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1 | @node Program Basics, Processes, Signal Handling, Top | |
2 | @c %MENU% Writing the beginning and end of your program | |
3 | @chapter The Basic Program/System Interface | |
4 | ||
5 | @cindex process | |
6 | @cindex program | |
7 | @cindex address space | |
8 | @cindex thread of control | |
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 | |
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, see @ref{Executing a File}. | |
35 | ||
36 | @menu | |
37 | * Program Arguments:: Parsing your program's command-line arguments | |
38 | * Environment Variables:: Less direct parameters affecting your program | |
39 | * Auxiliary Vector:: Least direct parameters affecting your program | |
40 | * System Calls:: Requesting service from the system | |
41 | * Program Termination:: Telling the system you're done; return status | |
42 | @end menu | |
43 | ||
44 | @node Program Arguments, Environment Variables, , Program Basics | |
45 | @section Program Arguments | |
46 | @cindex program arguments | |
47 | @cindex command line arguments | |
48 | @cindex arguments, to program | |
49 | ||
50 | @cindex program startup | |
51 | @cindex startup of program | |
52 | @cindex invocation of program | |
53 | @cindex @code{main} function | |
54 | @findex main | |
55 | The system starts a C program by calling the function @code{main}. It | |
56 | is up to you to write a function named @code{main}---otherwise, you | |
57 | won't even be able to link your program without errors. | |
58 | ||
59 | In @w{ISO C} you can define @code{main} either to take no arguments, or to | |
60 | take two arguments that represent the command line arguments to the | |
61 | program, like this: | |
62 | ||
63 | @smallexample | |
64 | int main (int @var{argc}, char *@var{argv}[]) | |
65 | @end smallexample | |
66 | ||
67 | @cindex argc (program argument count) | |
68 | @cindex argv (program argument vector) | |
69 | The command line arguments are the whitespace-separated tokens given in | |
70 | the shell command used to invoke the program; thus, in @samp{cat foo | |
71 | bar}, the arguments are @samp{foo} and @samp{bar}. The only way a | |
72 | program can look at its command line arguments is via the arguments of | |
73 | @code{main}. If @code{main} doesn't take arguments, then you cannot get | |
74 | at the command line. | |
75 | ||
76 | The value of the @var{argc} argument is the number of command line | |
77 | arguments. The @var{argv} argument is a vector of C strings; its | |
78 | elements are the individual command line argument strings. The file | |
79 | name of the program being run is also included in the vector as the | |
80 | first element; the value of @var{argc} counts this element. A null | |
81 | pointer always follows the last element: @code{@var{argv}[@var{argc}]} | |
82 | is this null pointer. | |
83 | ||
84 | For the command @samp{cat foo bar}, @var{argc} is 3 and @var{argv} has | |
85 | three elements, @code{"cat"}, @code{"foo"} and @code{"bar"}. | |
86 | ||
87 | In Unix systems you can define @code{main} a third way, using three arguments: | |
88 | ||
89 | @smallexample | |
90 | int main (int @var{argc}, char *@var{argv}[], char *@var{envp}[]) | |
91 | @end smallexample | |
92 | ||
93 | The first two arguments are just the same. The third argument | |
94 | @var{envp} gives the program's environment; it is the same as the value | |
95 | of @code{environ}. @xref{Environment Variables}. POSIX.1 does not | |
96 | allow this three-argument form, so to be portable it is best to write | |
97 | @code{main} to take two arguments, and use the value of @code{environ}. | |
98 | ||
99 | @menu | |
100 | * Argument Syntax:: By convention, options start with a hyphen. | |
101 | * Parsing Program Arguments:: Ways to parse program options and arguments. | |
102 | @end menu | |
103 | ||
104 | @node Argument Syntax, Parsing Program Arguments, , Program Arguments | |
105 | @subsection Program Argument Syntax Conventions | |
106 | @cindex program argument syntax | |
107 | @cindex syntax, for program arguments | |
108 | @cindex command argument syntax | |
109 | ||
110 | POSIX recommends these conventions for command line arguments. | |
111 | @code{getopt} (@pxref{Getopt}) and @code{argp_parse} (@pxref{Argp}) make | |
112 | it easy to implement them. | |
113 | ||
114 | @itemize @bullet | |
115 | @item | |
116 | Arguments are options if they begin with a hyphen delimiter (@samp{-}). | |
117 | ||
118 | @item | |
119 | Multiple options may follow a hyphen delimiter in a single token if | |
120 | the options do not take arguments. Thus, @samp{-abc} is equivalent to | |
121 | @samp{-a -b -c}. | |
122 | ||
123 | @item | |
124 | Option names are single alphanumeric characters (as for @code{isalnum}; | |
125 | @pxref{Classification of Characters}). | |
126 | ||
127 | @item | |
128 | Certain options require an argument. For example, the @samp{-o} command | |
129 | of the @code{ld} command requires an argument---an output file name. | |
130 | ||
131 | @item | |
132 | An option and its argument may or may not appear as separate tokens. (In | |
133 | other words, the whitespace separating them is optional.) Thus, | |
134 | @w{@samp{-o foo}} and @samp{-ofoo} are equivalent. | |
135 | ||
136 | @item | |
137 | Options typically precede other non-option arguments. | |
138 | ||
139 | The implementations of @code{getopt} and @code{argp_parse} in @theglibc{} | |
140 | normally make it appear as if all the option arguments were | |
141 | specified before all the non-option arguments for the purposes of | |
142 | parsing, even if the user of your program intermixed option and | |
143 | non-option arguments. They do this by reordering the elements of the | |
144 | @var{argv} array. This behavior is nonstandard; if you want to suppress | |
145 | it, define the @code{_POSIX_OPTION_ORDER} environment variable. | |
146 | @xref{Standard Environment}. | |
147 | ||
148 | @item | |
149 | The argument @samp{--} terminates all options; any following arguments | |
150 | are treated as non-option arguments, even if they begin with a hyphen. | |
151 | ||
152 | @item | |
153 | A token consisting of a single hyphen character is interpreted as an | |
154 | ordinary non-option argument. By convention, it is used to specify | |
155 | input from or output to the standard input and output streams. | |
156 | ||
157 | @item | |
158 | Options may be supplied in any order, or appear multiple times. The | |
