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15 .\" manual page may be incorrect or out-of-date.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 2016-03-15 "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 *" "wstatus" );
59 .BI "pid_t waitpid(pid_t " pid ", int *" wstatus ", 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 || /* Since glibc 2.12: */ _POSIX_C_SOURCE\ >=\ 200809L
77 || /* Glibc versions <= 2.19: */ _BSD_SOURCE
82 All of these system calls are used to wait for state changes
83 in a child of the calling process, and obtain information
84 about the child whose state has changed.
85 A state change is considered to be: the child terminated;
86 the child was stopped by a signal; or the child was resumed by a signal.
87 In the case of a terminated child, performing a wait allows
88 the system to release the resources associated with the child;
89 if a wait is not performed, then the terminated child remains in
90 a "zombie" state (see NOTES below).
92 If a child has already changed state, then these calls return immediately.
93 Otherwise, they block until either a child changes state or
94 a signal handler interrupts the call (assuming that system calls
95 are not automatically restarted using the
99 In the remainder of this page, a child whose state has changed
100 and which has not yet been waited upon by one of these system
103 .SS wait() and waitpid()
106 system call suspends execution of the calling process until one of its
113 waitpid(\-1, &wstatus, 0);
118 system call suspends execution of the calling process until a
121 argument has changed state.
124 waits only for terminated children, but this behavior is modifiable
127 argument, as described below.
133 meaning wait for any child process whose process group ID is
134 equal to the absolute value of
137 meaning wait for any child process.
139 meaning wait for any child process whose process group ID is
140 equal to that of the calling process.
142 meaning wait for the child whose process ID is equal to the
148 is an OR of zero or more of the following constants:
151 return immediately if no child has exited.
154 also return if a child has stopped
159 children which have stopped is provided
160 even if this option is not specified.
162 .BR WCONTINUED " (since Linux 2.6.10)"
163 also return if a stopped child has been resumed by delivery of
166 (For Linux-only options, see below.)
174 store status information in the \fIint\fP to which it points.
175 This integer can be inspected with the following macros (which
176 take the integer itself as an argument, not a pointer to it,
182 .BI WIFEXITED( wstatus )
183 returns true if the child terminated normally, that is,
188 or by returning from main().
190 .BI WEXITSTATUS( wstatus )
191 returns the exit status of the child.
192 This consists of the least significant 8 bits of the
194 argument that the child specified in a call to
198 or as the argument for a return statement in main().
199 This macro should be employed only if
203 .BI WIFSIGNALED( wstatus )
204 returns true if the child process was terminated by a signal.
206 .BI WTERMSIG( wstatus )
207 returns the number of the signal that caused the child process to
209 This macro should be employed only if
213 .BI WCOREDUMP( wstatus )
214 returns true if the child produced a core dump.
215 This macro should be employed only if
218 This macro is not specified in POSIX.1-2001 and is not available on
219 some UNIX implementations (e.g., AIX, SunOS).
220 Only use this enclosed in #ifdef WCOREDUMP ... #endif.
222 .BI WIFSTOPPED( wstatus )
223 returns true if the child process was stopped by delivery of a signal;
224 this is possible only if the call was done using
226 or when the child is being traced (see
229 .BI WSTOPSIG( wstatus )
230 returns the number of the signal which caused the child to stop.
231 This macro should be employed only if
235 .BI WIFCONTINUED( wstatus )
237 returns true if the child process was resumed by delivery of
242 system call (available since Linux 2.6.9) provides more precise
243 control over which child state changes to wait for.
249 arguments select the child(ren) to wait for, as follows:
250 .IP "\fIidtype\fP == \fBP_PID\fP"
251 Wait for the child whose process ID matches
253 .IP "\fIidtype\fP == \fBP_PGID\fP"
254 Wait for any child whose process group ID matches
256 .IP "\fIidtype\fP == \fBP_ALL\fP"
261 The child state changes to wait for are specified by ORing
262 one or more of the following flags in
266 Wait for children that have terminated.
269 Wait for children that have been stopped by delivery of a signal.
272 Wait for (previously stopped) children that have been
273 resumed by delivery of
276 The following flags may additionally be ORed in
284 Leave the child in a waitable state; a later wait call
285 can be used to again retrieve the child status information.
287 Upon successful return,
289 fills in the following fields of the
291 structure pointed to by
295 The process ID of the child.
298 The real user ID of the child.
299 (This field is not set on most other implementations.)
306 Either the exit status of the child, as given to
310 or the signal that caused the child to terminate, stop, or continue.
313 field can be used to determine how to interpret this field.
321 (child killed by signal);
323 (child killed by signal, and dumped core);
325 (child stopped by signal);
327 (traced child has trapped); or
336 and there were no children in a waitable state, then
338 returns 0 immediately and
341 structure pointed to by
344 .\" POSIX.1-2001 leaves this possibility unspecified; most
345 .\" implementations (including Linux) zero out the structure
346 .\" in this case, but at least one implementation (AIX 5.1)
347 .\" does not -- MTK Nov 04
348 To distinguish this case from that where a child was in a
349 waitable state, zero out the
351 field before the call and check for a nonzero value in this field
352 after the call returns.
355 on success, returns the process ID of the terminated child;
356 on error, \-1 is returned.
359 on success, returns the process ID of the child whose state has changed;
362 was specified and one or more child(ren) specified by
364 exist, but have not yet changed state, then 0 is returned.
365 On error, \-1 is returned.
368 returns 0 on success or
371 was specified and no child(ren) specified by
373 has yet changed state;
374 on error, \-1 is returned.
