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17c389fc | 1 | @node Program Basics, Processes, Signal Handling, Top |
7a68c94a | 2 | @c %MENU% Writing the beginning and end of your program |
17c389fc | 3 | @chapter The Basic Program/System Interface |
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4 | |
5 | @cindex process | |
17c389fc UD |
6 | @cindex program |
7 | @cindex address space | |
8 | @cindex thread of control | |
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9 | @dfn{Processes} are the primitive units for allocation of system |
10 | resources. Each process has its own address space and (usually) one | |
11 | thread of control. A process executes a program; you can have multiple | |
12 | processes executing the same program, but each process has its own copy | |
13 | of the program within its own address space and executes it | |
17c389fc UD |
14 | independently of the other copies. Though it may have multiple threads |
15 | of control within the same program and a program may be composed of | |
16 | multiple logically separate modules, a process always executes exactly | |
17 | one program. | |
18 | ||
19 | Note that we are using a specific definition of ``program'' for the | |
20 | purposes of this manual, which corresponds to a common definition in the | |
21 | context of Unix system. In popular usage, ``program'' enjoys a much | |
22 | broader definition; it can refer for example to a system's kernel, an | |
23 | editor macro, a complex package of software, or a discrete section of | |
24 | code executing within a process. | |
25 | ||
26 | Writing the program is what this manual is all about. This chapter | |
27 | explains the most basic interface between your program and the system | |
28 | that runs, or calls, it. This includes passing of parameters (arguments | |
29 | and environment) from the system, requesting basic services from the | |
30 | system, and telling the system the program is done. | |
31 | ||
32 | A program starts another program with the @code{exec} family of system calls. | |
33 | This chapter looks at program startup from the execee's point of view. To | |
34 | see the event from the execor's point of view, @xref{Executing a File}. | |
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35 | |
36 | @menu | |
37 | * Program Arguments:: Parsing your program's command-line arguments. | |
17c389fc UD |
38 | * Environment Variables:: Less direct parameters affecting your program |
39 | * System Calls:: Requesting service from the system | |
40 | * Program Termination:: Telling the system you're done; return status | |
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41 | @end menu |
42 | ||
43 | @node Program Arguments | |
44 | @section Program Arguments | |
45 | @cindex program arguments | |
46 | @cindex command line arguments | |
47 | @cindex arguments, to program | |
48 | ||
49 | @cindex program startup | |
50 | @cindex startup of program | |
51 | @cindex invocation of program | |
52 | @cindex @code{main} function | |
53 | @findex main | |
54 | The system starts a C program by calling the function @code{main}. It | |
55 | is up to you to write a function named @code{main}---otherwise, you | |
56 | won't even be able to link your program without errors. | |
57 | ||
f65fd747 | 58 | In @w{ISO C} you can define @code{main} either to take no arguments, or to |
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59 | take two arguments that represent the command line arguments to the |
60 | program, like this: | |
61 | ||
62 | @smallexample | |
63 | int main (int @var{argc}, char *@var{argv}[]) | |
64 | @end smallexample | |
65 | ||
66 | @cindex argc (program argument count) | |
67 | @cindex argv (program argument vector) | |
68 | The command line arguments are the whitespace-separated tokens given in | |
69 | the shell command used to invoke the program; thus, in @samp{cat foo | |
70 | bar}, the arguments are @samp{foo} and @samp{bar}. The only way a | |
71 | program can look at its command line arguments is via the arguments of | |
72 | @code{main}. If @code{main} doesn't take arguments, then you cannot get | |
73 | at the command line. | |
74 | ||
75 | The value of the @var{argc} argument is the number of command line | |
76 | arguments. The @var{argv} argument is a vector of C strings; its | |
77 | elements are the individual command line argument strings. The file | |
78 | name of the program being run is also included in the vector as the | |
79 | first element; the value of @var{argc} counts this element. A null | |
80 | pointer always follows the last element: @code{@var{argv}[@var{argc}]} | |
81 | is this null pointer. | |
82 | ||
83 | For the command @samp{cat foo bar}, @var{argc} is 3 and @var{argv} has | |
84 | three elements, @code{"cat"}, @code{"foo"} and @code{"bar"}. | |
85 | ||
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86 | In Unix systems you can define @code{main} a third way, using three arguments: |
87 | ||
88 | @smallexample | |
85857f93 | 89 | int main (int @var{argc}, char *@var{argv}[], char *@var{envp}[]) |
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90 | @end smallexample |
91 | ||
92 | The first two arguments are just the same. The third argument | |
17c389fc | 93 | @var{envp} gives the program's environment; it is the same as the value |
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94 | of @code{environ}. @xref{Environment Variables}. POSIX.1 does not |
95 | allow this three-argument form, so to be portable it is best to write | |
96 | @code{main} to take two arguments, and use the value of @code{environ}. | |
97 | ||
98 | @menu | |
2064087b | 99 | * Argument Syntax:: By convention, options start with a hyphen. |
b0de3e9e | 100 | * Parsing Program Arguments:: Ways to parse program options and arguments. |
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101 | @end menu |
102 | ||
052b6a6c | 103 | @node Argument Syntax, Parsing Program Arguments, , Program Arguments |
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104 | @subsection Program Argument Syntax Conventions |
105 | @cindex program argument syntax | |
