1 .\" Copyright 2005, 2012, 2016 Michael Kerrisk <mtk.manpages@gmail.com>
3 .\" %%%LICENSE_START(GPL_NOVERSION_ONELINE)
4 .\" Distributed under the GPL.
7 .TH FMEMOPEN 3 2017-09-15 "GNU" "Linux Programmer's Manual"
9 fmemopen \- open memory as stream
14 .BI "FILE *fmemopen(void *"buf ", size_t "size ", const char *" mode ");"
18 Feature Test Macro Requirements for glibc (see
19 .BR feature_test_macros (7)):
28 _POSIX_C_SOURCE\ >=\ 200809L
38 function opens a stream that permits the access specified by
40 The stream allows I/O to be performed on the string or memory buffer
46 argument specifies the semantics of I/O on the stream,
47 and is one of the following:
50 The stream is opened for reading.
53 The stream is opened for writing.
56 Append; open the stream for writing,
57 with the initial buffer position set to the first null byte.
60 Open the stream for reading and writing.
63 Open the stream for reading and writing.
64 The buffer contents are truncated
65 (i.e., \(aq\\0\(aq is placed in the first byte of the buffer).
68 Append; open the stream for reading and writing,
69 with the initial buffer position set to the first null byte.
71 The stream maintains the notion of a current position,
72 the location where the next I/O operation will be performed.
73 The current position is implicitly updated by I/O operations.
74 It can be explicitly updated using
78 In all modes other than append,
79 the initial position is set to the start of the buffer.
80 In append mode, if no null byte is found within the buffer,
81 then the initial position is
86 is specified as NULL, then
91 This is useful for an application that wants to write data to
92 a temporary buffer and then read it back again.
93 The initial position is set to the start of the buffer.
94 The buffer is automatically freed when the stream is closed.
95 Note that the caller has no way to obtain a pointer to the
96 temporary buffer allocated by this call (but see
97 .BR open_memstream (3)).
101 is not NULL, then it should point to a buffer of at least
103 bytes allocated by the caller.
105 When a stream that has been opened for writing is flushed
109 a null byte is written at the end of the buffer if there is space.
110 The caller should ensure that an extra byte is available in the
117 In a stream opened for reading,
118 null bytes (\(aq\\0\(aq) in the buffer do not cause read
119 operations to return an end-of-file indication.
120 A read from the buffer will indicate end-of-file
121 only when the current buffer position advances
123 bytes past the start of the buffer.
125 Write operations take place either at the current position
126 (for modes other than append), or at the current size of the stream
129 Attempts to write more than
131 bytes to the buffer result in an error.
132 By default, such errors will be visible
133 (by the absence of data) only when the
136 Disabling buffering with the following call
137 may be useful to detect errors at the time of an output operation:
139 setbuf(stream, NULL);
141 Upon successful completion,
146 Otherwise, NULL is returned and
148 is set to indicate the error.
151 was already available in glibc 1.0.x.
153 For an explanation of the terms used in this section, see
159 Interface Attribute Value
162 T} Thread safety MT-Safe
167 This function is not specified in POSIX.1-2001,
168 and is not widely available on other systems.
170 POSIX.1-2008 specifies that \(aqb\(aq in
173 However, Technical Corrigendum 1
174 .\" http://austingroupbugs.net/view.php?id=396
175 adjusts the standard to allow implementation-specific treatment for this case,
176 thus permitting the glibc treatment of \(aqb\(aq.
178 There is no file descriptor associated with the file stream
179 returned by this function
182 will return an error if called on the returned stream).
184 With version 2.22, binary mode (see below) was removed,
185 many longstanding bugs in the implementation of
187 were fixed, and a new versioned symbol was created for this interface.
190 From version 2.9 to 2.21, the glibc implementation of
192 supported a "binary" mode,
193 enabled by specifying the letter \(aqb\(aq as the second character in
196 writes don't implicitly add a terminating null byte, and
199 is relative to the end of the buffer (i.e., the value specified by the
201 argument), rather than the current string length.
203 An API bug afflicted the implementation of binary mode:
204 to specify binary mode, the \(aqb\(aq must be the
208 Thus, for example, "wb+" has the desired effect, but "w+b" does not.
209 This is inconsistent with the treatment of
210 .\" http://sourceware.org/bugzilla/show_bug.cgi?id=12836
215 Binary mode was removed in glibc 2.22; a \(aqb\(aq specified in
219 In versions of glibc before 2.22, if
221 is specified as zero,
225 .\" http://sourceware.org/bugzilla/show_bug.cgi?id=11216
226 It would be more consistent if this case successfully created
227 a stream that then returned end-of-file on the first attempt at reading;
228 since version 2.22, the glibc implementation provides that behavior.
230 In versions of glibc before 2.22,
231 specifying append mode ("a" or "a+") for
233 sets the initial buffer position to the first null byte, but
234 .\" http://sourceware.org/bugzilla/show_bug.cgi?id=13152
235 (if the current position is reset to a location other than
236 the end of the stream)
237 does not force subsequent writes to append at the end of the stream.
238 This bug is fixed in glibc 2.22.
240 In versions of glibc before 2.22, if the
244 specifies append ("a" or "a+"), and the
246 argument does not cover a null byte in
248 then, according to POSIX.1-2008,
249 the initial buffer position should be set to
250 the next byte after the end of the buffer.
251 However, in this case the glibc
252 .\" http://sourceware.org/bugzilla/show_bug.cgi?id=13151
254 sets the buffer position to \-1.
255 This bug is fixed in glibc 2.22.
257 In versions of glibc before 2.22,
258 .\" https://sourceware.org/bugzilla/show_bug.cgi?id=14292
265 was performed on a stream created by
271 from the end-of-stream position, instead of being added.
272 This bug is fixed in glibc 2.22.
274 The glibc 2.9 addition of "binary" mode for
276 .\" http://sourceware.org/bugzilla/show_bug.cgi?id=6544
277 silently changed the ABI: previously,
282 The program below uses
284 to open an input buffer, and
285 .BR open_memstream (3)
286 to open a dynamically sized output buffer.
287 The program scans its input string (taken from the program's
288 first command-line argument) reading integers,
289 and writes the squares of these integers to the output buffer.
290 An example of the output produced by this program is the following:
294 .RB "$" " ./a.out \(aq1 23 43\(aq"
295 size=11; ptr=1 529 1849
306 #define handle_error(msg) \\
307 do { perror(msg); exit(EXIT_FAILURE); } while (0)
310 main(int argc, char *argv[])
318 fprintf(stderr, "Usage: %s \(aq<num>...\(aq\\n", argv[0]);
322 in = fmemopen(argv[1], strlen(argv[1]), "r");
324 handle_error("fmemopen");
326 out = open_memstream(&ptr, &size);
328 handle_error("open_memstream");
331 s = fscanf(in, "%d", &v);
335 s = fprintf(out, "%d ", v * v);
337 handle_error("fprintf");
343 printf("size=%zu; ptr=%s\\n", size, ptr);
352 .BR open_memstream (3)