wait(2) — Linux manual page


WAIT(2)                   Linux Programmer's Manual                  WAIT(2)

NAME         top

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

SYNOPSIS         top

       #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)):

           Since glibc 2.26: _XOPEN_SOURCE >= 500 ||
               _POSIX_C_SOURCE >= 200809L
           Glibc 2.25 and earlier:
                   || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
                   || /* Glibc versions <= 2.19: */ _BSD_SOURCE

DESCRIPTION         top

       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 con‐

              return immediately if no child has exited.

              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

       (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()!):

              returns true if the child terminated normally, that is, by
              calling exit(3) or _exit(2), or by returning from main().

              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.

              returns true if the child process was terminated by a signal.

              returns the number of the signal that caused the child process
              to terminate.  This macro should be employed only if WIFSIG‐
              NALED returned true.

              returns true if the child produced a core dump (see core(5)).
              This macro should be employed only if WIFSIGNALED returned

              This macro is not specified in POSIX.1-2001 and is not avail‐
              able on some UNIX implementations (e.g., AIX, SunOS).  There‐
              fore, enclose its use inside #ifdef WCOREDUMP ... #endif.

              returns true if the child process was stopped by delivery of a
              signal; this is possible only if the call was done using WUN‐
              TRACED or when the child is being traced (see ptrace(2)).

              returns the number of the signal which caused the child to
              stop.  This macro should be employed only if WIFSTOPPED
              returned true.

              (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

       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:

              Wait for children that have terminated.

              Wait for children that have been stopped by delivery of a sig‐

              Wait for (previously stopped) children that have been resumed
              by delivery of SIGCONT.

       The following flags may additionally be ORed in options:

              As for waitpid().

              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.)

              Always set to SIGCHLD.

              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.

              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 implemen‐
       tation.  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 implementa‐
       tions follow the POSIX.1 specification on this point.

RETURN VALUE         top

       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

       Each of these calls sets errno to an appropriate value in the case of
       an error.

ERRORS         top

       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         top

       SVr4, 4.3BSD, POSIX.1-2001.

NOTES         top

       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.

   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 can also, since Linux 4.7, be used with

              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-

       __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.

   C library/kernel differences
       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.

BUGS         top

       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.

EXAMPLES         top

       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
           $ 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>

       main(int argc, char *argv[])
           pid_t cpid, w;
           int wstatus;

           cpid = fork();
           if (cpid == -1) {

           if (cpid == 0) {            /* Code executed by child */
               printf("Child PID is %ld\n", (long) getpid());
               if (argc == 1)
                   pause();                    /* Wait for signals */

           } else {                    /* Code executed by parent */
               do {
                   w = waitpid(cpid, &wstatus, WUNTRACED | WCONTINUED);
                   if (w == -1) {

                   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)) {
               } while (!WIFEXITED(wstatus) && !WIFSIGNALED(wstatus));

SEE ALSO         top

       _exit(2), clone(2), fork(2), kill(2), ptrace(2), sigaction(2),
       signal(2), wait4(2), pthread_create(3), core(5), credentials(7),

COLOPHON         top

       This page is part of release 5.08 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                            2020-04-11                          WAIT(2)

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