wait(2) System Calls Manual wait(2)
NAME
wait, waitpid, waitid - wait for process to change state
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <sys/wait.h>
pid_t wait(int *_Nullable wstatus);
pid_t waitpid(pid_t pid, int *_Nullable 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 <= 2.19: */ _BSD_SOURCE
DESCRIPTION
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 termi-
nated; 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 immedi-
ately. 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 thread 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 thread until
a child specified by pid argument has changed state. By default, wait-
pid() waits only for terminated children, but this behavior is modifi-
able 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 at the time of the call to
waitpid().
> 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 con-
stants:
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 fol-
lowing 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 call-
ing 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 speci-
fied 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 re-
turned true.
WCOREDUMP(wstatus)
returns true if the child produced a core dump (see core(5)).
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, en-
close 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 WUN-
TRACED 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 re-
sumed 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 fol-
lows:
idtype == P_PID
Wait for the child whose process ID matches id.
idtype == P_PIDFD (since Linux 5.4)
Wait for the child referred to by the PID file descriptor speci-
fied in id. (See pidfd_open(2) for further information on PID
file descriptors.)
idtype == P_PGID
Wait for any child whose process group ID matches id. Since
Linux 5.4, if id is zero, then wait for any child that is in the
same process group as the caller's process group at the time of
the call.
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 depends on the implementation.
To (portably) 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.
POSIX.1-2008 Technical Corrigendum 1 (2013) adds the requirement that
when WNOHANG is specified in options and there were no children in a
waitable state, then waitid() should zero out the si_pid and si_signo
fields of the structure. On Linux and other implementations that adhere
to this requirement, it is not necessary to zero out the si_pid field
before calling waitid(). However, not all implementations follow the
POSIX.1 specification on this point.
RETURN VALUE
wait(): on success, returns the process ID of the terminated child; on
failure, -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) speci-
fied by pid exist, but have not yet changed state, then 0 is returned.
On failure, -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 failure, -1 is re-
turned.
On failure, each of these calls sets errno to indicate the error.
ERRORS
EAGAIN The PID file descriptor specified in id is nonblocking and the
process that it refers to has not terminated.
ECHILD (for wait()) The calling process does not have any unwaited-for
children.
ECHILD (for waitpid() or waitid()) The process specified by pid (wait-
pid()) 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.
ESRCH (for wait() or waitpid()) pid is equal to INT_MIN.
VERSIONS
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 re-
source usage information about the child, in the same manner as
wait4(2). See getrusage(2) for details.
STANDARDS
POSIX.1-2008.
HISTORY
SVr4, 4.3BSD, POSIX.1-2001.
NOTES
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 set-
ting 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 con-
struct 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 termi-
nation.) 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.
BUGS
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 rely-
ing on this inconsistent, nonstandard, and unnecessary feature.
EXAMPLES
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 <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
int
main(int argc, char *argv[])
{
int wstatus;
pid_t cpid, w;
cpid = fork();
if (cpid == -1) {
perror("fork");
exit(EXIT_FAILURE);
}
if (cpid == 0) { /* Code executed by child */
printf("Child PID is %jd\n", (intmax_t) 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);
}
}
SEE ALSO
_exit(2), clone(2), fork(2), kill(2), ptrace(2), sigaction(2), sig-
nal(2), wait4(2), pthread_create(3), core(5), credentials(7), signal(7)
Linux man-pages 6.9.1 2024-06-15 wait(2)
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