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

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.\"     @(#)vfork.2	8.1 (Berkeley) 6/4/93
.\"	$Id: vfork.2,v 1.5 1996/12/23 23:09:41 wosch Exp $
.\"
.Dd June 4, 1993
.Dt VFORK 2
.Os BSD 4
.Sh NAME
.Nm vfork
.Nd spawn new process in a virtual memory efficient way
.Sh SYNOPSIS
.Fd #include <unistd.h>
.Ft int
.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 3
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 childs 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 SEE ALSO
.Xr execve 2 ,
.Xr fork 2 ,
.Xr sigvec 2 ,
.Xr wait 2
.Sh RETURN VALUES
Same as for
.Xr fork 2 .
.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
.Xr vfork 2
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.
.Sh HISTORY
The
.Fn vfork
function call appeared in
.Bx 3.0 .