159 | interpretation is left up to the particular application program. | |
160 | @end itemize | |
161 | ||
162 | @cindex long-named options | |
163 | GNU adds @dfn{long options} to these conventions. Long options consist | |
164 | of @samp{--} followed by a name made of alphanumeric characters and | |
165 | dashes. Option names are typically one to three words long, with | |
166 | hyphens to separate words. Users can abbreviate the option names as | |
167 | long as the abbreviations are unique. | |
168 | ||
169 | To specify an argument for a long option, write | |
170 | @samp{--@var{name}=@var{value}}. This syntax enables a long option to | |
171 | accept an argument that is itself optional. | |
172 | ||
173 | Eventually, @gnusystems{} will provide completion for long option names | |
174 | in the shell. | |
175 | ||
176 | @node Parsing Program Arguments, , Argument Syntax, Program Arguments | |
177 | @subsection Parsing Program Arguments | |
178 | ||
179 | @cindex program arguments, parsing | |
180 | @cindex command arguments, parsing | |
181 | @cindex parsing program arguments | |
182 | If the syntax for the command line arguments to your program is simple | |
183 | enough, you can simply pick the arguments off from @var{argv} by hand. | |
184 | But unless your program takes a fixed number of arguments, or all of the | |
185 | arguments are interpreted in the same way (as file names, for example), | |
186 | you are usually better off using @code{getopt} (@pxref{Getopt}) or | |
187 | @code{argp_parse} (@pxref{Argp}) to do the parsing. | |
188 | ||
189 | @code{getopt} is more standard (the short-option only version of it is a | |
190 | part of the POSIX standard), but using @code{argp_parse} is often | |
191 | easier, both for very simple and very complex option structures, because | |
192 | it does more of the dirty work for you. | |
193 | ||
194 | @menu | |
195 | * Getopt:: Parsing program options using @code{getopt}. | |
196 | * Argp:: Parsing program options using @code{argp_parse}. | |
197 | * Suboptions:: Some programs need more detailed options. | |
198 | * Suboptions Example:: This shows how it could be done for @code{mount}. | |
199 | @end menu | |
200 | ||
201 | @c Getopt and argp start at the @section level so that there's | |
202 | @c enough room for their internal hierarchy (mostly a problem with | |
203 | @c argp). -Miles | |
204 | ||
205 | @include getopt.texi | |
206 | @include argp.texi | |
207 | ||
208 | @node Suboptions, Suboptions Example, Argp, Parsing Program Arguments | |
209 | @c This is a @section so that it's at the same level as getopt and argp | |
210 | @subsubsection Parsing of Suboptions | |
211 | ||
212 | Having a single level of options is sometimes not enough. There might | |
213 | be too many options which have to be available or a set of options is | |
214 | closely related. | |
215 | ||
216 | For this case some programs use suboptions. One of the most prominent | |
217 | programs is certainly @code{mount}(8). The @code{-o} option take one | |
218 | argument which itself is a comma separated list of options. To ease the | |
219 | programming of code like this the function @code{getsubopt} is | |
220 | available. | |
221 | ||
222 | @comment stdlib.h | |
223 | @deftypefun int getsubopt (char **@var{optionp}, char *const *@var{tokens}, char **@var{valuep}) | |
224 | @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} | |
225 | @c getsubopt ok | |
226 | @c strchrnul dup ok | |
227 | @c memchr dup ok | |
228 | @c strncmp dup ok | |
229 | ||
230 | The @var{optionp} parameter must be a pointer to a variable containing | |
231 | the address of the string to process. When the function returns the | |
232 | reference is updated to point to the next suboption or to the | |
233 | terminating @samp{\0} character if there is no more suboption available. | |
234 | ||
235 | The @var{tokens} parameter references an array of strings containing the | |
236 | known suboptions. All strings must be @samp{\0} terminated and to mark | |
237 | the end a null pointer must be stored. When @code{getsubopt} finds a | |
238 | possible legal suboption it compares it with all strings available in | |
239 | the @var{tokens} array and returns the index in the string as the | |
240 | indicator. | |
241 | ||
242 | In case the suboption has an associated value introduced by a @samp{=} | |
243 | character, a pointer to the value is returned in @var{valuep}. The | |
244 | string is @samp{\0} terminated. If no argument is available | |
245 | @var{valuep} is set to the null pointer. By doing this the caller can | |
246 | check whether a necessary value is given or whether no unexpected value | |
247 | is present. | |
248 | ||
249 | In case the next suboption in the string is not mentioned in the | |
250 | @var{tokens} array the starting address of the suboption including a | |
251 | possible value is returned in @var{valuep} and the return value of the | |
252 | function is @samp{-1}. | |
253 | @end deftypefun | |
254 | ||
255 | @node Suboptions Example, , Suboptions, Parsing Program Arguments | |
256 | @subsection Parsing of Suboptions Example | |
257 | ||
258 | The code which might appear in the @code{mount}(8) program is a perfect | |
259 | example of the use of @code{getsubopt}: | |
260 | ||
261 | @smallexample | |
262 | @include subopt.c.texi | |
263 | @end smallexample | |
264 | ||
265 | ||
266 | @node Environment Variables, Auxiliary Vector, Program Arguments, Program Basics | |
267 | @section Environment Variables | |
268 | ||
269 | @cindex environment variable | |
270 | When a program is executed, it receives information about the context in | |
271 | which it was invoked in two ways. The first mechanism uses the | |
272 | @var{argv} and @var{argc} arguments to its @code{main} function, and is | |
273 | discussed in @ref{Program Arguments}. The second mechanism uses | |
274 | @dfn{environment variables} and is discussed in this section. | |
275 | ||
276 | The @var{argv} mechanism is typically used to pass command-line | |
277 | arguments specific to the particular program being invoked. The | |
278 | environment, on the other hand, keeps track of information that is | |
279 | shared by many programs, changes infrequently, and that is less | |
280 | frequently used. | |
281 | ||
282 | The environment variables discussed in this section are the same | |
283 | environment variables that you set using assignments and the | |
284 | @code{export} command in the shell. Programs executed from the shell | |
