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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | BUGS | EXAMPLE | SEE ALSO | COLOPHON |
WAIT(2) Linux Programmer's Manual WAIT(2)
wait, waitpid, waitid - wait for process to change state
#include <sys/types.h>
#include <sys/wait.h>
pid_t wait(int *wstatus);
pid_t waitpid(pid_t pid, int *wstatus, int options);
int waitid(idtype_t idtype, id_t id, siginfo_t *infop, int options);
/* This is the glibc and POSIX interface; see
NOTES for information on the raw system call. */
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
waitid():
Since glibc 2.26: _XOPEN_SOURCE >= 500 ||
_POSIX_C_SOURCE >= 200809L
Glibc 2.25 and earlier:
_XOPEN_SOURCE
|| /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
|| /* Glibc versions <= 2.19: */ _BSD_SOURCE
All of these system calls are used to wait for state changes in a
child of the calling process, and obtain information about the child
whose state has changed. A state change is considered to be: the
child terminated; the child was stopped by a signal; or the child was
resumed by a signal. In the case of a terminated child, performing a
wait allows the system to release the resources associated with the
child; if a wait is not performed, then the terminated child remains
in a "zombie" state (see NOTES below).
If a child has already changed state, then these calls return
immediately. Otherwise, they block until either a child changes
state or a signal handler interrupts the call (assuming that system
calls are not automatically restarted using the SA_RESTART flag of
sigaction(2)). In the remainder of this page, a child whose state
has changed and which has not yet been waited upon by one of these
system calls is termed waitable.
wait() and waitpid()
The wait() system call suspends execution of the calling process
until one of its children terminates. The call wait(&wstatus) is
equivalent to:
waitpid(-1, &wstatus, 0);
The waitpid() system call suspends execution of the calling process
until a child specified by pid argument has changed state. By
default, waitpid() waits only for terminated children, but this
behavior is modifiable via the options argument, as described below.
The value of pid can be:
< -1 meaning wait for any child process whose process group ID is
equal to the absolute value of pid.
-1 meaning wait for any child process.
0 meaning wait for any child process whose process group ID is
equal to that of the calling process.
> 0 meaning wait for the child whose process ID is equal to the
value of pid.
The value of options is an OR of zero or more of the following
constants:
WNOHANG return immediately if no child has exited.
WUNTRACED also return if a child has stopped (but not traced via
ptrace(2)). Status for traced children which have
stopped is provided even if this option is not specified.
WCONTINUED (since Linux 2.6.10)
also return if a stopped child has been resumed by
delivery of SIGCONT.
(For Linux-only options, see below.)
If wstatus is not NULL, wait() and waitpid() store status information
in the int to which it points. This integer can be inspected with
the following macros (which take the integer itself as an argument,
not a pointer to it, as is done in wait() and waitpid()!):
WIFEXITED(wstatus)
returns true if the child terminated normally, that is, by
calling exit(3) or _exit(2), or by returning from main().
WEXITSTATUS(wstatus)
returns the exit status of the child. This consists of the
least significant 8 bits of the status argument that the child
specified in a call to exit(3) or _exit(2) or as the argument
for a return statement in main(). This macro should be
employed only if WIFEXITED returned true.
WIFSIGNALED(wstatus)
returns true if the child process was terminated by a signal.
WTERMSIG(wstatus)
returns the number of the signal that caused the child process
to terminate. This macro should be employed only if
WIFSIGNALED returned true.
WCOREDUMP(wstatus)
returns true if the child produced a core dump. This macro
should be employed only if WIFSIGNALED returned true.
This macro is not specified in POSIX.1-2001 and is not
available on some UNIX implementations (e.g., AIX, SunOS).
Therefore, enclose its use inside #ifdef WCOREDUMP ... #endif.
WIFSTOPPED(wstatus)
returns true if the child process was stopped by delivery of a
signal; this is possible only if the call was done using
WUNTRACED or when the child is being traced (see ptrace(2)).
WSTOPSIG(wstatus)
returns the number of the signal which caused the child to
stop. This macro should be employed only if WIFSTOPPED
returned true.
WIFCONTINUED(wstatus)
(since Linux 2.6.10) returns true if the child process was
resumed by delivery of SIGCONT.
waitid()
The waitid() system call (available since Linux 2.6.9) provides more
precise control over which child state changes to wait for.