106 | @cindex syntax, for program arguments | |
107 | @cindex command argument syntax | |
108 | ||
109 | POSIX recommends these conventions for command line arguments. | |
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110 | @code{getopt} (@pxref{Getopt}) and @code{argp_parse} (@pxref{Argp}) make |
111 | it easy to implement them. | |
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112 | |
113 | @itemize @bullet | |
114 | @item | |
115 | Arguments are options if they begin with a hyphen delimiter (@samp{-}). | |
116 | ||
117 | @item | |
118 | Multiple options may follow a hyphen delimiter in a single token if | |
119 | the options do not take arguments. Thus, @samp{-abc} is equivalent to | |
120 | @samp{-a -b -c}. | |
121 | ||
122 | @item | |
123 | Option names are single alphanumeric characters (as for @code{isalnum}; | |
8b7fb588 | 124 | @pxref{Classification of Characters}). |
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125 | |
126 | @item | |
127 | Certain options require an argument. For example, the @samp{-o} command | |
128 | of the @code{ld} command requires an argument---an output file name. | |
129 | ||
130 | @item | |
131 | An option and its argument may or may not appear as separate tokens. (In | |
132 | other words, the whitespace separating them is optional.) Thus, | |
133 | @w{@samp{-o foo}} and @samp{-ofoo} are equivalent. | |
134 | ||
135 | @item | |
136 | Options typically precede other non-option arguments. | |
137 | ||
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138 | The implementations of @code{getopt} and @code{argp_parse} in the GNU C |
139 | library normally make it appear as if all the option arguments were | |
140 | specified before all the non-option arguments for the purposes of | |
141 | parsing, even if the user of your program intermixed option and | |
142 | non-option arguments. They do this by reordering the elements of the | |
143 | @var{argv} array. This behavior is nonstandard; if you want to suppress | |
144 | it, define the @code{_POSIX_OPTION_ORDER} environment variable. | |
145 | @xref{Standard Environment}. | |
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146 | |
147 | @item | |
148 | The argument @samp{--} terminates all options; any following arguments | |
149 | are treated as non-option arguments, even if they begin with a hyphen. | |
150 | ||
151 | @item | |
152 | A token consisting of a single hyphen character is interpreted as an | |
153 | ordinary non-option argument. By convention, it is used to specify | |
154 | input from or output to the standard input and output streams. | |
155 | ||
156 | @item | |
157 | Options may be supplied in any order, or appear multiple times. The | |
158 | interpretation is left up to the particular application program. | |
159 | @end itemize | |
160 | ||
161 | @cindex long-named options | |
162 | GNU adds @dfn{long options} to these conventions. Long options consist | |
163 | of @samp{--} followed by a name made of alphanumeric characters and | |
164 | dashes. Option names are typically one to three words long, with | |
165 | hyphens to separate words. Users can abbreviate the option names as | |
166 | long as the abbreviations are unique. | |
167 | ||
168 | To specify an argument for a long option, write | |
169 | @samp{--@var{name}=@var{value}}. This syntax enables a long option to | |
170 | accept an argument that is itself optional. | |
171 | ||
172 | Eventually, the GNU system will provide completion for long option names | |
173 | in the shell. | |
174 | ||
052b6a6c | 175 | @node Parsing Program Arguments, , Argument Syntax, Program Arguments |
b0de3e9e UD |
176 | @subsection Parsing Program Arguments |
177 | ||
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178 | @cindex program arguments, parsing |
179 | @cindex command arguments, parsing | |
180 | @cindex parsing program arguments | |
b0de3e9e UD |
181 | If the syntax for the command line arguments to your program is simple |
182 | enough, you can simply pick the arguments off from @var{argv} by hand. | |
183 | But unless your program takes a fixed number of arguments, or all of the | |
184 | arguments are interpreted in the same way (as file names, for example), | |
185 | you are usually better off using @code{getopt} (@pxref{Getopt}) or | |
186 | @code{argp_parse} (@pxref{Argp}) to do the parsing. | |
28f540f4 | 187 | |
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188 | @code{getopt} is more standard (the short-option only version of it is a |
189 | part of the POSIX standard), but using @code{argp_parse} is often | |
190 | easier, both for very simple and very complex option structures, because | |
191 | it does more of the dirty work for you. | |
28f540f4 | 192 | |
b0de3e9e UD |
193 | @menu |
194 | * Getopt:: Parsing program options using @code{getopt}. | |
195 | * Argp:: Parsing program options using @code{argp_parse}. | |
196 | * Suboptions:: Some programs need more detailed options. | |
197 | * Suboptions Example:: This shows how it could be done for @code{mount}. | |
198 | @end menu | |
28f540f4 | 199 | |
b0de3e9e UD |
200 | @c Getopt and argp start at the @section level so that there's |
201 | @c enough room for their internal hierarchy (mostly a problem with | |
202 | @c argp). -Miles | |
28f540f4 | 203 | |
b0de3e9e UD |
204 | @include getopt.texi |
205 | @include argp.texi | |
28f540f4 | 206 | |
b0de3e9e UD |
207 | @node Suboptions, Suboptions Example, Argp, Parsing Program Arguments |
208 | @c This is a @section so that it's at the same level as getopt and argp | |
052b6a6c | 209 | @subsubsection Parsing of Suboptions |
2064087b RM |
210 | |
211 | Having a single level of options is sometimes not enough. There might | |
212 | be too many options which have to be available or a set of options is | |
213 | closely related. | |
214 | ||
215 | For this case some programs use suboptions. One of the most prominent | |
216 | programs is certainly @code{mount}(8). The @code{-o} option take one | |
217 | argument which itself is a comma separated list of options. To ease the | |
218 | programming of code like this the function @code{getsubopt} is | |
