libc/sys/vfork.2

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.\"     @(#)vfork.2	8.1 (Berkeley) 6/4/93
.\" $FreeBSD: src/lib/libc/sys/vfork.2,v 1.12.2.6 2002/12/29 16:35:34 schweikh Exp $
.\"
.Dd November 16, 2012
.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
.Fn Vfork
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 .
.Fn Vfork
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
.Fn Vfork
returns 0 in the child's context and (later) the pid of the child in
the parent's context.
.Pp
.Fn Vfork
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 can't
.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 fork 2 ,
.Xr rfork 2 ,
.Xr sigaction 2 ,
.Xr wait 2 ,
.Xr _exit 2 ,
.Xr exit 3
.Sh HISTORY
The
.Fn vfork
function 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.