285 | inherit all of the environment variables from the shell. | |
286 | @c !!! xref to right part of bash manual when it exists | |
287 | ||
288 | @cindex environment | |
289 | Standard environment variables are used for information about the user's | |
290 | home directory, terminal type, current locale, and so on; you can define | |
291 | additional variables for other purposes. The set of all environment | |
292 | variables that have values is collectively known as the | |
293 | @dfn{environment}. | |
294 | ||
295 | Names of environment variables are case-sensitive and must not contain | |
296 | the character @samp{=}. System-defined environment variables are | |
297 | invariably uppercase. | |
298 | ||
299 | The values of environment variables can be anything that can be | |
300 | represented as a string. A value must not contain an embedded null | |
301 | character, since this is assumed to terminate the string. | |
302 | ||
303 | ||
304 | @menu | |
305 | * Environment Access:: How to get and set the values of | |
306 | environment variables. | |
307 | * Standard Environment:: These environment variables have | |
308 | standard interpretations. | |
309 | @end menu | |
310 | ||
311 | @node Environment Access | |
312 | @subsection Environment Access | |
313 | @cindex environment access | |
314 | @cindex environment representation | |
315 | ||
316 | The value of an environment variable can be accessed with the | |
317 | @code{getenv} function. This is declared in the header file | |
318 | @file{stdlib.h}. | |
319 | @pindex stdlib.h | |
320 | ||
321 | Libraries should use @code{secure_getenv} instead of @code{getenv}, so | |
322 | that they do not accidentally use untrusted environment variables. | |
323 | Modifications of environment variables are not allowed in | |
324 | multi-threaded programs. The @code{getenv} and @code{secure_getenv} | |
325 | functions can be safely used in multi-threaded programs. | |
326 | ||
327 | @comment stdlib.h | |
328 | @comment ISO | |
329 | @deftypefun {char *} getenv (const char *@var{name}) | |
330 | @safety{@prelim{}@mtsafe{@mtsenv{}}@assafe{}@acsafe{}} | |
331 | @c Unguarded access to __environ. | |
332 | This function returns a string that is the value of the environment | |
333 | variable @var{name}. You must not modify this string. In some non-Unix | |
334 | systems not using @theglibc{}, it might be overwritten by subsequent | |
335 | calls to @code{getenv} (but not by any other library function). If the | |
336 | environment variable @var{name} is not defined, the value is a null | |
337 | pointer. | |
338 | @end deftypefun | |
339 | ||
340 | @comment stdlib.h | |
341 | @comment GNU | |
342 | @deftypefun {char *} secure_getenv (const char *@var{name}) | |
343 | @safety{@prelim{}@mtsafe{@mtsenv{}}@assafe{}@acsafe{}} | |
344 | @c Calls getenv unless secure mode is enabled. | |
345 | This function is similar to @code{getenv}, but it returns a null | |
346 | pointer if the environment is untrusted. This happens when the | |
347 | program file has SUID or SGID bits set. General-purpose libraries | |
348 | should always prefer this function over @code{getenv} to avoid | |
349 | vulnerabilities if the library is referenced from a SUID/SGID program. | |
350 | ||
351 | This function is a GNU extension. | |
352 | @end deftypefun | |
353 | ||
354 | ||
355 | @comment stdlib.h | |
356 | @comment SVID | |
357 | @deftypefun int putenv (char *@var{string}) | |
358 | @safety{@prelim{}@mtunsafe{@mtasuconst{:@mtsenv{}}}@asunsafe{@ascuheap{} @asulock{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{}}} | |
359 | @c putenv @mtasuconst:@mtsenv @ascuheap @asulock @acucorrupt @aculock @acsmem | |
360 | @c strchr dup ok | |
361 | @c strndup dup @ascuheap @acsmem | |
362 | @c add_to_environ dup @mtasuconst:@mtsenv @ascuheap @asulock @acucorrupt @aculock @acsmem | |
363 | @c free dup @ascuheap @acsmem | |
364 | @c unsetenv dup @mtasuconst:@mtsenv @asulock @aculock | |
365 | The @code{putenv} function adds or removes definitions from the environment. | |
366 | If the @var{string} is of the form @samp{@var{name}=@var{value}}, the | |
367 | definition is added to the environment. Otherwise, the @var{string} is | |
368 | interpreted as the name of an environment variable, and any definition | |
369 | for this variable in the environment is removed. | |
370 | ||
371 | If the function is successful it returns @code{0}. Otherwise the return | |
372 | value is nonzero and @code{errno} is set to indicate the error. | |
373 | ||
374 | The difference to the @code{setenv} function is that the exact string | |
375 | given as the parameter @var{string} is put into the environment. If the | |
376 | user should change the string after the @code{putenv} call this will | |
377 | reflect automatically in the environment. This also requires that | |
378 | @var{string} not be an automatic variable whose scope is left before the | |
379 | variable is removed from the environment. The same applies of course to | |
380 | dynamically allocated variables which are freed later. | |
381 | ||
382 | This function is part of the extended Unix interface. You should define | |
383 | @var{_XOPEN_SOURCE} before including any header. | |
384 | @end deftypefun | |
385 | ||
386 | ||
387 | @comment stdlib.h | |
388 | @comment BSD | |
389 | @deftypefun int setenv (const char *@var{name}, const char *@var{value}, int @var{replace}) | |
390 | @safety{@prelim{}@mtunsafe{@mtasuconst{:@mtsenv{}}}@asunsafe{@ascuheap{} @asulock{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{}}} | |
391 | @c setenv @mtasuconst:@mtsenv @ascuheap @asulock @acucorrupt @aculock @acsmem | |
392 | @c add_to_environ @mtasuconst:@mtsenv @ascuheap @asulock @acucorrupt @aculock @acsmem | |
393 | @c strlen dup ok | |
394 | @c libc_lock_lock @asulock @aculock | |
395 | @c strncmp dup ok | |
396 | @c realloc dup @ascuheap @acsmem | |
397 | @c libc_lock_unlock @aculock | |
398 | @c malloc dup @ascuheap @acsmem | |
399 | @c free dup @ascuheap @acsmem | |
400 | @c mempcpy dup ok | |
401 | @c memcpy dup ok | |
402 | @c KNOWN_VALUE ok | |
403 | @c tfind(strcmp) [no @mtsrace guarded access] | |
404 | @c strcmp dup ok | |
405 | @c STORE_VALUE @ascuheap @acucorrupt @acsmem | |
406 | @c tsearch(strcmp) @ascuheap @acucorrupt @acsmem [no @mtsrace or @asucorrupt guarded access makes for mtsafe and @asulock] | |
407 | @c strcmp dup ok | |
408 | The @code{setenv} function can be used to add a new definition to the | |
409 | environment. The entry with the name @var{name} is replaced by the | |