219 | available. | |
220 | ||
221 | @comment stdlib.h | |
222 | @deftypefun int getsubopt (char **@var{optionp}, const char* const *@var{tokens}, char **@var{valuep}) | |
223 | ||
224 | The @var{optionp} parameter must be a pointer to a variable containing | |
225 | the address of the string to process. When the function returns the | |
226 | reference is updated to point to the next suboption or to the | |
227 | terminating @samp{\0} character if there is no more suboption available. | |
228 | ||
229 | The @var{tokens} parameter references an array of strings containing the | |
230 | known suboptions. All strings must be @samp{\0} terminated and to mark | |
231 | the end a null pointer must be stored. When @code{getsubopt} finds a | |
232 | possible legal suboption it compares it with all strings available in | |
233 | the @var{tokens} array and returns the index in the string as the | |
234 | indicator. | |
235 | ||
236 | In case the suboption has an associated value introduced by a @samp{=} | |
237 | character, a pointer to the value is returned in @var{valuep}. The | |
238 | string is @samp{\0} terminated. If no argument is available | |
239 | @var{valuep} is set to the null pointer. By doing this the caller can | |
240 | check whether a necessary value is given or whether no unexpected value | |
241 | is present. | |
242 | ||
243 | In case the next suboption in the string is not mentioned in the | |
244 | @var{tokens} array the starting address of the suboption including a | |
245 | possible value is returned in @var{valuep} and the return value of the | |
246 | function is @samp{-1}. | |
247 | @end deftypefun | |
248 | ||
b0de3e9e | 249 | @node Suboptions Example, , Suboptions, Parsing Program Arguments |
2064087b RM |
250 | @subsection Parsing of Suboptions Example |
251 | ||
252 | The code which might appear in the @code{mount}(8) program is a perfect | |
253 | example of the use of @code{getsubopt}: | |
254 | ||
255 | @smallexample | |
256 | @include subopt.c.texi | |
257 | @end smallexample | |
258 | ||
259 | ||
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260 | @node Environment Variables |
261 | @section Environment Variables | |
262 | ||
263 | @cindex environment variable | |
264 | When a program is executed, it receives information about the context in | |
265 | which it was invoked in two ways. The first mechanism uses the | |
266 | @var{argv} and @var{argc} arguments to its @code{main} function, and is | |
267 | discussed in @ref{Program Arguments}. The second mechanism uses | |
268 | @dfn{environment variables} and is discussed in this section. | |
269 | ||
270 | The @var{argv} mechanism is typically used to pass command-line | |
271 | arguments specific to the particular program being invoked. The | |
272 | environment, on the other hand, keeps track of information that is | |
273 | shared by many programs, changes infrequently, and that is less | |
274 | frequently used. | |
275 | ||
276 | The environment variables discussed in this section are the same | |
277 | environment variables that you set using assignments and the | |
278 | @code{export} command in the shell. Programs executed from the shell | |
279 | inherit all of the environment variables from the shell. | |
280 | @c !!! xref to right part of bash manual when it exists | |
281 | ||
282 | @cindex environment | |
283 | Standard environment variables are used for information about the user's | |
284 | home directory, terminal type, current locale, and so on; you can define | |
285 | additional variables for other purposes. The set of all environment | |
286 | variables that have values is collectively known as the | |
287 | @dfn{environment}. | |
288 | ||
289 | Names of environment variables are case-sensitive and must not contain | |
290 | the character @samp{=}. System-defined environment variables are | |
291 | invariably uppercase. | |
292 | ||
293 | The values of environment variables can be anything that can be | |
294 | represented as a string. A value must not contain an embedded null | |
295 | character, since this is assumed to terminate the string. | |
296 | ||
297 | ||
298 | @menu | |
2064087b | 299 | * Environment Access:: How to get and set the values of |
40a55d20 | 300 | environment variables. |
2064087b | 301 | * Standard Environment:: These environment variables have |
40a55d20 | 302 | standard interpretations. |
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303 | @end menu |
304 | ||
305 | @node Environment Access | |
306 | @subsection Environment Access | |
307 | @cindex environment access | |
308 | @cindex environment representation | |
309 | ||
310 | The value of an environment variable can be accessed with the | |
311 | @code{getenv} function. This is declared in the header file | |
40a55d20 UD |
312 | @file{stdlib.h}. All of the following functions can be safely used in |
313 | multi-threaded programs. It is made sure that concurrent modifications | |
314 | to the environment do not lead to errors. | |
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315 | @pindex stdlib.h |
316 | ||
317 | @comment stdlib.h | |
f65fd747 | 318 | @comment ISO |
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319 | @deftypefun {char *} getenv (const char *@var{name}) |
320 | This function returns a string that is the value of the environment | |
321 | variable @var{name}. You must not modify this string. In some non-Unix | |
322 | systems not using the GNU library, it might be overwritten by subsequent | |
323 | calls to @code{getenv} (but not by any other library function). If the | |
324 | environment variable @var{name} is not defined, the value is a null | |
325 | pointer. | |
326 | @end deftypefun | |
327 | ||
328 | ||
329 | @comment stdlib.h | |
330 | @comment SVID | |
14d9bd50 | 331 | @deftypefun int putenv (char *@var{string}) |
28f540f4 RM |
332 | The @code{putenv} function adds or removes definitions from the environment. |
333 | If the @var{string} is of the form @samp{@var{name}=@var{value}}, the | |
334 | definition is added to the environment. Otherwise, the @var{string} is | |
