1 .\" Hey Emacs! This file is -*- nroff -*- source.
3 .\" Copyright (c) 1993 by Thomas Koenig <ig25@rz.uni-karlsruhe.de>
4 .\" and Copyright (c) 2004 by Michael Kerrisk <mtk-manpages@gmx.net>
6 .\" Permission is granted to make and distribute verbatim copies of this
7 .\" manual provided the copyright notice and this permission notice are
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10 .\" Permission is granted to copy and distribute modified versions of this
11 .\" manual under the conditions for verbatim copying, provided that the
12 .\" entire resulting derived work is distributed under the terms of a
13 .\" permission notice identical to this one.
15 .\" Since the Linux kernel and libraries are constantly changing, this
16 .\" manual page may be incorrect or out-of-date. The author(s) assume no
17 .\" responsibility for errors or omissions, or for damages resulting from
18 .\" the use of the information contained herein. The author(s) may not
19 .\" have taken the same level of care in the production of this manual,
20 .\" which is licensed free of charge, as they might when working
23 .\" Formatted or processed versions of this manual, if unaccompanied by
24 .\" the source, must acknowledge the copyright and authors of this work.
27 .\" Modified Sat Jul 24 13:30:06 1993 by Rik Faith <faith@cs.unc.edu>
28 .\" Modified Sun Aug 21 17:42:42 1994 by Rik Faith <faith@cs.unc.edu>
29 .\" (Thanks to Koen Holtman <koen@win.tue.nl>)
30 .\" Modified Wed May 17 15:54:12 1995 by Rik Faith <faith@cs.unc.edu>
31 .\" To remove *'s from status in macros (Thanks to Michael Shields).
32 .\" Modified as suggested by Nick Duffek <nsd@bbc.com>, aeb, 960426
33 .\" Modified Mon Jun 23 14:09:52 1997 by aeb - add EINTR.
34 .\" Modified Thu Nov 26 02:12:45 1998 by aeb - add SIGCHLD stuff.
35 .\" Modified Mon Jul 24 21:37:38 2000 by David A. Wheeler
36 .\" <dwheeler@dwheeler.com> - noted thread issues.
37 .\" Modified 26 Jun 01 by Michael Kerrisk
38 .\" Added __WCLONE, __WALL, and __WNOTHREAD descriptions
39 .\" Modified 2001-09-25, aeb
40 .\" Modified 26 Jun 01 by Michael Kerrisk, <mtk-manpages@gmx.net>
41 .\" Updated notes on setting disposition of SIGCHLD to SIG_IGN
43 .\" Added waitid(2); added WCONTINUED and WIFCONTINUED()
44 .\" Added text on SA_NOCLDSTOP
45 .\" Updated discussion of SA_NOCLDWAIT to reflect 2.6 behavior
46 .\" Much other text rewritten
47 .\" 2005-05-10, mtk, __W* flags can't be used with waitid()
49 .TH WAIT 2 2007-07-26 "Linux" "Linux Programmer's Manual"
51 wait, waitpid, waitid \- wait for process to change state
53 .B #include <sys/types.h>
55 .B #include <sys/wait.h>
57 .BI "pid_t wait(int *" "status" );
59 .BI "pid_t waitpid(pid_t " pid ", int *" status ", int " options );
61 .BI "int waitid(idtype_t " idtype ", id_t " id \
62 ", siginfo_t *" infop ", int " options );
65 Feature Test Macro Requirements for glibc (see
66 .BR feature_test_macros (7)):
70 _SVID_SOURCE || _XOPEN_SOURCE
72 All of these system calls are used to wait for state changes
73 in a child of the calling process, and obtain information
74 about the child whose state has changed.
75 A state change is considered to be: the child terminated;
76 the child was stopped by a signal; or the child was resumed by a signal.
77 In the case of a terminated child, performing a wait allows
78 the system to release the resources associated with the child;
79 if a wait is not performed, then terminated the child remains in
80 a "zombie" state (see NOTES below).