375 .\" FIXME As reported by Vegard Nossum, if infop is NULL, then waitid()
376 .\" returns the PID of the child. Either this is a bug, or it is intended
377 .\" behavior that needs to be documented. See my Jan 2009 LKML mail
378 .\" "waitid() return value strangeness when infop is NULL".
380 Each of these calls sets
382 to an appropriate value in the case of an error.
388 The calling process does not have any unwaited-for children.
395 The process specified by
403 does not exist or is not a child of the calling process.
404 (This can happen for one's own child if the action for
408 See also the \fILinux Notes\fP section about threads.)
412 was not set and an unblocked signal or a
420 argument was invalid.
422 SVr4, 4.3BSD, POSIX.1-2001.
424 A child that terminates, but has not been waited for becomes a "zombie".
425 The kernel maintains a minimal set of information about the zombie
426 process (PID, termination status, resource usage information)
427 in order to allow the parent to later perform a wait to obtain
428 information about the child.
429 As long as a zombie is not removed from the system via a wait,
430 it will consume a slot in the kernel process table, and if
431 this table fills, it will not be possible to create further processes.
432 If a parent process terminates, then its "zombie" children (if any)
435 which automatically performs a wait to remove the zombies.
437 POSIX.1-2001 specifies that if the disposition of
447 then children that terminate do not become zombies and a call to
451 will block until all children have terminated, and then fail with
455 (The original POSIX standard left the behavior of setting
460 Note that even though the default disposition of
462 is "ignore", explicitly setting the disposition to
464 results in different treatment of zombie process children.)
466 Linux 2.6 conforms to the POSIX requirements.
467 However, Linux 2.4 (and earlier) does not:
474 is being ignored, the call behaves just as though
476 were not being ignored, that is, the call blocks until the next child
477 terminates and then returns the process ID and status of that child.
479 In the Linux kernel, a kernel-scheduled thread is not a distinct
480 construct from a process.
481 Instead, a thread is simply a process
482 that is created using the Linux-unique
484 system call; other routines such as the portable
485 .BR pthread_create (3)
486 call are implemented using
488 Before Linux 2.4, a thread was just a special case of a process,
489 and as a consequence one thread could not wait on the children
490 of another thread, even when the latter belongs to the same thread group.
491 However, POSIX prescribes such functionality, and since Linux 2.4
492 a thread can, and by default will, wait on children of other threads
493 in the same thread group.
495 The following Linux-specific
497 are for use with children created using
499 they cannot be used with
504 Wait for "clone" children only.
505 If omitted, then wait for "non-clone" children only.
506 (A "clone" child is one which delivers no signal, or a signal other than
508 to its parent upon termination.)
509 This option is ignored if
513 .BR __WALL " (since Linux 2.4)"
514 .\" since patch-2.3.48
515 Wait for all children, regardless of
516 type ("clone" or "non-clone").
518 .BR __WNOTHREAD " (since Linux 2.4)"
519 .\" since patch-2.4.0-test8
520 Do not wait for children of other threads in
521 the same thread group.
522 This was the default before Linux 2.4.
523 .SS C library/kernel differences
525 is actually a library function that (in glibc) is implemented as a call to
529 .\" A waitpid() system call remains, with a comment noting that
530 .\" "waitpid() should be * implemented by calling sys_wait4() from libc.a"
532 is a wrapper function that invokes
537 system call takes a fifth argument, of type
538 .IR "struct rusage\ *" .
539 If this argument is non-NULL,
540 then it is used to return resource usage information about the child,
541 in the same manner as
547 According to POSIX.1-2008, an application calling
553 structure (i.e., that it is a non-null pointer).
558 succeeds, and returns the process ID of the waited-for child.
559 Applications should avoid relying on this inconsistent,
560 nonstandard, and unnecessary feature.
562 .\" fork.2 refers to this example program.
563 The following program demonstrates the use of
567 The program creates a child process.
568 If no command-line argument is supplied to the program,
569 then the child suspends its execution using
571 to allow the user to send signals to the child.
572 Otherwise, if a command-line argument is supplied,
573 then the child exits immediately,
574 using the integer supplied on the command line as the exit status.
575 The parent process executes a loop that monitors the child using
577 and uses the W*() macros described above to analyze the wait status value.
579 The following shell session demonstrates the use of the program:
586 .RB "$" " kill \-STOP 32360"
588 .RB "$" " kill \-CONT 32360"
590 .RB "$" " kill \-TERM 32360"
599 #include <sys/wait.h>
605 main(int argc, char *argv[])
616 if (cpid == 0) { /* Code executed by child */
617 printf("Child PID is %ld\\n", (long) getpid());
619 pause(); /* Wait for signals */
620 _exit(atoi(argv[1]));
622 } else { /* Code executed by parent */
624 w = waitpid(cpid, &wstatus, WUNTRACED | WCONTINUED);
630 if (WIFEXITED(wstatus)) {
631 printf("exited, status=%d\\n", WEXITSTATUS(wstatus));
632 } else if (WIFSIGNALED(wstatus)) {
633 printf("killed by signal %d\\n", WTERMSIG(wstatus));
634 } else if (WIFSTOPPED(wstatus)) {
635 printf("stopped by signal %d\\n", WSTOPSIG(wstatus));
636 } else if (WIFCONTINUED(wstatus)) {
637 printf("continued\\n");
639 } while (!WIFEXITED(wstatus) && !WIFSIGNALED(wstatus));
653 .BR pthread_create (3),