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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

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_STOPPED (child
                   stopped by signal); 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)  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 3.15 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/.