functions | |
5043 | that a custom stream needs. | |
5044 | ||
5045 | You should define the function to read data from the cookie as: | |
5046 | ||
5047 | @smallexample | |
62ecda3d | 5048 | ssize_t @var{reader} (void *@var{cookie}, char *@var{buffer}, size_t @var{size}) |
28f540f4 RM |
5049 | @end smallexample |
5050 | ||
5051 | This is very similar to the @code{read} function; see @ref{I/O | |
5052 | Primitives}. Your function should transfer up to @var{size} bytes into | |
5053 | the @var{buffer}, and return the number of bytes read, or zero to | |
5054 | indicate end-of-file. You can return a value of @code{-1} to indicate | |
5055 | an error. | |
5056 | ||
5057 | You should define the function to write data to the cookie as: | |
5058 | ||
5059 | @smallexample | |
62ecda3d | 5060 | ssize_t @var{writer} (void *@var{cookie}, const char *@var{buffer}, size_t @var{size}) |
28f540f4 RM |
5061 | @end smallexample |
5062 | ||
5063 | This is very similar to the @code{write} function; see @ref{I/O | |
5064 | Primitives}. Your function should transfer up to @var{size} bytes from | |
5065 | the buffer, and return the number of bytes written. You can return a | |
5066 | value of @code{-1} to indicate an error. | |
5067 | ||
5068 | You should define the function to perform seek operations on the cookie | |
5069 | as: | |
5070 | ||
5071 | @smallexample | |
c1e36819 | 5072 | int @var{seeker} (void *@var{cookie}, off64_t *@var{position}, int @var{whence}) |
28f540f4 RM |
5073 | @end smallexample |
5074 | ||
5075 | For this function, the @var{position} and @var{whence} arguments are | |
c1e36819 | 5076 | interpreted as for @code{fgetpos}; see @ref{Portable Positioning}. |
28f540f4 | 5077 | |
19c3f208 | 5078 | After doing the seek operation, your function should store the resulting |
28f540f4 RM |
5079 | file position relative to the beginning of the file in @var{position}. |
5080 | Your function should return a value of @code{0} on success and @code{-1} | |
5081 | to indicate an error. | |
5082 | ||
5083 | You should define the function to do cleanup operations on the cookie | |
5084 | appropriate for closing the stream as: | |
5085 | ||
5086 | @smallexample | |
5087 | int @var{cleaner} (void *@var{cookie}) | |
5088 | @end smallexample | |
5089 | ||
5090 | Your function should return @code{-1} to indicate an error, and @code{0} | |
5091 | otherwise. | |
5092 | ||
5093 | @comment stdio.h | |
5094 | @comment GNU | |
5095 | @deftp {Data Type} cookie_read_function | |
5096 | This is the data type that the read function for a custom stream should have. | |
5097 | If you declare the function as shown above, this is the type it will have. | |
5098 | @end deftp | |
5099 | ||
5100 | @comment stdio.h | |
5101 | @comment GNU | |
5102 | @deftp {Data Type} cookie_write_function | |
5103 | The data type of the write function for a custom stream. | |
5104 | @end deftp | |
5105 | ||
5106 | @comment stdio.h | |
5107 | @comment GNU | |
5108 | @deftp {Data Type} cookie_seek_function | |
5109 | The data type of the seek function for a custom stream. | |
5110 | @end deftp | |
5111 | ||
5112 | @comment stdio.h | |
5113 | @comment GNU | |
5114 | @deftp {Data Type} cookie_close_function | |
5115 | The data type of the close function for a custom stream. | |
5116 | @end deftp | |
5117 | ||
5118 | @ignore | |
5119 | Roland says: | |
5120 | ||
5121 | @quotation | |
5122 | There is another set of functions one can give a stream, the | |
5123 | input-room and output-room functions. These functions must | |
5124 | understand stdio internals. To describe how to use these | |
5125 | functions, you also need to document lots of how stdio works | |
5126 | internally (which isn't relevant for other uses of stdio). | |
5127 | Perhaps I can write an interface spec from which you can write | |
5128 | good documentation. But it's pretty complex and deals with lots | |
5129 | of nitty-gritty details. I think it might be better to let this | |
5130 | wait until the rest of the manual is more done and polished. | |