335 | interpreted as the name of an environment variable, and any definition | |
336 | for this variable in the environment is removed. | |
337 | ||
66f8fa9b UD |
338 | The difference to the @code{setenv} function is that the exact string |
339 | given as the parameter @var{string} is put into the environment. If the | |
340 | user should change the string after the @code{putenv} call this will | |
341 | reflect in automatically in the environment. This also requires that | |
342 | @var{string} is no automatic variable which scope is left before the | |
d364e525 UD |
343 | variable is removed from the environment. The same applies of course to |
344 | dynamically allocated variables which are freed later. | |
66f8fa9b | 345 | |
40a55d20 UD |
346 | This function is part of the extended Unix interface. Since it was also |
347 | available in old SVID libraries you should define either | |
348 | @var{_XOPEN_SOURCE} or @var{_SVID_SOURCE} before including any header. | |
349 | @end deftypefun | |
350 | ||
351 | ||
352 | @comment stdlib.h | |
353 | @comment BSD | |
354 | @deftypefun int setenv (const char *@var{name}, const char *@var{value}, int @var{replace}) | |
355 | The @code{setenv} function can be used to add a new definition to the | |
356 | environment. The entry with the name @var{name} is replaced by the | |
357 | value @samp{@var{name}=@var{value}}. Please note that this is also true | |
66f8fa9b UD |
358 | if @var{value} is the empty string. To do this a new string is created |
359 | and the strings @var{name} and @var{value} are copied. A null pointer | |
360 | for the @var{value} parameter is illegal. If the environment already | |
361 | contains an entry with key @var{name} the @var{replace} parameter | |
362 | controls the action. If replace is zero, nothing happens. Otherwise | |
363 | the old entry is replaced by the new one. | |
40a55d20 UD |
364 | |
365 | Please note that you cannot remove an entry completely using this function. | |
366 | ||
0423ee17 UD |
367 | This function was originally part of the BSD library but is now part of |
368 | the Unix standard. | |
40a55d20 UD |
369 | @end deftypefun |
370 | ||
371 | @comment stdlib.h | |
372 | @comment BSD | |
0423ee17 | 373 | @deftypefun int unsetenv (const char *@var{name}) |
40a55d20 UD |
374 | Using this function one can remove an entry completely from the |
375 | environment. If the environment contains an entry with the key | |
376 | @var{name} this whole entry is removed. A call to this function is | |
377 | equivalent to a call to @code{putenv} when the @var{value} part of the | |
378 | string is empty. | |
379 | ||
0423ee17 UD |
380 | The function return @code{-1} if @var{name} is a null pointer, points to |
381 | an empty string, or points to a string containing a @code{=} character. | |
382 | It returns @code{0} if the call succeeded. | |
383 | ||
b912ca11 | 384 | This function was originally part of the BSD library but is now part of |
0423ee17 | 385 | the Unix standard. The BSD version had no return value, though. |
40a55d20 UD |
386 | @end deftypefun |
387 | ||
388 | There is one more function to modify the whole environment. This | |
389 | function is said to be used in the POSIX.9 (POSIX bindings for Fortran | |
390 | 77) and so one should expect it did made it into POSIX.1. But this | |
391 | never happened. But we still provide this function as a GNU extension | |
392 | to enable writing standard compliant Fortran environments. | |
393 | ||
394 | @comment stdlib.h | |
395 | @comment GNU | |
396 | @deftypefun int clearenv (void) | |
397 | The @code{clearenv} function removes all entries from the environment. | |
398 | Using @code{putenv} and @code{setenv} new entries can be added again | |
399 | later. | |
400 | ||
401 | If the function is successful it returns @code{0}. Otherwise the return | |
402 | value is nonzero. | |
28f540f4 RM |
403 | @end deftypefun |
404 | ||
28f540f4 RM |
405 | |
406 | You can deal directly with the underlying representation of environment | |
407 | objects to add more variables to the environment (for example, to | |
8b7fb588 UD |
408 | communicate with another program you are about to execute; |
409 | @pxref{Executing a File}). | |
28f540f4 RM |
410 | |
411 | @comment unistd.h | |
412 | @comment POSIX.1 | |
413 | @deftypevar {char **} environ | |
414 | The environment is represented as an array of strings. Each string is | |
415 | of the format @samp{@var{name}=@var{value}}. The order in which | |
416 | strings appear in the environment is not significant, but the same | |
417 | @var{name} must not appear more than once. The last element of the | |
418 | array is a null pointer. | |
419 | ||
420 | This variable is declared in the header file @file{unistd.h}. | |
421 | ||
422 | If you just want to get the value of an environment variable, use | |
423 | @code{getenv}. | |
424 | @end deftypevar | |
425 | ||
426 | Unix systems, and the GNU system, pass the initial value of | |
427 | @code{environ} as the third argument to @code{main}. | |
428 | @xref{Program Arguments}. | |
429 | ||
430 | @node Standard Environment | |
431 | @subsection Standard Environment Variables | |
432 | @cindex standard environment variables | |
433 | ||
434 | These environment variables have standard meanings. This doesn't mean | |
435 | that they are always present in the environment; but if these variables | |
436 | @emph{are} present, they have these meanings. You shouldn't try to use | |
437 | these environment variable names for some other purpose. | |
438 | ||
439 | @comment Extra blank lines make it look better. | |
440 | @table @code | |
441 | @item HOME | |
838e5ffe | 442 | @cindex @code{HOME} environment variable |
28f540f4 RM |
443 | @cindex home directory |
444 | ||
445 | This is a string representing the user's @dfn{home directory}, or | |
446 | initial default working directory. | |
447 | ||
448 | The user can set @code{HOME} to any value. | |
449 | If you need to make sure to obtain the proper home directory | |
450 | for a particular user, you should not use @code{HOME}; instead, | |