82 If a child has already changed state, then these calls return immediately.
83 Otherwise they block until either a child changes state or
84 a signal handler interrupts the call (assuming that system calls
85 are not automatically restarted using the
89 In the remainder of this page, a child whose state has changed
90 and which has not yet been waited upon by one of these system
93 .SS "wait() and waitpid()"
96 system call suspends execution of the current process until one of its
103 waitpid(\-1, &status, 0);
108 system call suspends execution of the current process until a
111 argument has changed state.
114 waits only for terminated children, but this behavior is modifiable
117 argument, as described below.
123 meaning wait for any child process whose process group ID is
124 equal to the absolute value of
127 meaning wait for any child process.
129 meaning wait for any child process whose process group ID is
130 equal to that of the calling process.
132 meaning wait for the child whose process ID is equal to the
138 is an OR of zero or more of the following constants:
141 return immediately if no child has exited.
144 also return if a child has stopped
149 children which have stopped is provided
150 even if this option is not specified.
154 also return if a stopped child has been resumed by delivery of
157 (For Linux-only options, see below.)
163 options are only effective if the
165 flag has not been set for the
176 store status information in the \fIint\fR to which it points.
177 This integer can be inspected with the following macros (which
178 take the integer itself as an argument, not a pointer to it,
184 .BI WIFEXITED( status )
185 returns true if the child terminated normally, that is,
190 or by returning from main().
192 .BI WEXITSTATUS( status )
193 returns the exit status of the child.
194 This consists of the least significant 8 bits of the
196 argument that the child specified in a call to
200 or as the argument for a return statement in main().
201 This macro should only be employed if
205 .BI WIFSIGNALED( status )
206 returns true if the child process was terminated by a signal.
208 .BI WTERMSIG( status )
209 returns the number of the signal that caused the child process to
211 This macro should only be employed if
215 .BI WCOREDUMP( status )
216 returns true if the child produced a core dump.
217 This macro should only be employed if
220 This macro is not specified in POSIX.1-2001 and is not available on
221 some Unix implementations (e.g., AIX, SunOS).
222 Only use this enclosed in #ifdef WCOREDUMP ... #endif.
224 .BI WIFSTOPPED( status )
225 returns true if the child process was stopped by delivery of a signal;
226 this is only possible if the call was done using
228 or when the child is being traced (see
231 .BI WSTOPSIG( status )
232 returns the number of the signal which caused the child to stop.
233 This macro should only be employed if
237 .BI WIFCONTINUED( status )
239 returns true if the child process was resumed by delivery of
244 system call (available since Linux 2.6.9) provides more precise
245 control over which child state changes to wait for.
251 arguments select the child(ren) to wait for, as follows:
252 .IP "\fIidtype\fP == \fBP_PID\fP"
253 Wait for the child whose process ID matches
255 .IP "\fIidtype\fP == \fBP_PGID\fP"
256 Wait for any child whose process group ID matches
258 .IP "\fIidtype\fP == \fBP_ALL\fP"
263 The child state changes to wait for are specified by ORing
264 one or more of the following flags in
268 Wait for children that have terminated.
271 Wait for children that have been stopped by delivery of a signal.
274 Wait for (previously stopped) children that have been
275 resumed by delivery of
278 The following flags may additionally be ORed in
286 Leave the child in a waitable state; a later wait call
287 can be used to again retrieve the child status information.
289 Upon successful return,
291 fills in the following fields of the
293 structure pointed to by
296 The process ID of the child.
298 The real user ID of the child.
299 (This field is not set on most other implementations.)
304 Either the exit status of the child, as given to
308 or the signal that caused the child to terminate, stop, or continue.
311 field can be used to determine how to interpret this field.
318 (child killed by signal);
320 (child stopped by signal); or
329 and there were no children in a waitable state, then
331 returns 0 immediately and
334 structure pointed to by
337 .\" POSIX.1-2001 leaves this possibility unspecified; most
338 .\" implementations (including Linux) zero out the structure
339 .\" in this case, but at at least one implementation (AIX 5.1)
340 .\" does not -- MTK Nov 04
341 To distinguish this case from that where a child was in a
342 waitable state, zero out the
344 field before the call and check for a non-zero value in this field
345 after the call returns.
348 on success, returns the process ID of the terminated child;
349 on error, \-1 is returned.
352 on success, returns the process ID of the child whose state has changed;
353 on error, \-1 is returned;
356 was specified and no child(ren) specified by
358 has yet changed state, then 0 is returned.
