1 .\" Copyright (c) 1993 by Thomas Koenig <ig25@rz.uni-karlsruhe.de>
2 .\" and Copyright (c) 2004 by Michael Kerrisk <mtk.manpages@gmail.com>
4 .\" %%%LICENSE_START(VERBATIM)
5 .\" Permission is granted to make and distribute verbatim copies of this
6 .\" manual provided the copyright notice and this permission notice are
7 .\" preserved on all copies.
9 .\" Permission is granted to copy and distribute modified versions of this
10 .\" manual under the conditions for verbatim copying, provided that the
11 .\" entire resulting derived work is distributed under the terms of a
12 .\" permission notice identical to this one.
14 .\" Since the Linux kernel and libraries are constantly changing, this
15 .\" manual page may be incorrect or out-of-date. The author(s) assume no
16 .\" responsibility for errors or omissions, or for damages resulting from
17 .\" the use of the information contained herein. The author(s) may not
18 .\" have taken the same level of care in the production of this manual,
19 .\" which is licensed free of charge, as they might when working
22 .\" Formatted or processed versions of this manual, if unaccompanied by
23 .\" the source, must acknowledge the copyright and authors of this work.
26 .\" Modified Sat Jul 24 13:30:06 1993 by Rik Faith <faith@cs.unc.edu>
27 .\" Modified Sun Aug 21 17:42:42 1994 by Rik Faith <faith@cs.unc.edu>
28 .\" (Thanks to Koen Holtman <koen@win.tue.nl>)
29 .\" Modified Wed May 17 15:54:12 1995 by Rik Faith <faith@cs.unc.edu>
30 .\" To remove *'s from status in macros (Thanks to Michael Shields).
31 .\" Modified as suggested by Nick Duffek <nsd@bbc.com>, aeb, 960426
32 .\" Modified Mon Jun 23 14:09:52 1997 by aeb - add EINTR.
33 .\" Modified Thu Nov 26 02:12:45 1998 by aeb - add SIGCHLD stuff.
34 .\" Modified Mon Jul 24 21:37:38 2000 by David A. Wheeler
35 .\" <dwheeler@dwheeler.com> - noted thread issues.
36 .\" Modified 26 Jun 01 by Michael Kerrisk
37 .\" Added __WCLONE, __WALL, and __WNOTHREAD descriptions
38 .\" Modified 2001-09-25, aeb
39 .\" Modified 26 Jun 01 by Michael Kerrisk, <mtk.manpages@gmail.com>
40 .\" Updated notes on setting disposition of SIGCHLD to SIG_IGN
42 .\" Added waitid(2); added WCONTINUED and WIFCONTINUED()
43 .\" Added text on SA_NOCLDSTOP
44 .\" Updated discussion of SA_NOCLDWAIT to reflect 2.6 behavior
45 .\" Much other text rewritten
46 .\" 2005-05-10, mtk, __W* flags can't be used with waitid()
47 .\" 2008-07-04, mtk, removed erroneous text about SA_NOCLDSTOP
49 .TH WAIT 2 2014-08-19 "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 );
63 /* This is the glibc and POSIX interface; see
64 NOTES for information on the raw system call. */
67 Feature Test Macro Requirements for glibc (see
68 .BR feature_test_macros (7)):
76 _XOPEN_SOURCE\ >=\ 500 ||
77 _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
79 || /* Since glibc 2.12: */ _POSIX_C_SOURCE\ >=\ 200809L
84 All of these system calls are used to wait for state changes
85 in a child of the calling process, and obtain information
86 about the child whose state has changed.
87 A state change is considered to be: the child terminated;
88 the child was stopped by a signal; or the child was resumed by a signal.
89 In the case of a terminated child, performing a wait allows
90 the system to release the resources associated with the child;
91 if a wait is not performed, then the terminated child remains in
92 a "zombie" state (see NOTES below).