The idtype and id arguments select the child(ren) to wait for, as
follows:
idtype == P_PID
Wait for the child whose process ID matches id.
idtype == P_PGID
Wait for any child whose process group ID matches id.
idtype == P_ALL
Wait for any child; id is ignored.
The child state changes to wait for are specified by ORing one or
more of the following flags in options:
WEXITED Wait for children that have terminated.
WSTOPPED Wait for children that have been stopped by delivery of a
signal.
WCONTINUED Wait for (previously stopped) children that have been
resumed by delivery of SIGCONT.
The following flags may additionally be ORed in options:
WNOHANG As for waitpid().
WNOWAIT Leave the child in a waitable state; a later wait call
can be used to again retrieve the child status
information.
Upon successful return, waitid() fills in the following fields of the
siginfo_t structure pointed to by infop:
si_pid The process ID of the child.
si_uid The real user ID of the child. (This field is not set on
most other implementations.)
si_signo Always set to SIGCHLD.
si_status Either the exit status of the child, as given to _exit(2)
(or exit(3)), or the signal that caused the child to
terminate, stop, or continue. The si_code field can be
used to determine how to interpret this field.
si_code Set to one of: CLD_EXITED (child called _exit(2));
CLD_KILLED (child killed by signal); CLD_DUMPED (child
killed by signal, and dumped core); CLD_STOPPED (child
stopped by signal); CLD_TRAPPED (traced child has
trapped); or CLD_CONTINUED (child continued by SIGCONT).
If WNOHANG was specified in options and there were no children in a
waitable state, then waitid() returns 0 immediately and the state of
the siginfo_t structure pointed to by infop is unspecified. To
distinguish this case from that where a child was in a waitable
state, zero out the si_pid field before the call and check for a
nonzero value in this field after the call returns.
wait(): on success, returns the process ID of the terminated child;
on error, -1 is returned.
waitpid(): on success, returns the process ID of the child whose
state has changed; if WNOHANG was specified and one or more
child(ren) specified by pid exist, but have not yet changed state,
then 0 is returned. On error, -1 is returned.
waitid(): returns 0 on success or if WNOHANG was specified and no
child(ren) specified by id has yet changed state; on error, -1 is
returned.
Each of these calls sets errno to an appropriate value in the case of
an error.
ECHILD (for wait()) The calling process does not have any unwaited-
for children.
ECHILD (for waitpid() or waitid()) The process specified by pid
(waitpid()) or idtype and id (waitid()) does not exist or is
not a child of the calling process. (This can happen for
one's own child if the action for SIGCHLD is set to SIG_IGN.
See also the Linux Notes section about threads.)
EINTR WNOHANG was not set and an unblocked signal or a SIGCHLD was
caught; see signal(7).
EINVAL The options argument was invalid.
SVr4, 4.3BSD, POSIX.1-2001.
A child that terminates, but has not been waited for becomes a
"zombie". The kernel maintains a minimal set of information about
the zombie process (PID, termination status, resource usage
information) in order to allow the parent to later perform a wait to
obtain information about the child. As long as a zombie is not
removed from the system via a wait, it will consume a slot in the
kernel process table, and if this table fills, it will not be
possible to create further processes. If a parent process
terminates, then its "zombie" children (if any) are adopted by
init(1), (or by the nearest "subreaper" process as defined through
the use of the prctl(2) PR_SET_CHILD_SUBREAPER operation); init(1)
automatically performs a wait to remove the zombies.
POSIX.1-2001 specifies that if the disposition of SIGCHLD is set to
SIG_IGN or the SA_NOCLDWAIT flag is set for SIGCHLD (see
sigaction(2)), then children that terminate do not become zombies and
a call to wait() or waitpid() will block until all children have
terminated, and then fail with errno set to ECHILD. (The original
POSIX standard left the behavior of setting SIGCHLD to SIG_IGN
unspecified. Note that even though the default disposition of
SIGCHLD is "ignore", explicitly setting the disposition to SIG_IGN
results in different treatment of zombie process children.)
Linux 2.6 conforms to the POSIX requirements. However, Linux 2.4
(and earlier) does not: if a wait() or waitpid() call is made while
SIGCHLD is being ignored, the call behaves just as though SIGCHLD
were not being ignored, that is, the call blocks until the next child
terminates and then returns the process ID and status of that child.