5131 | @end quotation | |
5132 | @end ignore | |
5133 | ||
5134 | @c ??? This section could use an example. | |
0501d603 UD |
5135 | |
5136 | ||
5137 | @node Formatted Messages | |
5138 | @section Formatted Messages | |
5139 | @cindex formatted messages | |
5140 | ||
5141 | On systems which are based on System V messages of programs (especially | |
5142 | the system tools) are printed in a strict form using the @code{fmtmsg} | |
5143 | function. The uniformity sometimes helps the user to interpret messages | |
cf29ffbe | 5144 | and the strictness tests of the @code{fmtmsg} function ensure that the |
0501d603 UD |
5145 | programmer follows some minimal requirements. |
5146 | ||
5147 | @menu | |
5148 | * Printing Formatted Messages:: The @code{fmtmsg} function. | |
5149 | * Adding Severity Classes:: Add more severity classes. | |
5150 | * Example:: How to use @code{fmtmsg} and @code{addseverity}. | |
5151 | @end menu | |
5152 | ||
5153 | ||
5154 | @node Printing Formatted Messages | |
5155 | @subsection Printing Formatted Messages | |
5156 | ||
5157 | Messages can be printed to standard error and/or to the console. To | |
cf29ffbe | 5158 | select the destination the programmer can use the following two values, |
0501d603 UD |
5159 | bitwise OR combined if wanted, for the @var{classification} parameter of |
5160 | @code{fmtmsg}: | |
5161 | ||
5162 | @vtable @code | |
5163 | @item MM_PRINT | |
5164 | Display the message in standard error. | |
5165 | @item MM_CONSOLE | |
5166 | Display the message on the system console. | |
5167 | @end vtable | |
5168 | ||
f2ea0f5b | 5169 | The erroneous piece of the system can be signalled by exactly one of the |
0501d603 UD |
5170 | following values which also is bitwise ORed with the |
5171 | @var{classification} parameter to @code{fmtmsg}: | |
5172 | ||
5173 | @vtable @code | |
5174 | @item MM_HARD | |
5175 | The source of the condition is some hardware. | |
5176 | @item MM_SOFT | |
5177 | The source of the condition is some software. | |
5178 | @item MM_FIRM | |
5179 | The source of the condition is some firmware. | |
5180 | @end vtable | |
5181 | ||
5182 | A third component of the @var{classification} parameter to @code{fmtmsg} | |
5183 | can describe the part of the system which detects the problem. This is | |
5184 | done by using exactly one of the following values: | |
5185 | ||
5186 | @vtable @code | |
5187 | @item MM_APPL | |
f2ea0f5b | 5188 | The erroneous condition is detected by the application. |
0501d603 | 5189 | @item MM_UTIL |
f2ea0f5b | 5190 | The erroneous condition is detected by a utility. |
0501d603 | 5191 | @item MM_OPSYS |
f2ea0f5b | 5192 | The erroneous condition is detected by the operating system. |
0501d603 UD |
5193 | @end vtable |
5194 | ||
5195 | A last component of @var{classification} can signal the results of this | |
5196 | message. Exactly one of the following values can be used: | |
5197 | ||
5198 | @vtable @code | |
5199 | @item MM_RECOVER | |
5200 | It is a recoverable error. | |
5201 | @item MM_NRECOV | |
5202 | It is a non-recoverable error. | |
5203 | @end vtable | |
5204 | ||
5205 | @comment fmtmsg.h | |
5206 | @comment XPG | |
5207 | @deftypefun int fmtmsg (long int @var{classification}, const char *@var{label}, int @var{severity}, const char *@var{text}, const char *@var{action}, const char *@var{tag}) | |
5208 | Display a message described by its parameters on the device(s) specified | |
5209 | in the @var{classification} parameter. The @var{label} parameter | |
5210 | identifies the source of the message. The string should consist of two | |
5211 | colon separated parts where the first part has not more than 10 and the | |
04b9968b | 5212 | second part not more than 14 characters. The @var{text} parameter |
f2ea0f5b | 5213 | describes the condition of the error, the @var{action} parameter possible |
0501d603 UD |
5214 | steps to recover from the error and the @var{tag} parameter is a |
5215 | reference to the online documentation where more information can be | |