410 | value @samp{@var{name}=@var{value}}. Please note that this is also true | |
411 | if @var{value} is the empty string. To do this a new string is created | |
412 | and the strings @var{name} and @var{value} are copied. A null pointer | |
413 | for the @var{value} parameter is illegal. If the environment already | |
414 | contains an entry with key @var{name} the @var{replace} parameter | |
415 | controls the action. If replace is zero, nothing happens. Otherwise | |
416 | the old entry is replaced by the new one. | |
417 | ||
418 | Please note that you cannot remove an entry completely using this function. | |
419 | ||
420 | If the function is successful it returns @code{0}. Otherwise the | |
421 | environment is unchanged and the return value is @code{-1} and | |
422 | @code{errno} is set. | |
423 | ||
424 | This function was originally part of the BSD library but is now part of | |
425 | the Unix standard. | |
426 | @end deftypefun | |
427 | ||
428 | @comment stdlib.h | |
429 | @comment BSD | |
430 | @deftypefun int unsetenv (const char *@var{name}) | |
431 | @safety{@prelim{}@mtunsafe{@mtasuconst{:@mtsenv{}}}@asunsafe{@asulock{}}@acunsafe{@aculock{}}} | |
432 | @c unsetenv @mtasuconst:@mtsenv @asulock @aculock | |
433 | @c strchr dup ok | |
434 | @c strlen dup ok | |
435 | @c libc_lock_lock @asulock @aculock | |
436 | @c strncmp dup ok | |
437 | @c libc_lock_unlock @aculock | |
438 | Using this function one can remove an entry completely from the | |
439 | environment. If the environment contains an entry with the key | |
440 | @var{name} this whole entry is removed. A call to this function is | |
441 | equivalent to a call to @code{putenv} when the @var{value} part of the | |
442 | string is empty. | |
443 | ||
444 | The function return @code{-1} if @var{name} is a null pointer, points to | |
445 | an empty string, or points to a string containing a @code{=} character. | |
446 | It returns @code{0} if the call succeeded. | |
447 | ||
448 | This function was originally part of the BSD library but is now part of | |
449 | the Unix standard. The BSD version had no return value, though. | |
450 | @end deftypefun | |
451 | ||
452 | There is one more function to modify the whole environment. This | |
453 | function is said to be used in the POSIX.9 (POSIX bindings for Fortran | |
454 | 77) and so one should expect it did made it into POSIX.1. But this | |
455 | never happened. But we still provide this function as a GNU extension | |
456 | to enable writing standard compliant Fortran environments. | |
457 | ||
458 | @comment stdlib.h | |
459 | @comment GNU | |
460 | @deftypefun int clearenv (void) | |
461 | @safety{@prelim{}@mtunsafe{@mtasuconst{:@mtsenv{}}}@asunsafe{@ascuheap{} @asulock{}}@acunsafe{@aculock{} @acsmem{}}} | |
462 | @c clearenv @mtasuconst:@mtsenv @ascuheap @asulock @aculock @acsmem | |
463 | @c libc_lock_lock @asulock @aculock | |
464 | @c free dup @ascuheap @acsmem | |
465 | @c libc_lock_unlock @aculock | |
466 | The @code{clearenv} function removes all entries from the environment. | |
467 | Using @code{putenv} and @code{setenv} new entries can be added again | |
468 | later. | |
469 | ||
470 | If the function is successful it returns @code{0}. Otherwise the return | |
471 | value is nonzero. | |
472 | @end deftypefun | |
473 | ||
474 | ||
475 | You can deal directly with the underlying representation of environment | |
476 | objects to add more variables to the environment (for example, to | |
477 | communicate with another program you are about to execute; | |
478 | @pxref{Executing a File}). | |
479 | ||
480 | @comment unistd.h | |
481 | @comment POSIX.1 | |
482 | @deftypevar {char **} environ | |
483 | The environment is represented as an array of strings. Each string is | |
484 | of the format @samp{@var{name}=@var{value}}. The order in which | |
485 | strings appear in the environment is not significant, but the same | |
486 | @var{name} must not appear more than once. The last element of the | |
487 | array is a null pointer. | |
488 | ||
489 | This variable is declared in the header file @file{unistd.h}. | |
490 | ||
491 | If you just want to get the value of an environment variable, use | |
492 | @code{getenv}. | |
493 | @end deftypevar | |
494 | ||
495 | Unix systems, and @gnusystems{}, pass the initial value of | |
496 | @code{environ} as the third argument to @code{main}. | |
497 | @xref{Program Arguments}. | |
498 | ||
499 | @node Standard Environment | |
500 | @subsection Standard Environment Variables | |
501 | @cindex standard environment variables | |
502 | ||
503 | These environment variables have standard meanings. This doesn't mean | |
504 | that they are always present in the environment; but if these variables | |
505 | @emph{are} present, they have these meanings. You shouldn't try to use | |
506 | these environment variable names for some other purpose. | |
507 | ||
508 | @comment Extra blank lines make it look better. | |
509 | @table @code | |
510 | @item HOME | |
511 | @cindex @code{HOME} environment variable | |
512 | @cindex home directory | |
513 | ||
514 | This is a string representing the user's @dfn{home directory}, or | |
515 | initial default working directory. | |
516 | ||
517 | The user can set @code{HOME} to any value. | |
518 | If you need to make sure to obtain the proper home directory | |
519 | for a particular user, you should not use @code{HOME}; instead, | |
520 | look up the user's name in the user database (@pxref{User Database}). | |
521 | ||
522 | For most purposes, it is better to use @code{HOME}, precisely because | |
523 | this lets the user specify the value. | |
524 | ||
525 | @c !!! also USER | |
526 | @item LOGNAME | |
527 | @cindex @code{LOGNAME} environment variable | |
528 | ||
529 | This is the name that the user used to log in. Since the value in the | |
530 | environment can be tweaked arbitrarily, this is not a reliable way to | |
531 | identify the user who is running a program; a function like | |
532 | @code{getlogin} (@pxref{Who Logged In}) is better for that purpose. | |
533 | ||
534 | For most purposes, it is better to use @code{LOGNAME}, precisely because | |
535 | this lets the user specify the value. | |
536 | ||
537 | @item PATH | |
538 | @cindex @code{PATH} environment variable | |
539 | ||
540 | A @dfn{path} is a sequence of directory names which is used for | |
541 | searching for a file. The variable @code{PATH} holds a path used | |
542 | for searching for programs to be run. | |
