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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 *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

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.  Other-
       wise 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 signifi-
              cant 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 deliv-
              ery of SIGCONT.

   waitid()
       The waitid() system call (available since Linux 2.6.9) provides more  precise  con-
       trol 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 fol-
       lowing 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 deliv-
                   ery 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 non-zero 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) speci-
       fied 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 ker-
       nel maintains a minimal set of information about the zombie process (PID,  termina-
       tion status, resource usage information) in order to allow the parent to later per-
       form 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  pro-
       cesses.   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 termi-
       nate 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  chil-
       dren.)  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" chil-
              dren 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  wait-
       pid(), 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 3.22 of the Linux man-pages project.  A description of
       the  project, and information about reporting bugs, can be found at http://www.ker-
       nel.org/doc/man-pages/.



Linux                             2009-04-21                           WAIT(2)

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