5216 | found. It should contain the @var{label} value and a unique | |
5217 | identification number. | |
5218 | ||
cf29ffbe | 5219 | Each of the parameters can be a special value which means this value |
0501d603 UD |
5220 | is to be omitted. The symbolic names for these values are: |
5221 | ||
5222 | @vtable @code | |
5223 | @item MM_NULLLBL | |
5224 | Ignore @var{label} parameter. | |
5225 | @item MM_NULLSEV | |
5226 | Ignore @var{severity} parameter. | |
5227 | @item MM_NULLMC | |
5228 | Ignore @var{classification} parameter. This implies that nothing is | |
5229 | actually printed. | |
5230 | @item MM_NULLTXT | |
5231 | Ignore @var{text} parameter. | |
5232 | @item MM_NULLACT | |
5233 | Ignore @var{action} parameter. | |
5234 | @item MM_NULLTAG | |
5235 | Ignore @var{tag} parameter. | |
5236 | @end vtable | |
5237 | ||
cf29ffbe | 5238 | There is another way certain fields can be omitted from the output to |
0501d603 | 5239 | standard error. This is described below in the description of |
0bc93a2f | 5240 | environment variables influencing the behavior. |
0501d603 UD |
5241 | |
5242 | The @var{severity} parameter can have one of the values in the following | |
5243 | table: | |
5244 | @cindex severity class | |
5245 | ||
5246 | @vtable @code | |
5247 | @item MM_NOSEV | |
5248 | Nothing is printed, this value is the same as @code{MM_NULLSEV}. | |
5249 | @item MM_HALT | |
5250 | This value is printed as @code{HALT}. | |
5251 | @item MM_ERROR | |
5252 | This value is printed as @code{ERROR}. | |
5253 | @item MM_WARNING | |
5254 | This value is printed as @code{WARNING}. | |
5255 | @item MM_INFO | |
5256 | This value is printed as @code{INFO}. | |
5257 | @end vtable | |
5258 | ||
5259 | The numeric value of these five macros are between @code{0} and | |
5260 | @code{4}. Using the environment variable @code{SEV_LEVEL} or using the | |
5261 | @code{addseverity} function one can add more severity levels with their | |
5262 | corresponding string to print. This is described below | |
5263 | (@pxref{Adding Severity Classes}). | |
5264 | ||
5265 | @noindent | |
5266 | If no parameter is ignored the output looks like this: | |
5267 | ||
5268 | @smallexample | |
5269 | @var{label}: @var{severity-string}: @var{text} | |
5270 | TO FIX: @var{action} @var{tag} | |
5271 | @end smallexample | |
5272 | ||
5273 | The colons, new line characters and the @code{TO FIX} string are | |
5274 | inserted if necessary, i.e., if the corresponding parameter is not | |
5275 | ignored. | |
5276 | ||
5277 | This function is specified in the X/Open Portability Guide. It is also | |
04b9968b | 5278 | available on all systems derived from System V. |
0501d603 | 5279 | |
cf29ffbe | 5280 | The function returns the value @code{MM_OK} if no error occurred. If |
0501d603 UD |
5281 | only the printing to standard error failed, it returns @code{MM_NOMSG}. |
5282 | If printing to the console fails, it returns @code{MM_NOCON}. If | |
cf29ffbe | 5283 | nothing is printed @code{MM_NOTOK} is returned. Among situations where |
0501d603 UD |
5284 | all outputs fail this last value is also returned if a parameter value |
5285 | is incorrect. | |
5286 | @end deftypefun | |
5287 | ||
0bc93a2f | 5288 | There are two environment variables which influence the behavior of |
0501d603 UD |
5289 | @code{fmtmsg}. The first is @code{MSGVERB}. It is used to control the |
5290 | output actually happening on standard error (@emph{not} the console | |
04b9968b | 5291 | output). Each of the five fields can explicitly be enabled. To do |
0501d603 | 5292 | this the user has to put the @code{MSGVERB} variable with a format like |
cf29ffbe | 5293 | the following in the environment before calling the @code{fmtmsg} function |
0501d603 UD |
5294 | the first time: |
5295 | ||
5296 | @smallexample | |
95fdc6a0 | 5297 | MSGVERB=@var{keyword}[:@var{keyword}[:@dots{}]] |
0501d603 UD |
5298 | @end smallexample |
5299 | ||
5300 | Valid @var{keyword}s are @code{label}, @code{severity}, @code{text}, | |
