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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.

       > 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_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_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.

EXAMPLE

       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 <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 %ld\n", (long) 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 5.02 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/.