wait, waitpid, waitid - wait for process to change state
#include <sys/types.h>
#include <sys/wait.h>
pid_t wait(int *wstatus);
pid_t waitpid(pid_t pid, int *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 versions <= 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.
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.
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
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(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.
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.
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.
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
$
#include <sys/wait.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
int
main(int argc, char *argv[])
{
pid_t cpid, w;
int wstatus;
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, &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)