wait


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 ter-
       minated; 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  immedi-
       ately.   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 mod-
       ifiable 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.

       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 call-
              ing 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 WUN-
              TRACED 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 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 termi-
                   nate, 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 distin-
       guish 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) speci-
       fied  by pid exist, but have not yet changed state, then 0 is returned.
              pid())  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 "zom-
       bie".  The kernel maintains a minimal set of information about the zom-
       bie  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  set-
       ting  the disposition to SIG_IGN results in different treatment of zom-
       bie process children.)  Linux 2.6 conforms to this specification.  How-
       ever,  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 con-
       struct 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():
              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  immedi-
       ately,  using the integer supplied on the command line as the exit sta-
       tus.  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 */
                       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),  sig-
       nal(2), wait4(2), pthread_create(3), credentials(7), signal(7)

COLOPHON
       This  page  is  part of release 3.23 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.



Linux                             2009-04-21                           WAIT(2)
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