wait, waitpid, waitid - wait for process to change state

#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
                   NOTES for information on the raw system call. */

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

    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_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.

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)). 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.

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) 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 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 to indicate the error.

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 <stdint.h>
#include <stdio.h>
#include <stdlib.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);
    }
}

_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)