wait(2) — Linux manual page


wait(2)                    System Calls Manual                   wait(2)

NAME         top

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

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #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

           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 <= 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

       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

              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 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()!):

              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

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

              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 ...

              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

              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

              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 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.

RETURN VALUE         top

       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.

ERRORS         top

       EAGAIN The PID file descriptor specified in id is nonblocking and
              the process that it refers to has not terminated.

       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.

       ESRCH  (for wait() or waitpid()) pid is equal to INT_MIN.

VERSIONS         top

   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.

STANDARDS         top


HISTORY         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

       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 waitid():

              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

       __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.

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 <stdint.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <sys/wait.h>
       #include <unistd.h>

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

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

           if (cpid == 0) {            /* Code executed by child */
               printf("Child PID is %jd\n", (intmax_t) 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),

Linux man-pages (unreleased)     (date)                          wait(2)

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