543 | ||
544 | The @code{execlp} and @code{execvp} functions (@pxref{Executing a File}) | |
545 | use this environment variable, as do many shells and other utilities | |
546 | which are implemented in terms of those functions. | |
547 | ||
548 | The syntax of a path is a sequence of directory names separated by | |
549 | colons. An empty string instead of a directory name stands for the | |
550 | current directory (@pxref{Working Directory}). | |
551 | ||
552 | A typical value for this environment variable might be a string like: | |
553 | ||
554 | @smallexample | |
555 | :/bin:/etc:/usr/bin:/usr/new/X11:/usr/new:/usr/local/bin | |
556 | @end smallexample | |
557 | ||
558 | This means that if the user tries to execute a program named @code{foo}, | |
559 | the system will look for files named @file{foo}, @file{/bin/foo}, | |
560 | @file{/etc/foo}, and so on. The first of these files that exists is | |
561 | the one that is executed. | |
562 | ||
563 | @c !!! also TERMCAP | |
564 | @item TERM | |
565 | @cindex @code{TERM} environment variable | |
566 | ||
567 | This specifies the kind of terminal that is receiving program output. | |
568 | Some programs can make use of this information to take advantage of | |
569 | special escape sequences or terminal modes supported by particular kinds | |
570 | of terminals. Many programs which use the termcap library | |
571 | (@pxref{Finding a Terminal Description,Find,,termcap,The Termcap Library | |
572 | Manual}) use the @code{TERM} environment variable, for example. | |
573 | ||
574 | @item TZ | |
575 | @cindex @code{TZ} environment variable | |
576 | ||
577 | This specifies the time zone. @xref{TZ Variable}, for information about | |
578 | the format of this string and how it is used. | |
579 | ||
580 | @item LANG | |
581 | @cindex @code{LANG} environment variable | |
582 | ||
583 | This specifies the default locale to use for attribute categories where | |
584 | neither @code{LC_ALL} nor the specific environment variable for that | |
585 | category is set. @xref{Locales}, for more information about | |
586 | locales. | |
587 | ||
588 | @ignore | |
589 | @c I doubt this really exists | |
590 | @item LC_ALL | |
591 | @cindex @code{LC_ALL} environment variable | |
592 | ||
593 | This is similar to the @code{LANG} environment variable. However, its | |
594 | value takes precedence over any values provided for the individual | |
595 | attribute category environment variables, or for the @code{LANG} | |
596 | environment variable. | |
597 | @end ignore | |
598 | ||
599 | @item LC_ALL | |
600 | @cindex @code{LC_ALL} environment variable | |
601 | ||
602 | If this environment variable is set it overrides the selection for all | |
603 | the locales done using the other @code{LC_*} environment variables. The | |
604 | value of the other @code{LC_*} environment variables is simply ignored | |
605 | in this case. | |
606 | ||
607 | @item LC_COLLATE | |
608 | @cindex @code{LC_COLLATE} environment variable | |
609 | ||
610 | This specifies what locale to use for string sorting. | |
611 | ||
612 | @item LC_CTYPE | |
613 | @cindex @code{LC_CTYPE} environment variable | |
614 | ||
615 | This specifies what locale to use for character sets and character | |
616 | classification. | |
617 | ||
618 | @item LC_MESSAGES | |
619 | @cindex @code{LC_MESSAGES} environment variable | |
620 | ||
621 | This specifies what locale to use for printing messages and to parse | |
622 | responses. | |
623 | ||
624 | @item LC_MONETARY | |
625 | @cindex @code{LC_MONETARY} environment variable | |
626 | ||
627 | This specifies what locale to use for formatting monetary values. | |
628 | ||
629 | @item LC_NUMERIC | |
630 | @cindex @code{LC_NUMERIC} environment variable | |
631 | ||
632 | This specifies what locale to use for formatting numbers. | |
633 | ||
634 | @item LC_TIME | |
635 | @cindex @code{LC_TIME} environment variable | |
636 | ||
637 | This specifies what locale to use for formatting date/time values. | |
638 | ||
639 | @item NLSPATH | |
640 | @cindex @code{NLSPATH} environment variable | |
641 | ||
642 | This specifies the directories in which the @code{catopen} function | |
643 | looks for message translation catalogs. | |
644 | ||
645 | @item _POSIX_OPTION_ORDER | |
646 | @cindex @code{_POSIX_OPTION_ORDER} environment variable. | |
647 | ||
648 | If this environment variable is defined, it suppresses the usual | |
649 | reordering of command line arguments by @code{getopt} and | |
650 | @code{argp_parse}. @xref{Argument Syntax}. | |
651 | ||
652 | @c !!! GNU also has COREFILE, CORESERVER, EXECSERVERS | |
653 | @end table | |
654 | ||
655 | @node Auxiliary Vector | |
656 | @section Auxiliary Vector | |
657 | @cindex auxiliary vector | |
658 | ||
659 | When a program is executed, it receives information from the operating | |
660 | system about the environment in which it is operating. The form of this | |
661 | information is a table of key-value pairs, where the keys are from the | |
662 | set of @samp{AT_} values in @file{elf.h}. Some of the data is provided | |
663 | by the kernel for libc consumption, and may be obtained by ordinary | |
664 | interfaces, such as @code{sysconf}. However, on a platform-by-platform | |
665 | basis there may be information that is not available any other way. | |
666 | ||
667 | @subsection Definition of @code{getauxval} | |
668 | @comment sys/auxv.h | |
669 | @deftypefun {unsigned long int} getauxval (unsigned long int @var{type}) | |
670 | @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} | |
671 | @c Reads from hwcap or iterates over constant auxv. | |
672 | This function is used to inquire about the entries in the auxiliary | |
673 | vector. The @var{type} argument should be one of the @samp{AT_} symbols | |
674 | defined in @file{elf.h}. If a matching entry is found, the value is | |
675 | returned; if the entry is not found, zero is returned and @code{errno} is | |
676 | set to @code{ENOENT}. | |
677 | @end deftypefun | |
678 | ||
679 | For some platforms, the key @code{AT_HWCAP} is the easiest way to inquire | |
680 | about any instruction set extensions available at runtime. In this case, | |
681 | there will (of necessity) be a platform-specific set of @samp{HWCAP_} | |
682 | values masked together that describe the capabilities of the cpu on which | |
683 | the program is being executed. | |
684 | ||
685 | @node System Calls | |
686 | @section System Calls | |
687 | ||
688 | @cindex system call | |
