man-pages/man2/wait.2

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.\" (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
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.\" 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.
.\" Modified Mon Jul 24 21:37:38 2000 by David A. Wheeler
.\"          <dwheeler@dwheeler.com> - noted thread issues.
.\" Modified 26 Jun 01 by Michael Kerrisk
.\"          Added __WCLONE, __WALL, and __WNOTHREAD descriptions
.\" Modified 2001-09-25, aeb
.\" Modified 26 Jun 01 by Michael Kerrisk, <mtk-manpages@gmx.net>
.\"	Updated notes on setting disposition of SIGCHLD to SIG_IGN
.\"
.TH WAIT 2  2000-07-24 "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
(but not traced), and whose status has not been reported.
Status for traced children which are stopped is provided
also without this option.
.PP
(For Linux-only options, see below.)
.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 )
returns true if the child terminated normally, that is,
by calling exit() or _exit(), or by returning from main().
.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 exit() or _exit() or as the argument for a return statement
in the main program.  This macro can only be evaluated if
.B WIFEXITED
returned true.
.TP
.BI WIFSIGNALED( status )
returns true if the child process terminated 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
or when the child is being traced (see
.BR ptrace (2)).
.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.
.LP
Some versions of Unix (e.g. Linux, Solaris, but not AIX, SunOS)
also define a macro
.BI WCOREDUMP( status )
to test whether the child process dumped core. Only use this
enclosed in #ifdef WCOREDUMP ... #endif.
.SH "RETURN VALUE"
The process ID of the child which exited, or zero if
.B WNOHANG
was used and no child was available, or \-1 on error (in which case
.I errno
is set to an appropriate value).
.SH ERRORS
.TP
.BR ECHILD " (for " wait )
The calling process does not have any unwaited-for children.
.TP
.BR ECHILD " (for " waitpid )
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. See also the LINUX NOTES section about threads.)
.TP
.B EINTR
.B WNOHANG
was not set and an unblocked signal or a
.B SIGCHLD
was caught.
.TP
.B EINVAL
The
.I options
argument was invalid.
.SH 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
.BR wait ()
or
.BR waitpid ()
will block until all children have exited, and then fail with
.I errno
set to ECHILD.
.LP
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
.BR 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.
.SH "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
.BR clone (2)
system call; other routines such as the portable
.BR pthread_create (3)
call are implemented using
.BR 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.
.LP
The following Linux-specific
.I options
are for use with children created using
.BR clone (2).
.TP
.B __WCLONE
.\" since 0.99pl10
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
.B SIGCHLD
to its parent upon termination.)
This option is ignored if
.B __WALL
is also specified.
.TP
.B __WALL
.\" since patch-2.3.48
(Since Linux 2.4) Wait for all children, regardless of
type ("clone" or "non-clone").
.TP
.B __WNOTHREAD
.\" since patch-2.4.0-test8
(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.
.SH "CONFORMING TO"
SVr4, POSIX.1
.SH "SEE ALSO"
.BR clone (2),
.BR ptrace (2),
.BR signal (2),
.BR wait4 (2),
.BR pthread_create (3),
.BR signal (7)