waitpid(2)



WAIT(2)                    Linux Programmer's Manual                   WAIT(2)

NAME
       wait, waitpid - 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);

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 terminated the 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 current 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  current  process
       until a child specified by pid argument has changed state.  By default,
       waitpid() waits only for terminated children,  but  this  behaviour  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  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.)

       The  WUNTRACED  and  WCONTINUED  options  are  only  effective  if  the
       SA_NOCLDSTOP  flag  has not been set for the SIGCHLD signal (see sigac-
       tion(2)).

       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() or _exit() 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  sig-
              nal.

       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 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; on error, -1 is returned; if WNOHANG was specified and no
       child(ren) specified by pid has yet changed state, then 0 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.

       EINVAL The options argument was invalid.

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
       behaviour of setting SIGCHLD to SIG_IGN unspecified.)  Linux  2.6  con-
       forms  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 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(2).
       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(2),  and  uses  the  W*() macros described above to analyse 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
       $

       #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);
           }
       }

CONFORMING TO
       SVr4, 4.3BSD, POSIX.1-2001.

SEE ALSO
       _exit(2), clone(2), fork(2),  kill(2),  ptrace(2),  sigaction(2),  sig-
       nal(2), wait4(2), pthread_create(3), signal(7)

Linux                             2004-11-11                           WAIT(2)

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