Linux notes
In the Linux kernel, a kernel-scheduled thread is not a distinct
construct from a process. Instead, a thread is simply a process that
is created using the Linux-unique clone(2) system call; other
routines such as the portable pthread_create(3) call are implemented
using clone(2). Before Linux 2.4, a thread was just a special case
of a process, and as a consequence one thread could not wait on the
children of another thread, even when the latter belongs to the same
thread group. However, POSIX prescribes such functionality, and
since Linux 2.4 a thread can, and by default will, wait on children
of other threads in the same thread group.
The following Linux-specific options are for use with children
created using clone(2); they can also, since Linux 4.7, be used with
waitid():
__WCLONE
Wait for "clone" children only. If omitted, then wait for
"non-clone" children only. (A "clone" child is one which
delivers no signal, or a signal other than SIGCHLD to its
parent upon termination.) This option is ignored if __WALL is
also specified.
__WALL (since Linux 2.4)
Wait for all children, regardless of type ("clone" or "non-
clone").
__WNOTHREAD (since Linux 2.4)
Do not wait for children of other threads in the same thread
group. This was the default before Linux 2.4.
Since Linux 4.7, the __WALL flag is automatically implied if the
child is being ptraced.
C library/kernel differences
wait() is actually a library function that (in glibc) is implemented
as a call to wait4(2).
On some architectures, there is no waitpid() system call; instead,
this interface is implemented via a C library wrapper function that
calls wait4(2).
The raw waitid() system call takes a fifth argument, of type struct
rusage *. If this argument is non-NULL, then it is used to return
resource usage information about the child, in the same manner as
wait4(2). See getrusage(2) for details.
According to POSIX.1-2008, an application calling waitid() must
ensure that infop points to a siginfo_t structure (i.e., that it is a
non-null pointer). On Linux, if infop is NULL, waitid() succeeds,
and returns the process ID of the waited-for child. Applications
should avoid relying on this inconsistent, nonstandard, and
unnecessary feature.
The following program demonstrates the use of fork(2) and waitpid().
The program creates a child process. If no command-line argument is
supplied to the program, then the child suspends its execution using
pause(2), to allow the user to send signals to the child. Otherwise,
if a command-line argument is supplied, then the child exits
immediately, using the integer supplied on the command line as the
exit status. The parent process executes a loop that monitors the
child using waitpid(), and uses the W*() macros described above to
analyze the wait status value.
The following shell session demonstrates the use of the program:
$ ./a.out &
Child PID is 32360
[1] 32359
$ kill -STOP 32360
stopped by signal 19
$ kill -CONT 32360
continued
$ kill -TERM 32360
killed by signal 15
[1]+ Done ./a.out
$
Program source
#include <sys/wait.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
int
main(int argc, char *argv[])
{
pid_t cpid, w;
int wstatus;
cpid = fork();
if (cpid == -1) {
perror("fork");
exit(EXIT_FAILURE);
}
if (cpid == 0) { /* Code executed by child */
printf("Child PID is %ld\n", (long) getpid());
if (argc == 1)
pause(); /* Wait for signals */
_exit(atoi(argv[1]));
} else { /* Code executed by parent */
do {
w = waitpid(cpid, &wstatus, WUNTRACED | WCONTINUED);
if (w == -1) {
perror("waitpid");
exit(EXIT_FAILURE);
}
if (WIFEXITED(wstatus)) {
printf("exited, status=%d\n", WEXITSTATUS(wstatus));
} else if (WIFSIGNALED(wstatus)) {
printf("killed by signal %d\n", WTERMSIG(wstatus));
} else if (WIFSTOPPED(wstatus)) {
printf("stopped by signal %d\n", WSTOPSIG(wstatus));
} else if (WIFCONTINUED(wstatus)) {
printf("continued\n");
}
} while (!WIFEXITED(wstatus) && !WIFSIGNALED(wstatus));
exit(EXIT_SUCCESS);
}
}
_exit(2), clone(2), fork(2), kill(2), ptrace(2), sigaction(2),
signal(2), wait4(2), pthread_create(3), credentials(7), signal(7)
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-05-03 WAIT(2)
Pages that refer to this page: intro(1), watch(1), clone(2), _exit(2), fork(2), getrusage(2), kill(2), prctl(2), ptrace(2), sigaction(2), syscalls(2), times(2), vfork(2), wait4(2), clock(3), exit(3), pmafm(3), posix_spawn(3), pthread_exit(3), sd-event(3), sd_event_add_child(3), system(3), proc(5), systemd.exec(5), credentials(7), man-pages(7), pthreads(7), signal(7), signal-safety(7), user_namespaces(7)