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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 *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

       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(&status) is equivalent

	   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‐

       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()!):

	      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 only be
	      employed 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 only be employed if WIFSIGNALED
	      returned true.

	      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.

	      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)).

	      returns the number of the signal which caused the child to stop.
	      This macro should only be employed 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 non-zero 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
       returned.  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(8), 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 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

   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():

	      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.

       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 status;

	   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, &status, WUNTRACED | WCONTINUED);
		   if (w == -1) {

		   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)) {
	       } while (!WIFEXITED(status) && !WIFSIGNALED(status));

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

       This  page  is  part of release 3.22 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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