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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 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.

       > 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  con-
       stants:

       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 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 idtype and id arguments select the child(ren) to wait for, as  fol-
       lows:

       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 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 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) speci-
       fied 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 (wait-
              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 pro-
       cess, 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 termi-
              nation.)  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 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 */
               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), sig-
       nal(2), wait4(2), pthread_create(3), credentials(7), signal(7)

COLOPHON
       This page is part of release 3.25 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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