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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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wait(2) System Calls Manual wait(2)
wait, waitpid, waitid - wait for process to change state
Standard C library (libc, -lc)
#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
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.
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.
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.
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.
POSIX.1-2008.
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.
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 <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);
}
}
_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)
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user-space interface documentation) project. Information about
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for this manual page, see
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Linux man-pages 6.9.1 2024-06-15 wait(2)
Pages that refer to this page: intro(1), waitpid(1), clone(2), _exit(2), fork(2), getrusage(2), io_uring_enter2(2), io_uring_enter(2), kill(2), pidfd_open(2), PR_SET_CHILD_SUBREAPER(2const), ptrace(2), reboot(2), seccomp(2), seccomp_unotify(2), sigaction(2), syscalls(2), times(2), vfork(2), wait4(2), clock(3), exit(3), id_t(3type), io_uring_prep_waitid(3), __pmprocessexec(3), __pmprocesspipe(3), pmrecord(3), posix_spawn(3), pthread_exit(3), sd-event(3), sd_event_add_child(3), sd_event_add_inotify(3), system(3), proc_pid_stat(5), systemd.exec(5), credentials(7), man-pages(7), pthreads(7), signal(7), signal-safety(7), user_namespaces(7)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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