689 | A system call is a request for service that a program makes of the | |
690 | kernel. The service is generally something that only the kernel has | |
691 | the privilege to do, such as doing I/O. Programmers don't normally | |
692 | need to be concerned with system calls because there are functions in | |
693 | @theglibc{} to do virtually everything that system calls do. | |
694 | These functions work by making system calls themselves. For example, | |
695 | there is a system call that changes the permissions of a file, but | |
696 | you don't need to know about it because you can just use @theglibc{}'s | |
697 | @code{chmod} function. | |
698 | ||
699 | @cindex kernel call | |
700 | System calls are sometimes called kernel calls. | |
701 | ||
702 | However, there are times when you want to make a system call explicitly, | |
703 | and for that, @theglibc{} provides the @code{syscall} function. | |
704 | @code{syscall} is harder to use and less portable than functions like | |
705 | @code{chmod}, but easier and more portable than coding the system call | |
706 | in assembler instructions. | |
707 | ||
708 | @code{syscall} is most useful when you are working with a system call | |
709 | which is special to your system or is newer than @theglibc{} you | |
710 | are using. @code{syscall} is implemented in an entirely generic way; | |
711 | the function does not know anything about what a particular system | |
712 | call does or even if it is valid. | |
713 | ||
714 | The description of @code{syscall} in this section assumes a certain | |
715 | protocol for system calls on the various platforms on which @theglibc{} | |
716 | runs. That protocol is not defined by any strong authority, but | |
717 | we won't describe it here either because anyone who is coding | |
718 | @code{syscall} probably won't accept anything less than kernel and C | |
719 | library source code as a specification of the interface between them | |
720 | anyway. | |
721 | ||
722 | ||
723 | @code{syscall} is declared in @file{unistd.h}. | |
724 | ||
725 | @comment unistd.h | |
726 | @comment ??? | |
727 | @deftypefun {long int} syscall (long int @var{sysno}, @dots{}) | |
728 | @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} | |
729 | ||
730 | @code{syscall} performs a generic system call. | |
731 | ||
732 | @cindex system call number | |
733 | @var{sysno} is the system call number. Each kind of system call is | |
734 | identified by a number. Macros for all the possible system call numbers | |
735 | are defined in @file{sys/syscall.h} | |
736 | ||
737 | The remaining arguments are the arguments for the system call, in | |
738 | order, and their meanings depend on the kind of system call. Each kind | |
739 | of system call has a definite number of arguments, from zero to five. | |
740 | If you code more arguments than the system call takes, the extra ones to | |
741 | the right are ignored. | |
742 | ||
743 | The return value is the return value from the system call, unless the | |
744 | system call failed. In that case, @code{syscall} returns @code{-1} and | |
745 | sets @code{errno} to an error code that the system call returned. Note | |
746 | that system calls do not return @code{-1} when they succeed. | |
747 | @cindex errno | |
748 | ||
749 | If you specify an invalid @var{sysno}, @code{syscall} returns @code{-1} | |
750 | with @code{errno} = @code{ENOSYS}. | |
751 | ||
752 | Example: | |
753 | ||
754 | @smallexample | |
755 | ||
756 | #include <unistd.h> | |
757 | #include <sys/syscall.h> | |
758 | #include <errno.h> | |
759 | ||
760 | @dots{} | |
761 | ||
762 | int rc; | |
763 | ||
764 | rc = syscall(SYS_chmod, "/etc/passwd", 0444); | |
765 | ||
766 | if (rc == -1) | |
767 | fprintf(stderr, "chmod failed, errno = %d\n", errno); | |
768 | ||
769 | @end smallexample | |
770 | ||
771 | This, if all the compatibility stars are aligned, is equivalent to the | |
772 | following preferable code: | |
773 | ||
774 | @smallexample | |
775 | ||
776 | #include <sys/types.h> | |
777 | #include <sys/stat.h> | |
778 | #include <errno.h> | |
779 | ||
780 | @dots{} | |
781 | ||
782 | int rc; | |
783 | ||
784 | rc = chmod("/etc/passwd", 0444); | |
785 | if (rc == -1) | |
786 | fprintf(stderr, "chmod failed, errno = %d\n", errno); | |
787 | ||
788 | @end smallexample | |
789 | ||
790 | @end deftypefun | |
791 | ||
792 | ||
793 | @node Program Termination | |
794 | @section Program Termination | |
795 | @cindex program termination | |
796 | @cindex process termination | |
797 | ||
798 | @cindex exit status value | |
799 | The usual way for a program to terminate is simply for its @code{main} | |
800 | function to return. The @dfn{exit status value} returned from the | |
801 | @code{main} function is used to report information back to the process's | |
802 | parent process or shell. | |
803 | ||
804 | A program can also terminate normally by calling the @code{exit} | |
805 | function. | |
806 | ||
807 | In addition, programs can be terminated by signals; this is discussed in | |
808 | more detail in @ref{Signal Handling}. The @code{abort} function causes | |
809 | a signal that kills the program. | |
810 | ||
811 | @menu | |
812 | * Normal Termination:: If a program calls @code{exit}, a | |
813 | process terminates normally. | |
814 | * Exit Status:: The @code{exit status} provides information | |
815 | about why the process terminated. | |
816 | * Cleanups on Exit:: A process can run its own cleanup | |
817 | functions upon normal termination. | |
818 | * Aborting a Program:: The @code{abort} function causes | |
819 | abnormal program termination. | |
820 | * Termination Internals:: What happens when a process terminates. | |
821 | @end menu | |
822 | ||
823 | @node Normal Termination | |
824 | @subsection Normal Termination | |
825 | ||
826 | A process terminates normally when its program signals it is done by | |
827 | calling @code{exit}. Returning from @code{main} is equivalent to | |
828 | calling @code{exit}, and the value that @code{main} returns is used as | |
829 | the argument to @code{exit}. | |
830 | ||
831 | @comment stdlib.h | |
832 | @comment ISO | |
833 | @deftypefun void exit (int @var{status}) | |
834 | @safety{@prelim{}@mtunsafe{@mtasurace{:exit}}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{} @aculock{}}} | |
835 | @c Access to the atexit/on_exit list, the libc_atexit hook and tls dtors | |
836 | @c is not guarded. Streams must be flushed, and that triggers the usual | |
837 | @c AS and AC issues with streams. | |