451 | look up the user's name in the user database (@pxref{User Database}). | |
452 | ||
453 | For most purposes, it is better to use @code{HOME}, precisely because | |
454 | this lets the user specify the value. | |
455 | ||
456 | @c !!! also USER | |
457 | @item LOGNAME | |
838e5ffe | 458 | @cindex @code{LOGNAME} environment variable |
28f540f4 RM |
459 | |
460 | This is the name that the user used to log in. Since the value in the | |
461 | environment can be tweaked arbitrarily, this is not a reliable way to | |
17c389fc | 462 | identify the user who is running a program; a function like |
28f540f4 RM |
463 | @code{getlogin} (@pxref{Who Logged In}) is better for that purpose. |
464 | ||
465 | For most purposes, it is better to use @code{LOGNAME}, precisely because | |
466 | this lets the user specify the value. | |
467 | ||
468 | @item PATH | |
838e5ffe | 469 | @cindex @code{PATH} environment variable |
28f540f4 RM |
470 | |
471 | A @dfn{path} is a sequence of directory names which is used for | |
472 | searching for a file. The variable @code{PATH} holds a path used | |
473 | for searching for programs to be run. | |
474 | ||
475 | The @code{execlp} and @code{execvp} functions (@pxref{Executing a File}) | |
476 | use this environment variable, as do many shells and other utilities | |
477 | which are implemented in terms of those functions. | |
478 | ||
479 | The syntax of a path is a sequence of directory names separated by | |
1b82a4a8 | 480 | colons. An empty string instead of a directory name stands for the |
28f540f4 RM |
481 | current directory (@pxref{Working Directory}). |
482 | ||
483 | A typical value for this environment variable might be a string like: | |
484 | ||
485 | @smallexample | |
486 | :/bin:/etc:/usr/bin:/usr/new/X11:/usr/new:/usr/local/bin | |
487 | @end smallexample | |
488 | ||
489 | This means that if the user tries to execute a program named @code{foo}, | |
490 | the system will look for files named @file{foo}, @file{/bin/foo}, | |
491 | @file{/etc/foo}, and so on. The first of these files that exists is | |
492 | the one that is executed. | |
493 | ||
494 | @c !!! also TERMCAP | |
495 | @item TERM | |
838e5ffe | 496 | @cindex @code{TERM} environment variable |
28f540f4 RM |
497 | |
498 | This specifies the kind of terminal that is receiving program output. | |
499 | Some programs can make use of this information to take advantage of | |
500 | special escape sequences or terminal modes supported by particular kinds | |
501 | of terminals. Many programs which use the termcap library | |
502 | (@pxref{Finding a Terminal Description,Find,,termcap,The Termcap Library | |
503 | Manual}) use the @code{TERM} environment variable, for example. | |
504 | ||
505 | @item TZ | |
838e5ffe | 506 | @cindex @code{TZ} environment variable |
28f540f4 RM |
507 | |
508 | This specifies the time zone. @xref{TZ Variable}, for information about | |
509 | the format of this string and how it is used. | |
510 | ||
511 | @item LANG | |
838e5ffe | 512 | @cindex @code{LANG} environment variable |
28f540f4 RM |
513 | |
514 | This specifies the default locale to use for attribute categories where | |
515 | neither @code{LC_ALL} nor the specific environment variable for that | |
516 | category is set. @xref{Locales}, for more information about | |
517 | locales. | |
518 | ||
519 | @ignore | |
520 | @c I doubt this really exists | |
521 | @item LC_ALL | |
838e5ffe | 522 | @cindex @code{LC_ALL} environment variable |
28f540f4 RM |
523 | |
524 | This is similar to the @code{LANG} environment variable. However, its | |
525 | value takes precedence over any values provided for the individual | |
526 | attribute category environment variables, or for the @code{LANG} | |
527 | environment variable. | |
528 | @end ignore | |
529 | ||
40a55d20 | 530 | @item LC_ALL |
838e5ffe | 531 | @cindex @code{LC_ALL} environment variable |
40a55d20 UD |
532 | |
533 | If this environment variable is set it overrides the selection for all | |
534 | the locales done using the other @code{LC_*} environment variables. The | |
535 | value of the other @code{LC_*} environment variables is simply ignored | |
536 | in this case. | |
537 | ||
28f540f4 | 538 | @item LC_COLLATE |
838e5ffe | 539 | @cindex @code{LC_COLLATE} environment variable |
28f540f4 RM |
540 | |
541 | This specifies what locale to use for string sorting. | |
542 | ||
543 | @item LC_CTYPE | |
838e5ffe | 544 | @cindex @code{LC_CTYPE} environment variable |
28f540f4 RM |
545 | |
546 | This specifies what locale to use for character sets and character | |
547 | classification. | |
548 | ||
40a55d20 | 549 | @item LC_MESSAGES |
838e5ffe | 550 | @cindex @code{LC_MESSAGES} environment variable |
40a55d20 UD |
551 | |
552 | This specifies what locale to use for printing messages and to parse | |
f2ea0f5b | 553 | responses. |
40a55d20 | 554 | |
28f540f4 | 555 | @item LC_MONETARY |
838e5ffe | 556 | @cindex @code{LC_MONETARY} environment variable |
28f540f4 RM |
557 | |
558 | This specifies what locale to use for formatting monetary values. | |
559 | ||
560 | @item LC_NUMERIC | |
838e5ffe | 561 | @cindex @code{LC_NUMERIC} environment variable |
28f540f4 RM |
562 | |
563 | This specifies what locale to use for formatting numbers. | |
564 | ||
565 | @item LC_TIME | |
838e5ffe | 566 | @cindex @code{LC_TIME} environment variable |
28f540f4 RM |
567 | |
568 | This specifies what locale to use for formatting date/time values. | |
569 | ||
40a55d20 | 570 | @item NLSPATH |
838e5ffe | 571 | @cindex @code{NLSPATH} environment variable |
40a55d20 UD |
572 | |
573 | This specifies the directories in which the @code{catopen} function | |
574 | looks for message translation catalogs. | |
575 | ||
28f540f4 | 576 | @item _POSIX_OPTION_ORDER |
838e5ffe | 577 | @cindex @code{_POSIX_OPTION_ORDER} environment variable. |
28f540f4 RM |
578 | |
579 | If this environment variable is defined, it suppresses the usual | |
b0de3e9e UD |
580 | reordering of command line arguments by @code{getopt} and |
581 | @code{argp_parse}. @xref{Argument Syntax}. | |
28f540f4 RM |
582 | |
583 | @c !!! GNU also has COREFILE, CORESERVER, EXECSERVERS | |
584 | @end table | |
