.\" Hey Emacs! This file is -*- nroff -*- source.
.\"
.\" (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
.\"
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one
.\" 
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date.  The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein.  The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\" 
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\" License.
.\"
.\" Modified Sat Jul 24 13:30:06 1993 by Rik Faith <faith@cs.unc.edu>
.\" Modified Sun Aug 21 17:42:42 1994 by Rik Faith <faith@cs.unc.edu>
.\"          (Thanks to Koen Holtman <koen@win.tue.nl>)
.\" Modified Wed May 17 15:54:12 1995 by Rik Faith <faith@cs.unc.edu>
.\"           To remove *'s from status in macros (Thanks to Michael Shields).
.\" Modified as suggested by Nick Duffek <nsd@bbc.com>, aeb, 960426
.\" Modified Mon Jun 23 14:09:52 1997 by aeb - add EINTR.
.\" Modified Thu Nov 26 02:12:45 1998 by aeb - add SIGCHLD stuff.
.\"
.TH WAIT 2  "23 June 1997" "Linux" "Linux Programmer's Manual"
.SH NAME
wait, waitpid \- wait for process termination
.SH SYNOPSIS
.B #include <sys/types.h>
.br
.B #include <sys/wait.h>
.sp
.BI "pid_t wait(int *" "status" ")"
.br
.BI "pid_t waitpid(pid_t " pid ", int *" status ", int " options );
.SH DESCRIPTION
The
.B 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
.B waitpid
function suspends execution of the current process until a
child as specified by the
.I 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
.I 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
.I pid
can be one of:
.IP "< \-1"
which means to wait for any child process whose process group ID is
equal to the absolute value of
.IR pid .
.IP \-1
which means to wait for any child process; this is the same
behaviour which
.B wait
exhibits.
.IP 0
which means to wait for any child process whose process group ID is
equal to that of the calling process.
.IP "> 0"
which means to wait for the child whose process ID is equal to the
value of
.IR pid .
.PP
The value of
.I options
is an OR of zero or more of the following constants:
.TP
.B WNOHANG
which means to return immediately if no child has exited.
.TP
.B WUNTRACED
which means to also return for children which are stopped, and whose
status has not been reported.
.PP
If
.I status
is not
.BR NULL ,
.B wait
or
.B waitpid
store status information in the location pointed to by
.IR status .

This status can be evaluated with the following macros (these macros take
the stat buffer (an \fBint\fR) as an argument \(em not a pointer to the
buffer!):
.TP
.BI WIFEXITED( status )
is non\-zero if the child exited normally.
.TP
.BI 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
.B exit()
or as the argument for a
.B return
statement in the main program.  This macro can only be evaluated if
.B WIFEXITED
returned non\-zero.
.TP
.BI WIFSIGNALED( status )
returns true if the child process exited because of a signal which was
not caught.
.TP
.BI WTERMSIG( status )
returns the number of the signal that caused the child process to
terminate. This macro can only be evaluated if
.B WIFSIGNALED
returned non\-zero.
.TP
.BI 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
.BR WUNTRACED .
.TP
.BI WSTOPSIG( status )
returns the number of the signal which caused the child to stop.  This
macro can only be evaluated if
.B WIFSTOPPED
returned non\-zero.
.SH "RETURN VALUE"
The process ID of the child which exited, \-1 on error or zero if
.B WNOHANG
was used and no child was available (in which case,
.I errno
is set to an appropriate value).
.SH "ERRORS"
.TP
.B ECHILD
if the process specified in
.I 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.)
.TP
.B EINVAL
if the
.I options
argument was invalid.
.TP
.B ERESTARTSYS
if
.B WNOHANG
was not set and an unblocked signal or a
.B SIGCHLD
was caught. This error is returned by the system call.
The library interface is not allowed to return
.BR ERESTARTSYS ,
but will return
.BR EINTR .
.SH "NOTES"
The Single Unix Specification describes a flag SA_NOCLDWAIT (not present
under Linux) such that if either this flag is set, or the action for
SIGCHLD is set to SIG_IGN (which, by the way, is not allowed by POSIX),
then children that exit do not become zombies and a call to
.I wait()
or
.I waitpid()
will block until all children have exited, and then fail with
.I errno
set to ECHILD.
.SH "CONFORMING TO"
SVr4, POSIX.1
.SH "SEE ALSO"
.BR signal (2),
.BR wait4 (2),
.BR signal (7)