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.\" $OpenBSD: vfork.2,v 1.20 2015/09/10 17:55:21 schwarze Exp $
.\" $NetBSD: vfork.2,v 1.8 1997/07/10 07:54:13 mikel Exp $
.\"
.\" Copyright (c) 1980, 1991, 1993
.\" The Regents of the University of California. All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\" 3. Neither the name of the University nor the names of its contributors
.\" may be used to endorse or promote products derived from this software
.\" without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\" @(#)vfork.2 8.1 (Berkeley) 6/4/93
.\"
.Dd $Mdocdate: September 10 2015 $
.Dt VFORK 2
.Os
.Sh NAME
.Nm vfork
.Nd spawn new process and block parent
.Sh SYNOPSIS
.In unistd.h
.Ft pid_t
.Fn vfork void
.Sh DESCRIPTION
.Fn vfork
was originally used to create new processes without fully copying the address
space of the old process, which is horrendously inefficient in a paged
environment.
It was useful when the purpose of
.Xr fork 2
would have been to create a new system context for an
.Xr execve 2 .
Since
.Xr fork 2
is now efficient, even in the above case, the need for
.Fn vfork
has diminished.
.Fn vfork
differs from
.Xr fork 2
in that the parent is suspended until the child makes a call to
.Xr execve 2
or an exit (either by a call to
.Xr _exit 2
or abnormally).
In addition, fork handlers established using
.Xr pthread_atfork 3
are not called when a multithreaded program calls
.Fn vfork .
.Pp
.Fn vfork
returns 0 in the child's context and (later) the PID of the child in
the parent's context.
.Sh RETURN VALUES
Same as for
.Xr fork 2 .
.Sh SEE ALSO
.Xr execve 2 ,
.Xr fork 2 ,
.Xr sigaction 2 ,
.Xr wait 2
.Sh HISTORY
The
.Fn vfork
function call appeared in
.Bx 3.0
with the additional semantics that the child process ran in the
memory of the parent until it called
.Xr execve 2
or exited.
That sharing of memory was removed in
.Bx 4.4 ,
leaving just the semantics of blocking the parent until the child calls
.Xr execve 2
or exits.
On many other systems the original behavior has been restored,
making this interface particularly unportable.
.Sh BUGS
To avoid a possible deadlock situation, processes that are children
in the middle of a
.Fn vfork
are never sent
.Dv SIGTTOU
or
.Dv SIGTTIN
signals; rather, output or
.Xr ioctl 2
calls are allowed and input attempts result in an end-of-file indication.
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