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

       wait, waitpid, waitid - wait for process to change state

       #include <sys/types.h>
       #include <sys/wait.h>

       pid_t wait(int *wstatus);

       pid_t waitpid(pid_t pid, int *wstatus, int options);

       int waitid(idtype_t idtype, id_t id, siginfo_t *infop, int options);
		       /* This is the glibc and POSIX interface; see
			  NOTES for information on the raw system call. */

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

	       || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
	       || /* Glibc versions <= 2.19: */ _BSD_SOURCE

       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

   wait() and waitpid()
       The wait() system call suspends execution of the calling process	 until
       one  of its children terminates.	 The call wait(&wstatus) is equivalent

	   waitpid(-1, &wstatus, 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-

       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 wstatus 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()!):

	      returns true if the child terminated normally, that is, by call-
	      ing exit(3) or _exit(2), or by returning from main().

	      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 be employed
	      only if WIFEXITED returned true.

	      returns true if the child process was terminated by a signal.

	      returns the number of the signal that caused the	child  process
	      to terminate.  This macro should be employed only if WIFSIGNALED
	      returned true.

	      returns true if the child produced  a  core  dump.   This	 macro
	      should be employed only 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).   Therefore,
	      enclose its use inside #ifdef WCOREDUMP ... #endif.

	      returns  true  if the child process was stopped by delivery of a
	      signal; this is possible only if the call was  done  using  WUN-
	      TRACED or when the child is being traced (see ptrace(2)).

	      returns the number of the signal which caused the child to stop.
	      This macro should be employed only if WIFSTOPPED returned true.

	      (since Linux 2.6.10) returns  true  if  the  child  process  was
	      resumed by delivery of SIGCONT.

       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-

       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

       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.

       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

       Each  of	 these calls sets errno to an appropriate value in the case of
       an error.

       ECHILD (for wait()) The calling process does not have any  unwaited-for

       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.

       SVr4, 4.3BSD, POSIX.1-2001.

       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(1), which automatically performs a wait to  remove  the

       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 the POSIX requirements.  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
       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 can also, since Linux 4.7, be used with waitid():

	      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-

       __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.

       Since Linux 4.7, the __WALL flag is automatically implied if the	 child
       is being ptraced.

   C library/kernel differences
       wait() is actually a library function that (in glibc) is implemented as
       a call to wait4(2).

       On some architectures, there is no waitpid() system call; instead, this
       interface  is  implemented  via a C library wrapper function that calls

       The raw waitid() system call takes a fifth  argument,  of  type	struct
       rusage *.   If  this  argument  is  non-NULL, then it is used to return
       resource usage information about the  child,  in	 the  same  manner  as
       wait4(2).  See getrusage(2) for details.

       According  to POSIX.1-2008, an application calling waitid() must ensure
       that infop points to a siginfo_t structure (i.e., that it is a non-null
       pointer).   On  Linux, if infop is NULL, waitid() succeeds, and returns
       the process ID of the  waited-for  child.   Applications	 should	 avoid
       relying on this inconsistent, nonstandard, and unnecessary feature.

       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
	   $ 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>

       main(int argc, char *argv[])
	   pid_t cpid, w;
	   int wstatus;

	   cpid = fork();
	   if (cpid == -1) {

	   if (cpid == 0) {	       /* Code executed by child */
	       printf("Child PID is %ld\n", (long) getpid());
	       if (argc == 1)
		   pause();		       /* Wait for signals */

	   } else {		       /* Code executed by parent */
	       do {
		   w = waitpid(cpid, &wstatus, WUNTRACED | WCONTINUED);
		   if (w == -1) {

		   if (WIFEXITED(wstatus)) {
		       printf("exited, status=%d\n", WEXITSTATUS(wstatus));
		   } else if (WIFSIGNALED(wstatus)) {
		       printf("killed by signal %d\n", WTERMSIG(wstatus));
		   } else if (WIFSTOPPED(wstatus)) {
		       printf("stopped by signal %d\n", WSTOPSIG(wstatus));
		   } else if (WIFCONTINUED(wstatus)) {
	       } while (!WIFEXITED(wstatus) && !WIFSIGNALED(wstatus));

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

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Linux				  2016-12-12			       WAIT(2)