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

NAME
       wait, waitpid - wait for	process	termination

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

       pid_t wait(int *status);
       pid_t waitpid(pid_t pid,	int *status, int options);

DESCRIPTION
       The  wait  function  suspends  execution	of the current process until a
       child has exited, or until a signal is delivered	 whose	action	is  to
       terminate  the  current	process	or to call a signal handling function.
       If a child has already exited by	the time  of  the  call	 (a  so-called
       "zombie"	 process),  the	 function  returns  immediately.   Any	system
       resources used by the child are freed.

       The waitpid function suspends execution of the current process until  a
       child as	specified by the pid argument has exited, or until a signal is
       delivered whose action is to terminate the current process or to	call a
       signal  handling	 function.  If a child as requested by pid has already
       exited by the time of the call  (a  so-called  "zombie"	process),  the
       function	 returns  immediately.	Any system resources used by the child
       are freed.

       The value of pid	can be one of:

       < -1   which means to wait for any child	process	whose process group ID
	      is equal to the absolute value of	pid.

       -1     which  means to wait for any child process; this is the same be-
	      haviour which wait exhibits.

       0      which means to wait for any child	process	whose process group ID
	      is equal to that of the calling process.

       > 0    which  means  to 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
	      which means to return immediately	if no child has	exited.

       WUNTRACED
	      which  means  to also return for children	which are stopped, and
	      whose status has not been	reported.

       (For Linux-only options,	see below.)

       If status is not	NULL, wait or waitpid store status information in  the
       location	pointed	to by status.

       This  status  can  be evaluated with the	following macros (these	macros
       take the	stat buffer (an	int) as	an argument -- not a  pointer  to  the
       buffer!):

       WIFEXITED(status)
	      is non-zero if the child exited normally.

       WEXITSTATUS(status)
	      evaluates	to the least significant eight bits of the return code
	      of the child which terminated, which may have been  set  as  the
	      argument	to  a  call  to	exit() or as the argument for a	return
	      statement	in the main program.  This macro can only be evaluated
	      if WIFEXITED returned non-zero.

       WIFSIGNALED(status)
	      returns  true  if	 the  child process exited because of a	signal
	      which was	not caught.

       WTERMSIG(status)
	      returns the number of the	signal that caused the	child  process
	      to  terminate.  This  macro can only be evaluated	if WIFSIGNALED
	      returned non-zero.

       WIFSTOPPED(status)
	      returns true if the child	process	which  caused  the  return  is
	      currently	 stopped;  this	 is only possible if the call was done
	      using WUNTRACED.

       WSTOPSIG(status)
	      returns the number of the	signal which caused the	child to stop.
	      This   macro  can	 only  be  evaluated  if  WIFSTOPPED  returned
	      non-zero.

       Some versions of	Unix (e.g. Linux, Solaris, but not  AIX,  SunOS)  also
       define  a  macro	 WCOREDUMP(status)  to	test whether the child process
       dumped core. Only use this enclosed in #ifdef WCOREDUMP ... #endif.

RETURN VALUE
       The process ID of the child which exited, or zero if WNOHANG  was  used
       and  no child was available, or -1 on error (in which case errno	is set
       to an appropriate value).

ERRORS
       ECHILD if the process specified in pid 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.)

       EINVAL if the options argument was invalid.

       EINTR  if  WNOHANG was not set and an unblocked signal or a SIGCHLD was
	      caught.

NOTES
       The Single Unix Specification describes a flag SA_NOCLDWAIT  (not  sup-
       ported under Linux) such	that if	either this flag is set, or the	action
       for SIGCHLD is set to SIG_IGN then children that	 exit  do  not	become
       zombies and a call to wait() or waitpid() will block until all children
       have exited, and	then fail with errno set to ECHILD.

       The original POSIX standard left	the behaviour of  setting  SIGCHLD  to
       SIG_IGN	unspecified.   Later  standards,  including  SUSv2  and	 POSIX
       1003.1-2001 specify the behaviour just described	as  an	XSI-compliance
       option.	 Linux	does  not conform to the second	of the two points just
       described: if a wait() or waitpid() call	is made	while SIGCHLD is being
       ignored,	the call behaves just as though	SIGCHLD	were not being igored,
       that is,	the call blocks	until  the  next  child	 terminates  and  then
       returns the PID 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).

       __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.

CONFORMING TO
       SVr4, POSIX.1

SEE ALSO
       clone(2), signal(2), wait4(2), pthread_create(3), signal(7)

Linux				  2000-07-24			       WAIT(2)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | NOTES | LINUX NOTES | CONFORMING TO | SEE ALSO

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