838 | The @code{exit} function tells the system that the program is done, which | |
839 | causes it to terminate the process. | |
840 | ||
841 | @var{status} is the program's exit status, which becomes part of the | |
842 | process' termination status. This function does not return. | |
843 | @end deftypefun | |
844 | ||
845 | Normal termination causes the following actions: | |
846 | ||
847 | @enumerate | |
848 | @item | |
849 | Functions that were registered with the @code{atexit} or @code{on_exit} | |
850 | functions are called in the reverse order of their registration. This | |
851 | mechanism allows your application to specify its own ``cleanup'' actions | |
852 | to be performed at program termination. Typically, this is used to do | |
853 | things like saving program state information in a file, or unlocking | |
854 | locks in shared data bases. | |
855 | ||
856 | @item | |
857 | All open streams are closed, writing out any buffered output data. See | |
858 | @ref{Closing Streams}. In addition, temporary files opened | |
859 | with the @code{tmpfile} function are removed; see @ref{Temporary Files}. | |
860 | ||
861 | @item | |
862 | @code{_exit} is called, terminating the program. @xref{Termination Internals}. | |
863 | @end enumerate | |
864 | ||
865 | @node Exit Status | |
866 | @subsection Exit Status | |
867 | @cindex exit status | |
868 | ||
869 | When a program exits, it can return to the parent process a small | |
870 | amount of information about the cause of termination, using the | |
871 | @dfn{exit status}. This is a value between 0 and 255 that the exiting | |
872 | process passes as an argument to @code{exit}. | |
873 | ||
874 | Normally you should use the exit status to report very broad information | |
875 | about success or failure. You can't provide a lot of detail about the | |
876 | reasons for the failure, and most parent processes would not want much | |
877 | detail anyway. | |
878 | ||
879 | There are conventions for what sorts of status values certain programs | |
880 | should return. The most common convention is simply 0 for success and 1 | |
881 | for failure. Programs that perform comparison use a different | |
882 | convention: they use status 1 to indicate a mismatch, and status 2 to | |
883 | indicate an inability to compare. Your program should follow an | |
884 | existing convention if an existing convention makes sense for it. | |
885 | ||
886 | A general convention reserves status values 128 and up for special | |
887 | purposes. In particular, the value 128 is used to indicate failure to | |
888 | execute another program in a subprocess. This convention is not | |
889 | universally obeyed, but it is a good idea to follow it in your programs. | |
890 | ||
891 | @strong{Warning:} Don't try to use the number of errors as the exit | |
892 | status. This is actually not very useful; a parent process would | |
893 | generally not care how many errors occurred. Worse than that, it does | |
894 | not work, because the status value is truncated to eight bits. | |
895 | Thus, if the program tried to report 256 errors, the parent would | |
896 | receive a report of 0 errors---that is, success. | |
897 | ||
898 | For the same reason, it does not work to use the value of @code{errno} | |
899 | as the exit status---these can exceed 255. | |
900 | ||
901 | @strong{Portability note:} Some non-POSIX systems use different | |
902 | conventions for exit status values. For greater portability, you can | |
903 | use the macros @code{EXIT_SUCCESS} and @code{EXIT_FAILURE} for the | |
904 | conventional status value for success and failure, respectively. They | |
905 | are declared in the file @file{stdlib.h}. | |
906 | @pindex stdlib.h | |
907 | ||
908 | @comment stdlib.h | |
909 | @comment ISO | |
910 | @deftypevr Macro int EXIT_SUCCESS | |
911 | This macro can be used with the @code{exit} function to indicate | |
912 | successful program completion. | |
913 | ||
914 | On POSIX systems, the value of this macro is @code{0}. On other | |
915 | systems, the value might be some other (possibly non-constant) integer | |
916 | expression. | |
917 | @end deftypevr | |
918 | ||
919 | @comment stdlib.h | |
920 | @comment ISO | |
921 | @deftypevr Macro int EXIT_FAILURE | |
922 | This macro can be used with the @code{exit} function to indicate | |
923 | unsuccessful program completion in a general sense. | |
924 | ||
925 | On POSIX systems, the value of this macro is @code{1}. On other | |
926 | systems, the value might be some other (possibly non-constant) integer | |
927 | expression. Other nonzero status values also indicate failures. Certain | |
928 | programs use different nonzero status values to indicate particular | |
929 | kinds of "non-success". For example, @code{diff} uses status value | |
930 | @code{1} to mean that the files are different, and @code{2} or more to | |
931 | mean that there was difficulty in opening the files. | |
932 | @end deftypevr | |
933 | ||
934 | Don't confuse a program's exit status with a process' termination status. | |
935 | There are lots of ways a process can terminate besides having its program | |
936 | finish. In the event that the process termination @emph{is} caused by program | |
937 | termination (i.e., @code{exit}), though, the program's exit status becomes | |
938 | part of the process' termination status. | |
939 | ||
940 | @node Cleanups on Exit | |
941 | @subsection Cleanups on Exit | |
942 | ||
943 | Your program can arrange to run its own cleanup functions if normal | |
944 | termination happens. If you are writing a library for use in various | |
945 | application programs, then it is unreliable to insist that all | |
946 | applications call the library's cleanup functions explicitly before | |
947 | exiting. It is much more robust to make the cleanup invisible to the | |
948 | application, by setting up a cleanup function in the library itself | |
949 | using @code{atexit} or @code{on_exit}. | |
950 | ||
951 | @comment stdlib.h | |
952 | @comment ISO | |
953 | @deftypefun int atexit (void (*@var{function}) (void)) | |
954 | @safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{} @asulock{}}@acunsafe{@aculock{} @acsmem{}}} | |
955 | @c atexit @ascuheap @asulock @aculock @acsmem | |
956 | @c cxa_atexit @ascuheap @asulock @aculock @acsmem | |
957 | @c __internal_atexit @ascuheap @asulock @aculock @acsmem | |
958 | @c __new_exitfn @ascuheap @asulock @aculock @acsmem | |
959 | @c __libc_lock_lock @asulock @aculock | |
960 | @c calloc dup @ascuheap @acsmem | |
