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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 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 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, 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 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 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 (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, 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_PIDFD (since Linux 5.4)
Wait for the child referred to by the PID file descriptor specified 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) specified 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 returned.
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 (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.
ESRCH (for wait() or waitpid()) pid is equal to INT_MIN.
STANDARDS
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 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.
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 relying 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/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), signal(2), wait4(2),
pthread_create(3), core(5), credentials(7), signal(7)