585 | ||
17c389fc UD |
586 | @node System Calls |
587 | @section System Calls | |
588 | ||
589 | @cindex system call | |
590 | A system call is a request for service that a program makes of the | |
591 | kernel. The service is generally something that only the kernel has | |
592 | the privilege to do, such as doing I/O. Programmers don't normally | |
593 | need to be concerned with system calls because there are functions in | |
594 | the GNU C library to do virtually everything that system calls do. | |
595 | These functions work by making system calls themselves. For example, | |
d364e525 | 596 | there is a system call that changes the permissions of a file, but |
17c389fc UD |
597 | you don't need to know about it because you can just use the GNU C |
598 | library's @code{chmod} function. | |
599 | ||
600 | @cindex kernel call | |
601 | System calls are sometimes called kernel calls. | |
602 | ||
603 | However, there are times when you want to make a system call explicitly, | |
604 | and for that, the GNU C library provides the @code{syscall} function. | |
605 | @code{syscall} is harder to use and less portable than functions like | |
606 | @code{chmod}, but easier and more portable than coding the system call | |
607 | in assembler instructions. | |
608 | ||
609 | @code{syscall} is most useful when you are working with a system call | |
610 | which is special to your system or is newer than the GNU C library you | |
611 | are using. @code{syscall} is implemented in an entirely generic way; | |
612 | the function does not know anything about what a particular system | |
613 | call does or even if it is valid. | |
614 | ||
615 | The description of @code{syscall} in this section assumes a certain | |
616 | protocol for system calls on the various platforms on which the GNU C | |
617 | library runs. That protocol is not defined by any strong authority, but | |
618 | we won't describe it here either because anyone who is coding | |
619 | @code{syscall} probably won't accept anything less than kernel and C | |
620 | library source code as a specification of the interface between them | |
621 | anyway. | |
622 | ||
623 | ||
624 | @code{syscall} is declared in @file{unistd.h}. | |
625 | ||
626 | @comment unistd.h | |
627 | @comment ??? | |
4c78249d | 628 | @deftypefun {long int} syscall (long int @var{sysno}, ...) |
17c389fc UD |
629 | |
630 | @code{syscall} performs a generic system call. | |
631 | ||
632 | @cindex system call number | |
633 | @var{sysno} is the system call number. Each kind of system call is | |
634 | identified by a number. Macros for all the possible system call numbers | |
635 | are defined in @file{sys/syscall.h} | |
636 | ||
637 | The remaining arguments are the arguments for the system call, in | |
638 | order, and their meanings depend on the kind of system call. Each kind | |
639 | of system call has a definite number of arguments, from zero to five. | |
640 | If you code more arguments than the system call takes, the extra ones to | |
641 | the right are ignored. | |
642 | ||
643 | The return value is the return value from the system call, unless the | |
644 | system call failed. In that case, @code{syscall} returns @code{-1} and | |
645 | sets @code{errno} to an error code that the system call returned. Note | |
646 | that system calls do not return @code{-1} when they succeed. | |
647 | @cindex errno | |
648 | ||
649 | If you specify an invalid @var{sysno}, @code{syscall} returns @code{-1} | |
650 | with @code{errno} = @code{ENOSYS}. | |
651 | ||
652 | Example: | |
653 | ||
654 | @smallexample | |
655 | ||
656 | #include <unistd.h> | |
657 | #include <sys/syscall.h> | |
658 | #include <errno.h> | |
659 | ||
95fdc6a0 | 660 | @dots{} |
17c389fc UD |
661 | |
662 | int rc; | |
663 | ||
664 | rc = syscall(SYS_chmod, "/etc/passwd", 0444); | |
665 | ||
d364e525 | 666 | if (rc == -1) |
17c389fc UD |
667 | fprintf(stderr, "chmod failed, errno = %d\n", errno); |
668 | ||
669 | @end smallexample | |
670 | ||
671 | This, if all the compatibility stars are aligned, is equivalent to the | |
672 | following preferable code: | |
673 | ||
674 | @smallexample | |
675 | ||
676 | #include <sys/types.h> | |
677 | #include <sys/stat.h> | |
678 | #include <errno.h> | |
679 | ||
95fdc6a0 | 680 | @dots{} |
17c389fc UD |
681 | |
682 | int rc; | |
683 | ||
684 | rc = chmod("/etc/passwd", 0444); | |
685 | if (rc == -1) | |
686 | fprintf(stderr, "chmod failed, errno = %d\n", errno); | |
687 | ||
688 | @end smallexample | |
689 | ||
690 | @end deftypefun | |
691 | ||
692 | ||
28f540f4 RM |
693 | @node Program Termination |
694 | @section Program Termination | |
695 | @cindex program termination | |
696 | @cindex process termination | |
697 | ||
698 | @cindex exit status value | |
699 | The usual way for a program to terminate is simply for its @code{main} | |
700 | function to return. The @dfn{exit status value} returned from the | |
701 | @code{main} function is used to report information back to the process's | |
702 | parent process or shell. | |
703 | ||
704 | A program can also terminate normally by calling the @code{exit} | |
705 | function. | |
706 | ||
707 | In addition, programs can be terminated by signals; this is discussed in | |
708 | more detail in @ref{Signal Handling}. The @code{abort} function causes | |
709 | a signal that kills the program. | |
710 | ||
711 | @menu | |
712 | * Normal Termination:: If a program calls @code{exit}, a | |
713 | process terminates normally. | |
1b82a4a8 RM |
714 | * Exit Status:: The @code{exit status} provides information |
715 | about why the process terminated. | |
28f540f4 | 716 | * Cleanups on Exit:: A process can run its own cleanup |
1b82a4a8 | 717 | functions upon normal termination. |
28f540f4 | 718 | * Aborting a Program:: The @code{abort} function causes |
1b82a4a8 | 719 | abnormal program termination. |
28f540f4 RM |
720 | * Termination Internals:: What happens when a process terminates. |
721 | @end menu | |
722 | ||
723 | @node Normal Termination | |
724 | @subsection Normal Termination | |
725 | ||
17c389fc UD |
726 | A process terminates normally when its program signals it is done by |