94 If a child has already changed state, then these calls return immediately.
95 Otherwise, they block until either a child changes state or
96 a signal handler interrupts the call (assuming that system calls
97 are not automatically restarted using the
101 In the remainder of this page, a child whose state has changed
102 and which has not yet been waited upon by one of these system
105 .SS wait() and waitpid()
108 system call suspends execution of the calling process until one of its
115 waitpid(\-1, &status, 0);
120 system call suspends execution of the calling process until a
123 argument has changed state.
126 waits only for terminated children, but this behavior is modifiable
129 argument, as described below.
135 meaning wait for any child process whose process group ID is
136 equal to the absolute value of
139 meaning wait for any child process.
141 meaning wait for any child process whose process group ID is
142 equal to that of the calling process.
144 meaning wait for the child whose process ID is equal to the
150 is an OR of zero or more of the following constants:
153 return immediately if no child has exited.
156 also return if a child has stopped
161 children which have stopped is provided
162 even if this option is not specified.
164 .BR WCONTINUED " (since Linux 2.6.10)"
165 also return if a stopped child has been resumed by delivery of
168 (For Linux-only options, see below.)
176 store status information in the \fIint\fP 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 be employed only 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 be employed only if
215 .BI WCOREDUMP( status )
216 returns true if the child produced a core dump.
217 This macro should be employed only 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 possible only 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 be employed only 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
297 The process ID of the child.
300 The real user ID of the child.
301 (This field is not set on most other implementations.)
308 Either the exit status of the child, as given to
312 or the signal that caused the child to terminate, stop, or continue.
315 field can be used to determine how to interpret this field.
323 (child killed by signal);
325 (child killed by signal, and dumped core);
327 (child stopped by signal);
329 (traced child has trapped); or
338 and there were no children in a waitable state, then
340 returns 0 immediately and
343 structure pointed to by
346 .\" POSIX.1-2001 leaves this possibility unspecified; most
347 .\" implementations (including Linux) zero out the structure
348 .\" in this case, but at least one implementation (AIX 5.1)
349 .\" does not -- MTK Nov 04
350 To distinguish this case from that where a child was in a
351 waitable state, zero out the
353 field before the call and check for a nonzero value in this field
354 after the call returns.
357 on success, returns the process ID of the terminated child;
358 on error, \-1 is returned.
361 on success, returns the process ID of the child whose state has changed;
364 was specified and one or more child(ren) specified by
366 exist, but have not yet changed state, then 0 is returned.
367 On error, \-1 is returned.
370 returns 0 on success or
373 was specified and no child(ren) specified by
375 has yet changed state;
376 on error, \-1 is returned.
377 .\" FIXME As reported by Vegard Nossum, if infop is NULL, then waitid()
378 .\" returns the PID of the child. Either this is a bug, or it is intended
379 .\" behavior that needs to be documented. See my Jan 2009 LKML mail
380 .\" "waitid() return value strangeness when infop is NULL".
382 Each of these calls sets
384 to an appropriate value in the case of an error.
390 The calling process does not have any unwaited-for children.
397 The process specified by
405 does not exist or is not a child of the calling process.
406 (This can happen for one's own child if the action for
410 See also the \fILinux Notes\fP section about threads.)
414 was not set and an unblocked signal or a
422 argument was invalid.
424 SVr4, 4.3BSD, POSIX.1-2001.
426 A child that terminates, but has not been waited for becomes a "zombie".
427 The kernel maintains a minimal set of information about the zombie
428 process (PID, termination status, resource usage information)
429 in order to allow the parent to later perform a wait to obtain
430 information about the child.
431 As long as a zombie is not removed from the system via a wait,
432 it will consume a slot in the kernel process table, and if
433 this table fills, it will not be possible to create further processes.
434 If a parent process terminates, then its "zombie" children (if any)
437 which automatically performs a wait to remove the zombies.
439 POSIX.1-2001 specifies that if the disposition of
449 then children that terminate do not become zombies and a call to
453 will block until all children have terminated, and then fail with
457 (The original POSIX standard left the behavior of setting
462 Note that even though the default disposition of
464 is "ignore", explicitly setting the disposition to
466 results in different treatment of zombie process children.)