5301 | @code{action}, and @code{tag}. If the environment variable is not given | |
5302 | or is the empty string, a not supported keyword is given or the value is | |
5303 | somehow else invalid, no part of the message is masked out. | |
5304 | ||
0bc93a2f | 5305 | The second environment variable which influences the behavior of |
0501d603 | 5306 | @code{fmtmsg} is @code{SEV_LEVEL}. This variable and the change in the |
0bc93a2f | 5307 | behavior of @code{fmtmsg} is not specified in the X/Open Portability |
0501d603 | 5308 | Guide. It is available in System V systems, though. It can be used to |
cf29ffbe | 5309 | introduce new severity levels. By default, only the five severity levels |
0501d603 UD |
5310 | described above are available. Any other numeric value would make |
5311 | @code{fmtmsg} print nothing. | |
5312 | ||
5313 | If the user puts @code{SEV_LEVEL} with a format like | |
5314 | ||
5315 | @smallexample | |
95fdc6a0 | 5316 | SEV_LEVEL=[@var{description}[:@var{description}[:@dots{}]]] |
0501d603 UD |
5317 | @end smallexample |
5318 | ||
5319 | @noindent | |
5320 | in the environment of the process before the first call to | |
5321 | @code{fmtmsg}, where @var{description} has a value of the form | |
5322 | ||
5323 | @smallexample | |
5324 | @var{severity-keyword},@var{level},@var{printstring} | |
5325 | @end smallexample | |
5326 | ||
5327 | The @var{severity-keyword} part is not used by @code{fmtmsg} but it has | |
5328 | to be present. The @var{level} part is a string representation of a | |
5329 | number. The numeric value must be a number greater than 4. This value | |
5330 | must be used in the @var{severity} parameter of @code{fmtmsg} to select | |
5331 | this class. It is not possible to overwrite any of the predefined | |
5332 | classes. The @var{printstring} is the string printed when a message of | |
5333 | this class is processed by @code{fmtmsg} (see above, @code{fmtsmg} does | |
5334 | not print the numeric value but instead the string representation). | |
5335 | ||
5336 | ||
5337 | @node Adding Severity Classes | |
5338 | @subsection Adding Severity Classes | |
5339 | @cindex severity class | |
5340 | ||
04b9968b | 5341 | There is another possibility to introduce severity classes besides using |
0501d603 UD |
5342 | the environment variable @code{SEV_LEVEL}. This simplifies the task of |
5343 | introducing new classes in a running program. One could use the | |
5344 | @code{setenv} or @code{putenv} function to set the environment variable, | |
cf29ffbe | 5345 | but this is toilsome. |
0501d603 UD |
5346 | |
5347 | @deftypefun int addseverity (int @var{severity}, const char *@var{string}) | |
04b9968b | 5348 | This function allows the introduction of new severity classes which can be |
0501d603 UD |
5349 | addressed by the @var{severity} parameter of the @code{fmtmsg} function. |
5350 | The @var{severity} parameter of @code{addseverity} must match the value | |
04b9968b | 5351 | for the parameter with the same name of @code{fmtmsg}, and @var{string} |
0501d603 UD |
5352 | is the string printed in the actual messages instead of the numeric |
5353 | value. | |
5354 | ||
5355 | If @var{string} is @code{NULL} the severity class with the numeric value | |
5356 | according to @var{severity} is removed. | |
5357 | ||
cf29ffbe UD |
5358 | It is not possible to overwrite or remove one of the default severity |
5359 | classes. All calls to @code{addseverity} with @var{severity} set to one | |
5360 | of the values for the default classes will fail. | |
5361 | ||
0501d603 UD |
5362 | The return value is @code{MM_OK} if the task was successfully performed. |
5363 | If the return value is @code{MM_NOTOK} something went wrong. This could | |
5364 | mean that no more memory is available or a class is not available when | |
5365 | it has to be removed. | |
5366 | ||
5367 | This function is not specified in the X/Open Portability Guide although | |
cf29ffbe | 5368 | the @code{fmtsmg} function is. It is available on System V systems. |
0501d603 UD |
