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 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.
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_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_STOPPED
(child stopped by signal); 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.
RETURN VALUE
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.
ERRORS
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.
CONFORMING TO
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(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.
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 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.
EXAMPLE
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 <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);
}
}
This page is part of release 3.14 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/.