727 | calling @code{exit}. Returning from @code{main} is equivalent to | |
728 | calling @code{exit}, and the value that @code{main} returns is used as | |
729 | the argument to @code{exit}. | |
28f540f4 RM |
730 | |
731 | @comment stdlib.h | |
f65fd747 | 732 | @comment ISO |
28f540f4 | 733 | @deftypefun void exit (int @var{status}) |
17c389fc UD |
734 | The @code{exit} function tells the system that the program is done, which |
735 | causes it to terminate the process. | |
736 | ||
737 | @var{status} is the program's exit status, which becomes part of the | |
738 | process' termination status. This function does not return. | |
28f540f4 RM |
739 | @end deftypefun |
740 | ||
741 | Normal termination causes the following actions: | |
742 | ||
743 | @enumerate | |
1b82a4a8 | 744 | @item |
28f540f4 RM |
745 | Functions that were registered with the @code{atexit} or @code{on_exit} |
746 | functions are called in the reverse order of their registration. This | |
747 | mechanism allows your application to specify its own ``cleanup'' actions | |
748 | to be performed at program termination. Typically, this is used to do | |
749 | things like saving program state information in a file, or unlocking | |
750 | locks in shared data bases. | |
751 | ||
1b82a4a8 | 752 | @item |
28f540f4 RM |
753 | All open streams are closed, writing out any buffered output data. See |
754 | @ref{Closing Streams}. In addition, temporary files opened | |
755 | with the @code{tmpfile} function are removed; see @ref{Temporary Files}. | |
756 | ||
1b82a4a8 | 757 | @item |
28f540f4 RM |
758 | @code{_exit} is called, terminating the program. @xref{Termination Internals}. |
759 | @end enumerate | |
760 | ||
761 | @node Exit Status | |
762 | @subsection Exit Status | |
763 | @cindex exit status | |
764 | ||
765 | When a program exits, it can return to the parent process a small | |
766 | amount of information about the cause of termination, using the | |
767 | @dfn{exit status}. This is a value between 0 and 255 that the exiting | |
768 | process passes as an argument to @code{exit}. | |
769 | ||
770 | Normally you should use the exit status to report very broad information | |
771 | about success or failure. You can't provide a lot of detail about the | |
772 | reasons for the failure, and most parent processes would not want much | |
773 | detail anyway. | |
774 | ||
775 | There are conventions for what sorts of status values certain programs | |
776 | should return. The most common convention is simply 0 for success and 1 | |
777 | for failure. Programs that perform comparison use a different | |
778 | convention: they use status 1 to indicate a mismatch, and status 2 to | |
779 | indicate an inability to compare. Your program should follow an | |
780 | existing convention if an existing convention makes sense for it. | |
781 | ||
782 | A general convention reserves status values 128 and up for special | |
783 | purposes. In particular, the value 128 is used to indicate failure to | |
784 | execute another program in a subprocess. This convention is not | |
785 | universally obeyed, but it is a good idea to follow it in your programs. | |
786 | ||
787 | @strong{Warning:} Don't try to use the number of errors as the exit | |
788 | status. This is actually not very useful; a parent process would | |
789 | generally not care how many errors occurred. Worse than that, it does | |
790 | not work, because the status value is truncated to eight bits. | |
791 | Thus, if the program tried to report 256 errors, the parent would | |
792 | receive a report of 0 errors---that is, success. | |
793 | ||
794 | For the same reason, it does not work to use the value of @code{errno} | |
795 | as the exit status---these can exceed 255. | |
796 | ||
797 | @strong{Portability note:} Some non-POSIX systems use different | |
798 | conventions for exit status values. For greater portability, you can | |
799 | use the macros @code{EXIT_SUCCESS} and @code{EXIT_FAILURE} for the | |
800 | conventional status value for success and failure, respectively. They | |
801 | are declared in the file @file{stdlib.h}. | |
802 | @pindex stdlib.h | |
803 | ||
804 | @comment stdlib.h | |
f65fd747 | 805 | @comment ISO |
28f540f4 RM |
806 | @deftypevr Macro int EXIT_SUCCESS |
807 | This macro can be used with the @code{exit} function to indicate | |
808 | successful program completion. | |
809 | ||
810 | On POSIX systems, the value of this macro is @code{0}. On other | |
811 | systems, the value might be some other (possibly non-constant) integer | |
812 | expression. | |
813 | @end deftypevr | |
814 | ||
815 | @comment stdlib.h | |
f65fd747 | 816 | @comment ISO |
28f540f4 RM |
817 | @deftypevr Macro int EXIT_FAILURE |
818 | This macro can be used with the @code{exit} function to indicate | |
819 | unsuccessful program completion in a general sense. | |
820 | ||
821 | On POSIX systems, the value of this macro is @code{1}. On other | |
822 | systems, the value might be some other (possibly non-constant) integer | |
f166d865 | 823 | expression. Other nonzero status values also indicate failures. Certain |
28f540f4 RM |
824 | programs use different nonzero status values to indicate particular |
825 | kinds of "non-success". For example, @code{diff} uses status value | |
826 | @code{1} to mean that the files are different, and @code{2} or more to | |
827 | mean that there was difficulty in opening the files. | |
828 | @end deftypevr | |
829 | ||
17c389fc UD |
830 | Don't confuse a program's exit status with a process' termination status. |
831 | There are lots of ways a process can terminate besides having it's program | |
832 | finish. In the event that the process termination @emph{is} caused by program | |
32c075e1 | 833 | termination (i.e. @code{exit}), though, the program's exit status becomes |
17c389fc UD |
834 | part of the process' termination status. |
835 | ||
28f540f4 RM |
836 | @node Cleanups on Exit |
837 | @subsection Cleanups on Exit | |
838 | ||
839 | Your program can arrange to run its own cleanup functions if normal | |