361 returns 0 on success or
364 was specified and no child(ren) specified by
366 has yet changed state;
367 on error, \-1 is returned.
369 Each of these calls sets
371 to an appropriate value in the case of an error.
377 The calling process does not have any unwaited-for children.
384 The process specified by
392 does not exist or is not a child of the calling process.
393 (This can happen for one's own child if the action for
397 See also the \fILinux Notes\fP section about threads.)
401 was not set and an unblocked signal or a
408 argument was invalid.
410 SVr4, 4.3BSD, POSIX.1-2001.
412 A child that terminates, but has not been waited for becomes a "zombie".
413 The kernel maintains a minimal set of information about the zombie
414 process (PID, termination status, resource usage information)
415 in order to allow the parent to later perform a wait to obtain
416 information about the child.
417 As long as a zombie is not removed from the system via a wait,
418 it will consume a slot in the kernel process table, and if
419 this table fills, it will not be possible to create further processes.
420 If a parent process terminates, then its "zombie" children (if any)
423 which automatically performs a wait to remove the zombies.
425 POSIX.1-2001 specifies that if the disposition of
435 then children that terminate do not become zombies and a call to
439 will block until all children have terminated, and then fail with
443 (The original POSIX standard left the behavior of setting
448 Linux 2.6 conforms to this specification.
449 However, Linux 2.4 (and earlier) does not:
456 is being ignored, the call behaves just as though
458 were not being ignored, that is, the call blocks until the next child
459 terminates and then returns the process ID and status of that child.
461 In the Linux kernel, a kernel-scheduled thread is not a distinct
462 construct from a process.
463 Instead, a thread is simply a process
464 that is created using the Linux-unique
466 system call; other routines such as the portable
467 .BR pthread_create (3)
468 call are implemented using
470 Before Linux 2.4, a thread was just a special case of a process,
471 and as a consequence one thread could not wait on the children
472 of another thread, even when the latter belongs to the same thread group.
473 However, POSIX prescribes such functionality, and since Linux 2.4
474 a thread can, and by default will, wait on children of other threads
475 in the same thread group.
477 The following Linux specific
479 are for use with children created using
481 they cannot be used with
486 Wait for "clone" children only.
487 If omitted then wait for "non-clone" children only.
488 (A "clone" child is one which delivers no signal, or a signal other than
490 to its parent upon termination.)
491 This option is ignored if
496 .\" since patch-2.3.48
497 (Since Linux 2.4) Wait for all children, regardless of
498 type ("clone" or "non-clone").
501 .\" since patch-2.4.0-test8
502 (Since Linux 2.4) Do not wait for children of other threads in
503 the same thread group.
504 This was the default before Linux 2.4.
506 .\" fork.2 refers to this example program.
507 The following program demonstrates the use of
511 The program creates a child process.
512 If no command-line argument is supplied to the program,
513 then the child suspends its execution using
515 to allow the user to send signals to the child.
516 Otherwise, if a command-line argument is supplied,
517 then the child exits immediately,
518 using the integer supplied on the command line as the exit status.
519 The parent process executes a loop that monitors the child using
521 and uses the W*() macros described above to analyze the wait status value.
523 The following shell session demonstrates the use of the program:
538 #include <sys/wait.h>
544 main(int argc, char *argv[])
555 if (cpid == 0) { /* Code executed by child */
556 printf("Child PID is %ld\\n", (long) getpid());
558 pause(); /* Wait for signals */
559 _exit(atoi(argv[1]));
561 } else { /* Code executed by parent */
563 w = waitpid(cpid, &status, WUNTRACED | WCONTINUED);
569 if (WIFEXITED(status)) {
570 printf("exited, status=%d\\n", WEXITSTATUS(status));
571 } else if (WIFSIGNALED(status)) {
572 printf("killed by signal %d\\n", WTERMSIG(status));
573 } else if (WIFSTOPPED(status)) {
574 printf("stopped by signal %d\\n", WSTOPSIG(status));
575 } else if (WIFCONTINUED(status)) {
576 printf("continued\\n");
578 } while (!WIFEXITED(status) && !WIFSIGNALED(status));
592 .BR pthread_create (3),