NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | 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 *status);
pid_t waitpid(pid_t pid, int *status, int options);
int waitid(idtype_t idtype, id_t id, siginfo_t *infop, int options);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
waitid(): _SVID_SOURCE || _XOPEN_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.
The wait() system call suspends execution of the calling process until one of
its children terminates. The call wait(&status) is equivalent to:
waitpid(-1, &status, 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 status 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(status)
returns true if the child terminated normally, that is, by calling
exit(3) or _exit(2), or by returning from main().
WEXITSTATUS(status)
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 only be employed if WIFEXITED returned
true.
WIFSIGNALED(status)
returns true if the child process was terminated by a signal.
WTERMSIG(status)
returns the number of the signal that caused the child process to
terminate. This macro should only be employed if WIFSIGNALED returned
true.
WCOREDUMP(status)
returns true if the child produced a core dump. This macro should only
be employed 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). Only use this enclosed in #ifdef WCOREDUMP ...
#endif.
WIFSTOPPED(status)
returns true if the child process was stopped by delivery of a signal;
this is only possible if the call was done using WUNTRACED or when the
child is being traced (see ptrace(2)).
WSTOPSIG(status)
returns the number of the signal which caused the child to stop. This
macro should only be employed if WIFSTOPPED returned true.
WIFCONTINUED(status)
(since Linux 2.6.10) returns true if the child process was resumed by
delivery of SIGCONT.
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 non-zero 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(8), which 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 this
specification. 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.
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 cannot 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.
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
$
#include <sys/wait.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
int
main(int argc, char *argv[])
{
pid_t cpid, w;
int status;
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, &status, WUNTRACED | WCONTINUED);
if (w == -1) {
perror("waitpid");
exit(EXIT_FAILURE);
}
if (WIFEXITED(status)) {
printf("exited, status=%d\n", WEXITSTATUS(status));
} else if (WIFSIGNALED(status)) {
printf("killed by signal %d\n", WTERMSIG(status));
} else if (WIFSTOPPED(status)) {
printf("stopped by signal %d\n", WSTOPSIG(status));
} else if (WIFCONTINUED(status)) {
printf("continued\n");
}
} while (!WIFEXITED(status) && !WIFSIGNALED(status));
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 3.23 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2009-04-21 WAIT(2)