5369 | @end deftypefun |
5370 | ||
5371 | ||
5372 | @node Example | |
5373 | @subsection How to use @code{fmtmsg} and @code{addseverity} | |
5374 | ||
5375 | Here is a simple example program to illustrate the use of the both | |
5376 | functions described in this section. | |
5377 | ||
5378 | @smallexample | |
5379 | @include fmtmsgexpl.c.texi | |
5380 | @end smallexample | |
5381 | ||
04b9968b UD |
5382 | The second call to @code{fmtmsg} illustrates a use of this function as |
5383 | it usually occurs on System V systems, which heavily use this function. | |
5384 | It seems worthwhile to give a short explanation here of how this system | |
5385 | works on System V. The value of the | |
0bc93a2f | 5386 | @var{label} field (@code{UX:cat}) says that the error occurred in the |
0501d603 UD |
5387 | Unix program @code{cat}. The explanation of the error follows and the |
5388 | value for the @var{action} parameter is @code{"refer to manual"}. One | |
04b9968b | 5389 | could be more specific here, if necessary. The @var{tag} field contains, |
0501d603 UD |
5390 | as proposed above, the value of the string given for the @var{label} |
5391 | parameter, and additionally a unique ID (@code{001} in this case). For | |
5392 | a GNU environment this string could contain a reference to the | |
5393 | corresponding node in the Info page for the program. | |
5394 | ||
5395 | @noindent | |
5396 | Running this program without specifying the @code{MSGVERB} and | |
5397 | @code{SEV_LEVEL} function produces the following output: | |
5398 | ||
5399 | @smallexample | |
5400 | UX:cat: NOTE2: invalid syntax | |
5401 | TO FIX: refer to manual UX:cat:001 | |
5402 | @end smallexample | |
5403 | ||
5404 | We see the different fields of the message and how the extra glue (the | |
5405 | colons and the @code{TO FIX} string) are printed. But only one of the | |
5406 | three calls to @code{fmtmsg} produced output. The first call does not | |
5407 | print anything because the @var{label} parameter is not in the correct | |
8b7fb588 UD |
5408 | form. The string must contain two fields, separated by a colon |
5409 | (@pxref{Printing Formatted Messages}). The third @code{fmtmsg} call | |
0501d603 UD |
5410 | produced no output since the class with the numeric value @code{6} is |
5411 | not defined. Although a class with numeric value @code{5} is also not | |
04b9968b | 5412 | defined by default, the call to @code{addseverity} introduces it and |
0ea554bf | 5413 | the second call to @code{fmtmsg} produces the above output. |
0501d603 UD |
5414 | |
5415 | When we change the environment of the program to contain | |
5416 | @code{SEV_LEVEL=XXX,6,NOTE} when running it we get a different result: | |
5417 | ||
5418 | @smallexample | |
5419 | UX:cat: NOTE2: invalid syntax | |
5420 | TO FIX: refer to manual UX:cat:001 | |
5421 | label:foo: NOTE: text | |
5422 | TO FIX: action tag | |
5423 | @end smallexample | |
5424 | ||
04b9968b | 5425 | Now the third call to @code{fmtmsg} produced some output and we see how |
0501d603 UD |
5426 | the string @code{NOTE} from the environment variable appears in the |
5427 | message. | |
5428 | ||
04b9968b | 5429 | Now we can reduce the output by specifying which fields we are |
0501d603 UD |
5430 | interested in. If we additionally set the environment variable |
5431 | @code{MSGVERB} to the value @code{severity:label:action} we get the | |
5432 | following output: | |
5433 | ||
5434 | @smallexample | |
5435 | UX:cat: NOTE2 | |
5436 | TO FIX: refer to manual | |
5437 | label:foo: NOTE | |
5438 | TO FIX: action | |
5439 | @end smallexample | |
5440 | ||
5441 | @noindent | |
5442 | I.e., the output produced by the @var{text} and the @var{tag} parameters | |
cf29ffbe | 5443 | to @code{fmtmsg} vanished. Please also note that now there is no colon |
0501d603 | 5444 | after the @code{NOTE} and @code{NOTE2} strings in the output. This is |
04b9968b | 5445 | not necessary since there is no more output on this line because the text |
0501d603 | 5446 | is missing. |