freebsd-skq/lib/libc/sys/vfork.2

.\" 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.
.\" 4. 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
.\" $FreeBSD$
.\"
.Dd June 4, 1993
.Dt VFORK 2
.Os
.Sh NAME
.Nm vfork
.Nd spawn new process in a virtual memory efficient way
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft pid_t
.Fn vfork void
.Sh DESCRIPTION
The
.Fn vfork
system call
can be used to create new processes without fully copying the address
space of the old process, which is horrendously inefficient in a paged
environment.
It is useful when the purpose of
.Xr fork 2
would have been to create a new system context for an
.Xr execve 2 .
The
.Fn vfork
system call
differs from
.Xr fork 2
in that the child borrows the parent's memory and thread of
control until a call to
.Xr execve 2
or an exit (either by a call to
.Xr _exit 2
or abnormally).
The parent process is suspended while the child is using its resources.
.Pp
The
.Fn vfork
system call
returns 0 in the child's context and (later) the pid of the child in
the parent's context.
.Pp
The
.Fn vfork
system call
can normally be used just like
.Xr fork 2 .
It does not work, however, to return while running in the child's context
from the procedure that called
.Fn vfork
since the eventual return from
.Fn vfork
would then return to a no longer existent stack frame.
Be careful, also, to call
.Xr _exit 2
rather than
.Xr exit 3
if you cannot
.Xr execve 2 ,
since
.Xr exit 3
will flush and close standard I/O channels, and thereby mess up the
parent processes standard I/O data structures.
(Even with
.Xr fork 2
it is wrong to call
.Xr exit 3
since buffered data would then be flushed twice.)
.Sh RETURN VALUES
Same as for
.Xr fork 2 .
.Sh SEE ALSO
.Xr execve 2 ,
.Xr _exit 2 ,
.Xr fork 2 ,
.Xr rfork 2 ,
.Xr sigvec 2 ,
.Xr wait 2 ,
.Xr exit 3
.Sh HISTORY
The
.Fn vfork
system call appeared in
.Bx 2.9 .
.Sh BUGS
This system call will be eliminated when proper system sharing
mechanisms are implemented.
Users should not depend on the memory
sharing semantics of
.Fn vfork
as it will, in that case, be made synonymous to
.Xr fork 2 .
.Pp
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.