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RedHat 9 (Linux i386) - man page for waitpid (redhat section 2)

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  termi-
       nate  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  behaviour  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 constants:

       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 evalu-
	      ated 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 WCORE-
       DUMP(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  supported  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 construct from a process.
       Instead, a thread is simply a process that is created using the Linux-unique clone(2) sys-
       tem 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 con-
       sequence 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 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.

CONFORMING TO
       SVr4, POSIX.1

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

Linux					    2000-07-24					  WAIT(2)


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