wait

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



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

SYNOPSIS
       #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

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 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.

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; 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.

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; see signal(7).

       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(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 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.

   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
       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
       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 <sys/wait.h>
       #include <stdint.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 %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);
           }
       }

SEE ALSO
       _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)

COLOPHON
       This page is part of release 5.09 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
       https://www.kernel.org/doc/man-pages/.



Linux                              2020-11-01                            WAIT(2)