840 | termination happens. If you are writing a library for use in various | |
841 | application programs, then it is unreliable to insist that all | |
842 | applications call the library's cleanup functions explicitly before | |
843 | exiting. It is much more robust to make the cleanup invisible to the | |
844 | application, by setting up a cleanup function in the library itself | |
845 | using @code{atexit} or @code{on_exit}. | |
846 | ||
847 | @comment stdlib.h | |
f65fd747 | 848 | @comment ISO |
28f540f4 RM |
849 | @deftypefun int atexit (void (*@var{function}) (void)) |
850 | The @code{atexit} function registers the function @var{function} to be | |
851 | called at normal program termination. The @var{function} is called with | |
852 | no arguments. | |
853 | ||
854 | The return value from @code{atexit} is zero on success and nonzero if | |
1b82a4a8 | 855 | the function cannot be registered. |
28f540f4 RM |
856 | @end deftypefun |
857 | ||
858 | @comment stdlib.h | |
859 | @comment SunOS | |
860 | @deftypefun int on_exit (void (*@var{function})(int @var{status}, void *@var{arg}), void *@var{arg}) | |
861 | This function is a somewhat more powerful variant of @code{atexit}. It | |
862 | accepts two arguments, a function @var{function} and an arbitrary | |
863 | pointer @var{arg}. At normal program termination, the @var{function} is | |
864 | called with two arguments: the @var{status} value passed to @code{exit}, | |
865 | and the @var{arg}. | |
866 | ||
867 | This function is included in the GNU C library only for compatibility | |
868 | for SunOS, and may not be supported by other implementations. | |
869 | @end deftypefun | |
870 | ||
871 | Here's a trivial program that illustrates the use of @code{exit} and | |
872 | @code{atexit}: | |
873 | ||
874 | @smallexample | |
875 | @include atexit.c.texi | |
876 | @end smallexample | |
877 | ||
878 | @noindent | |
879 | When this program is executed, it just prints the message and exits. | |
880 | ||
881 | @node Aborting a Program | |
882 | @subsection Aborting a Program | |
883 | @cindex aborting a program | |
884 | ||
885 | You can abort your program using the @code{abort} function. The prototype | |
886 | for this function is in @file{stdlib.h}. | |
887 | @pindex stdlib.h | |
888 | ||
889 | @comment stdlib.h | |
f65fd747 | 890 | @comment ISO |
28f540f4 RM |
891 | @deftypefun void abort (void) |
892 | The @code{abort} function causes abnormal program termination. This | |
893 | does not execute cleanup functions registered with @code{atexit} or | |
894 | @code{on_exit}. | |
895 | ||
896 | This function actually terminates the process by raising a | |
897 | @code{SIGABRT} signal, and your program can include a handler to | |
898 | intercept this signal; see @ref{Signal Handling}. | |
899 | @end deftypefun | |
900 | ||
901 | @c Put in by rms. Don't remove. | |
902 | @cartouche | |
903 | @strong{Future Change Warning:} Proposed Federal censorship regulations | |
904 | may prohibit us from giving you information about the possibility of | |
905 | calling this function. We would be required to say that this is not an | |
906 | acceptable way of terminating a program. | |
907 | @end cartouche | |
908 | ||
909 | @node Termination Internals | |
910 | @subsection Termination Internals | |
911 | ||
912 | The @code{_exit} function is the primitive used for process termination | |
913 | by @code{exit}. It is declared in the header file @file{unistd.h}. | |
914 | @pindex unistd.h | |
915 | ||
916 | @comment unistd.h | |
917 | @comment POSIX.1 | |
918 | @deftypefun void _exit (int @var{status}) | |
919 | The @code{_exit} function is the primitive for causing a process to | |
920 | terminate with status @var{status}. Calling this function does not | |
921 | execute cleanup functions registered with @code{atexit} or | |
922 | @code{on_exit}. | |
923 | @end deftypefun | |
924 | ||
e518937a UD |
925 | @comment stdlib.h |
926 | @comment ISO | |
927 | @deftypefun void _Exit (int @var{status}) | |
928 | The @code{_Exit} function is the @w{ISO C} equivalent to @code{_exit}. | |
929 | The @w{ISO C} committee members were not sure whether the definitions of | |
930 | @code{_exit} and @code{_Exit} were compatible so they have not used the | |
931 | POSIX name. | |
932 | ||
ec751a23 | 933 | This function was introduced in @w{ISO C99} and is declared in |
e518937a UD |
934 | @file{stdlib.h}. |
935 | @end deftypefun | |
936 | ||
17c389fc UD |
937 | When a process terminates for any reason---either because the program |
938 | terminates, or as a result of a signal---the | |
28f540f4 RM |
939 | following things happen: |
940 | ||
941 | @itemize @bullet | |
942 | @item | |
943 | All open file descriptors in the process are closed. @xref{Low-Level I/O}. | |
944 | Note that streams are not flushed automatically when the process | |
8b7fb588 | 945 | terminates; see @ref{I/O on Streams}. |
28f540f4 RM |
946 | |
947 | @item | |
17c389fc UD |
948 | A process exit status is saved to be reported back to the parent process |
949 | via @code{wait} or @code{waitpid}; see @ref{Process Completion}. If the | |
950 | program exited, this status includes as its low-order 8 bits the program | |
951 | exit status. | |
952 | ||
28f540f4 RM |
953 | |
954 | @item | |
955 | Any child processes of the process being terminated are assigned a new | |
956 | parent process. (On most systems, including GNU, this is the @code{init} | |
957 | process, with process ID 1.) | |
958 | ||
959 | @item | |
960 | A @code{SIGCHLD} signal is sent to the parent process. | |
961 | ||
962 | @item | |
963 | If the process is a session leader that has a controlling terminal, then | |
964 | a @code{SIGHUP} signal is sent to each process in the foreground job, | |
965 | and the controlling terminal is disassociated from that session. | |
966 | @xref{Job Control}. | |
967 | ||
968 | @item | |
969 | If termination of a process causes a process group to become orphaned, | |
970 | and any member of that process group is stopped, then a @code{SIGHUP} | |
971 | signal and a @code{SIGCONT} signal are sent to each process in the | |
972 | group. @xref{Job Control}. | |
973 | @end itemize |