wait(2) - Linux manual page (original) (raw)
wait(2) System Calls Manual wait(2)
NAME top
wait, waitpid, waitid - wait for process to change stateLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#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
VERSIONS 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_SOURCEDESCRIPTION top
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)](../man2/sigaction.2.html)). 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)](../man2/ptrace.2.html)). 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)](../man3/exit.3.html) or [_exit(2)](../man2/%5Fexit.2.html), 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)](../man3/exit.3.html) or [_exit(2)](../man2/%5Fexit.2.html) 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)](../man5/core.5.html)). 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)](../man2/ptrace.2.html)).
**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)](../man2/pidfd%5Fopen.2.html) 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 _siginfot_ structure pointed to by _infop_:
_sipid_ The process ID of the child.
_siuid_ The real user ID of the child. (This field is not set on
most other implementations.)
_sisigno_
Always set to **SIGCHLD**.
_sistatus_
Either the exit status of the child, as given to [_exit(2)](../man2/%5Fexit.2.html)
(or [exit(3)](../man3/exit.3.html)), or the signal that caused the child to
terminate, stop, or continue. The _sicode_ field can be
used to determine how to interpret this field.
_sicode_
Set to one of: **CLD_EXITED** (child called [_exit(2)](../man2/%5Fexit.2.html));
**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 _siginfot_ 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 _sipid_ 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
_sipid_ and _sisigno_ fields of the structure. On Linux and other
implementations that adhere to this requirement, it is not
necessary to zero out the _sipid_ field before calling **waitid**().
However, not all implementations follow the POSIX.1 specification
on this point.RETURN VALUE top
**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](../man3/errno.3.html)_ to indicate the error.ERRORS top
**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)](../man7/signal.7.html).
**EINVAL** The _options_ argument was invalid.
**ESRCH** (for **wait**() or **waitpid**()) _pid_ is equal to **INT_MIN**.VERSIONS top
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)](../man2/wait4.2.html).
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)](../man2/wait4.2.html). See [getrusage(2)](../man2/getrusage.2.html) for details.STANDARDS top
POSIX.1-2008.HISTORY top
SVr4, 4.3BSD, POSIX.1-2001.NOTES top
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)](../man1/init.1.html), (or by the nearest "subreaper" process as defined through
the use of the [prctl(2)](../man2/prctl.2.html) **PR_SET_CHILD_SUBREAPER** operation); [init(1)](../man1/init.1.html)
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)](../man2/sigaction.2.html)), 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](../man3/errno.3.html)_ 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)](../man2/clone.2.html); 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.BUGS top
According to POSIX.1-2008, an application calling **waitid**() must
ensure that _infop_ points to a _siginfot_ 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 top
The following program demonstrates the use of [fork(2)](../man2/fork.2.html) 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)](../man2/pause.2.html), 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 top
[_exit(2)](../man2/%5Fexit.2.html), [clone(2)](../man2/clone.2.html), [fork(2)](../man2/fork.2.html), [kill(2)](../man2/kill.2.html), [ptrace(2)](../man2/ptrace.2.html), [sigaction(2)](../man2/sigaction.2.html),
[signal(2)](../man2/signal.2.html), [wait4(2)](../man2/wait4.2.html), [pthread_create(3)](../man3/pthread%5Fcreate.3.html), [core(5)](../man5/core.5.html), [credentials(7)](../man7/credentials.7.html),
[signal(7)](../man7/signal.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 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)