Linux Command Reference

DESCRIPTION

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 terminated the 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 current 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 current process until a child specified by pid argument has changed state. By default, waitpid() waits only for terminated children, but this behaviour 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.)

The WUNTRACED and WCONTINUED options are only effective if the SA_NOCLDSTOP flag has not been set for the SIGCHLD signal (see sigaction(2)).

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; on error, −1 is returned; if WNOHANG was specified and no child(ren) specified by pid has yet changed state, then 0 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.
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 behaviour of setting SIGCHLD to SIG_IGN unspecified.) 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(2). 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(2), and uses the W*() macros described above to analyse 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);
    }
}

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