468 Linux 2.6 conforms to the POSIX requirements.
469 However, Linux 2.4 (and earlier) does not:
476 is being ignored, the call behaves just as though
478 were not being ignored, that is, the call blocks until the next child
479 terminates and then returns the process ID and status of that child.
481 In the Linux kernel, a kernel-scheduled thread is not a distinct
482 construct from a process.
483 Instead, a thread is simply a process
484 that is created using the Linux-unique
486 system call; other routines such as the portable
487 .BR pthread_create (3)
488 call are implemented using
490 Before Linux 2.4, a thread was just a special case of a process,
491 and as a consequence one thread could not wait on the children
492 of another thread, even when the latter belongs to the same thread group.
493 However, POSIX prescribes such functionality, and since Linux 2.4
494 a thread can, and by default will, wait on children of other threads
495 in the same thread group.
497 The following Linux-specific
499 are for use with children created using
501 they cannot be used with
506 Wait for "clone" children only.
507 If omitted, then wait for "non-clone" children only.
508 (A "clone" child is one which delivers no signal, or a signal other than
510 to its parent upon termination.)
511 This option is ignored if
515 .BR __WALL " (since Linux 2.4)"
516 .\" since patch-2.3.48
517 Wait for all children, regardless of
518 type ("clone" or "non-clone").
520 .BR __WNOTHREAD " (since Linux 2.4)"
521 .\" since patch-2.4.0-test8
522 Do not wait for children of other threads in
523 the same thread group.
524 This was the default before Linux 2.4.
525 .SS C library/kernel ABI differences
528 system call takes a fifth argument, of type
529 .IR "struct rusage\ *" .
530 If this argument is non-NULL,
531 then it is used to return resource usage information about the child,
532 in the same manner as
538 According to POSIX.1-2008, an application calling
544 structure (i.e., that it is a non-null pointer).
549 succeeds, and returns the process ID of the waited-for child.
550 Applications should avoid relying on this inconsistent,
551 nonstandard, and unnecessary feature.
553 .\" fork.2 refers to this example program.
554 The following program demonstrates the use of
558 The program creates a child process.
559 If no command-line argument is supplied to the program,
560 then the child suspends its execution using
562 to allow the user to send signals to the child.
563 Otherwise, if a command-line argument is supplied,
564 then the child exits immediately,
565 using the integer supplied on the command line as the exit status.
566 The parent process executes a loop that monitors the child using
568 and uses the W*() macros described above to analyze the wait status value.
570 The following shell session demonstrates the use of the program:
577 .RB "$" " kill \-STOP 32360"
579 .RB "$" " kill \-CONT 32360"
581 .RB "$" " kill \-TERM 32360"
590 #include <sys/wait.h>
596 main(int argc, char *argv[])
607 if (cpid == 0) { /* Code executed by child */
608 printf("Child PID is %ld\\n", (long) getpid());
610 pause(); /* Wait for signals */
611 _exit(atoi(argv[1]));
613 } else { /* Code executed by parent */
615 w = waitpid(cpid, &status, WUNTRACED | WCONTINUED);
621 if (WIFEXITED(status)) {
622 printf("exited, status=%d\\n", WEXITSTATUS(status));
623 } else if (WIFSIGNALED(status)) {
624 printf("killed by signal %d\\n", WTERMSIG(status));
625 } else if (WIFSTOPPED(status)) {
626 printf("stopped by signal %d\\n", WSTOPSIG(status));
627 } else if (WIFCONTINUED(status)) {
628 printf("continued\\n");
630 } while (!WIFEXITED(status) && !WIFSIGNALED(status));
644 .BR pthread_create (3),
648 This page is part of release 3.79 of the Linux
651 A description of the project,
652 information about reporting bugs,
653 and the latest version of this page,
655 \%http://www.kernel.org/doc/man\-pages/.