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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 *status);

       pid_t waitpid(pid_t pid, int *status, int options);

       int waitid(idtype_t idtype, id_t id, siginfo_t *infop, int options);

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

       waitid():
           _SVID_SOURCE || _XOPEN_SOURCE >= 500 ||
           _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
           || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L

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 process until
       one of its children terminates.  The call wait(&status)  is  equivalent
       to:

           waitpid(-1, &status, 0);

       The  waitpid()  system  call  suspends execution of the calling process
       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 status 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(status)
              returns true if the  child  terminated  normally,  that  is,  by
              calling exit(3) or _exit(2), or by returning from main().

       WEXITSTATUS(status)
              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  only  be
              employed if WIFEXITED returned true.

       WIFSIGNALED(status)
              returns true if the child process was terminated by a signal.

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

       WCOREDUMP(status)
              returns  true  if  the  child  produced a core dump.  This macro
              should only be employed  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).   Only  use  this
              enclosed in #ifdef WCOREDUMP ... #endif.

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

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

       WIFCONTINUED(status)
              (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  is  unspecified.   To
       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.

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(8), which 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 this specification.
       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 cannot 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.

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 status;

           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, &status, WUNTRACED | WCONTINUED);
                   if (w == -1) {
                       perror("waitpid");
                       exit(EXIT_FAILURE);
                   }

                   if (WIFEXITED(status)) {
                       printf("exited, status=%d\n", WEXITSTATUS(status));
                   } else if (WIFSIGNALED(status)) {
                       printf("killed by signal %d\n", WTERMSIG(status));
                   } else if (WIFSTOPPED(status)) {
                       printf("stopped by signal %d\n", WSTOPSIG(status));
                   } else if (WIFCONTINUED(status)) {
                       printf("continued\n");
                   }
               } while (!WIFEXITED(status) && !WIFSIGNALED(status));
               exit(EXIT_SUCCESS);
           }
       }

SEE ALSO

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

COLOPHON

       This  page  is  part of release 3.35 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://man7.org/linux/man-pages/.