961 | @c __libc_lock_unlock @aculock | |
962 | @c atomic_write_barrier dup ok | |
963 | The @code{atexit} function registers the function @var{function} to be | |
964 | called at normal program termination. The @var{function} is called with | |
965 | no arguments. | |
966 | ||
967 | The return value from @code{atexit} is zero on success and nonzero if | |
968 | the function cannot be registered. | |
969 | @end deftypefun | |
970 | ||
971 | @comment stdlib.h | |
972 | @comment SunOS | |
973 | @deftypefun int on_exit (void (*@var{function})(int @var{status}, void *@var{arg}), void *@var{arg}) | |
974 | @safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{} @asulock{}}@acunsafe{@aculock{} @acsmem{}}} | |
975 | @c on_exit @ascuheap @asulock @aculock @acsmem | |
976 | @c new_exitfn dup @ascuheap @asulock @aculock @acsmem | |
977 | @c atomic_write_barrier dup ok | |
978 | This function is a somewhat more powerful variant of @code{atexit}. It | |
979 | accepts two arguments, a function @var{function} and an arbitrary | |
980 | pointer @var{arg}. At normal program termination, the @var{function} is | |
981 | called with two arguments: the @var{status} value passed to @code{exit}, | |
982 | and the @var{arg}. | |
983 | ||
984 | This function is included in @theglibc{} only for compatibility | |
985 | for SunOS, and may not be supported by other implementations. | |
986 | @end deftypefun | |
987 | ||
988 | Here's a trivial program that illustrates the use of @code{exit} and | |
989 | @code{atexit}: | |
990 | ||
991 | @smallexample | |
992 | @include atexit.c.texi | |
993 | @end smallexample | |
994 | ||
995 | @noindent | |
996 | When this program is executed, it just prints the message and exits. | |
997 | ||
998 | @node Aborting a Program | |
999 | @subsection Aborting a Program | |
1000 | @cindex aborting a program | |
1001 | ||
1002 | You can abort your program using the @code{abort} function. The prototype | |
1003 | for this function is in @file{stdlib.h}. | |
1004 | @pindex stdlib.h | |
1005 | ||
1006 | @comment stdlib.h | |
1007 | @comment ISO | |
1008 | @deftypefun void abort (void) | |
1009 | @safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} | |
1010 | @c The implementation takes a recursive lock and attempts to support | |
1011 | @c calls from signal handlers, but if we're in the middle of flushing or | |
1012 | @c using streams, we may encounter them in inconsistent states. | |
1013 | The @code{abort} function causes abnormal program termination. This | |
1014 | does not execute cleanup functions registered with @code{atexit} or | |
1015 | @code{on_exit}. | |
1016 | ||
1017 | This function actually terminates the process by raising a | |
1018 | @code{SIGABRT} signal, and your program can include a handler to | |
1019 | intercept this signal; see @ref{Signal Handling}. | |
1020 | @end deftypefun | |
1021 | ||
1022 | @c Put in by rms. Don't remove. | |
1023 | @cartouche | |
1024 | @strong{Future Change Warning:} Proposed Federal censorship regulations | |
1025 | may prohibit us from giving you information about the possibility of | |
1026 | calling this function. We would be required to say that this is not an | |
1027 | acceptable way of terminating a program. | |
1028 | @end cartouche | |
1029 | ||
1030 | @node Termination Internals | |
1031 | @subsection Termination Internals | |
1032 | ||
1033 | The @code{_exit} function is the primitive used for process termination | |
1034 | by @code{exit}. It is declared in the header file @file{unistd.h}. | |
1035 | @pindex unistd.h | |
1036 | ||
1037 | @comment unistd.h | |
1038 | @comment POSIX.1 | |
1039 | @deftypefun void _exit (int @var{status}) | |
1040 | @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} | |
1041 | @c Direct syscall (exit_group or exit); calls __task_terminate on hurd, | |
1042 | @c and abort in the generic posix implementation. | |
1043 | The @code{_exit} function is the primitive for causing a process to | |
1044 | terminate with status @var{status}. Calling this function does not | |
1045 | execute cleanup functions registered with @code{atexit} or | |
1046 | @code{on_exit}. | |
1047 | @end deftypefun | |
1048 | ||
1049 | @comment stdlib.h | |
1050 | @comment ISO | |
1051 | @deftypefun void _Exit (int @var{status}) | |
1052 | @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} | |
1053 | @c Alias for _exit. | |
1054 | The @code{_Exit} function is the @w{ISO C} equivalent to @code{_exit}. | |
1055 | The @w{ISO C} committee members were not sure whether the definitions of | |
1056 | @code{_exit} and @code{_Exit} were compatible so they have not used the | |
1057 | POSIX name. | |
1058 | ||
1059 | This function was introduced in @w{ISO C99} and is declared in | |
1060 | @file{stdlib.h}. | |
1061 | @end deftypefun | |
1062 | ||
1063 | When a process terminates for any reason---either because the program | |
1064 | terminates, or as a result of a signal---the | |
1065 | following things happen: | |
1066 | ||
1067 | @itemize @bullet | |
1068 | @item | |
1069 | All open file descriptors in the process are closed. @xref{Low-Level I/O}. | |
1070 | Note that streams are not flushed automatically when the process | |
1071 | terminates; see @ref{I/O on Streams}. | |
1072 | ||
1073 | @item | |
1074 | A process exit status is saved to be reported back to the parent process | |
1075 | via @code{wait} or @code{waitpid}; see @ref{Process Completion}. If the | |
1076 | program exited, this status includes as its low-order 8 bits the program | |
1077 | exit status. | |
1078 | ||
1079 | ||
1080 | @item | |
1081 | Any child processes of the process being terminated are assigned a new | |
1082 | parent process. (On most systems, including GNU, this is the @code{init} | |
1083 | process, with process ID 1.) | |
1084 | ||
1085 | @item | |
1086 | A @code{SIGCHLD} signal is sent to the parent process. | |
1087 | ||
1088 | @item | |
1089 | If the process is a session leader that has a controlling terminal, then | |
1090 | a @code{SIGHUP} signal is sent to each process in the foreground job, | |
1091 | and the controlling terminal is disassociated from that session. | |
1092 | @xref{Job Control}. | |
1093 | ||
1094 | @item | |
1095 | If termination of a process causes a process group to become orphaned, | |
1096 | and any member of that process group is stopped, then a @code{SIGHUP} | |
1097 | signal and a @code{SIGCONT} signal are sent to each process in the | |
1098 | group. @xref{Job Control}. | |
1099 | @end itemize |