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	1	.\" $NetBSD: vfork.2,v 1.6 1995/02/27 12:39:30 cgd Exp $
	2	.\"
	3	.\" Copyright (c) 1980, 1991, 1993
	4	.\" The Regents of the University of California. All rights reserved.
	5	.\"
	6	.\" Redistribution and use in source and binary forms, with or without
	7	.\" modification, are permitted provided that the following conditions
	8	.\" are met:
	9	.\" 1. Redistributions of source code must retain the above copyright
	10	.\" notice, this list of conditions and the following disclaimer.
	11	.\" 2. Redistributions in binary form must reproduce the above copyright
	12	.\" notice, this list of conditions and the following disclaimer in the
	13	.\" documentation and/or other materials provided with the distribution.
	14	.\" 3. All advertising materials mentioning features or use of this software
	15	.\" must display the following acknowledgement:
	16	.\" This product includes software developed by the University of
	17	.\" California, Berkeley and its contributors.
	18	.\" 4. Neither the name of the University nor the names of its contributors
	19	.\" may be used to endorse or promote products derived from this software
	20	.\" without specific prior written permission.
	21	.\"
	22	.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
	23	.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
	24	.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
	25	.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
	26	.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
	27	.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
	28	.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
	29	.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
	30	.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
	31	.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
	32	.\" SUCH DAMAGE.
	33	.\"
	34	.\" @(#)vfork.2 8.1 (Berkeley) 6/4/93
	35	.\"
	36	.Dd June 4, 1993
	37	.Dt VFORK 2
	38	.Os BSD 4
	39	.Sh NAME
	40	.Nm vfork
	41	.Nd spawn new process in a virtual memory efficient way
	42	.Sh SYNOPSIS
	43	.Fd #include <unistd.h>
	44	.Ft pid_t
	45	.Fo vfork
	46	.Fa void
	47	.Fc
	48	.Sh DESCRIPTION
	49	.Fn Vfork
	50	can be used to create new processes without fully copying the address
	51	space of the old process, which is horrendously inefficient in a paged
	52	environment. It is useful when the purpose of
	53	.Xr fork 2
	54	would have been to create a new system context for an
	55	.Xr execve .
	56	.Fn Vfork
	57	differs from
	58	.Xr fork
	59	in that the child borrows the parent's memory and thread of
	60	control until a call to
	61	.Xr execve 2
	62	or an exit (either by a call to
	63	.Xr exit 2
	64	or abnormally.)
	65	The parent process is suspended while the child is using its resources.
	66	.Pp
	67	.Fn Vfork
	68	returns 0 in the child's context and (later) the pid of the child in
	69	the parent's context.
	70	.Pp
	71	.Fn Vfork
	72	can normally be used just like
	73	.Xr fork .
	74	It does not work, however, to return while running in the childs context
	75	from the procedure that called
	76	.Fn vfork
	77	since the eventual return from
	78	.Fn vfork
	79	would then return to a no longer existent stack frame.
	80	Be careful, also, to call
	81	.Xr _exit
	82	rather than
	83	.Xr exit
	84	if you can't
	85	.Xr execve ,
	86	since
	87	.Xr exit
	88	will flush and close standard I/O channels, and thereby mess up the
	89	parent processes standard I/O data structures.
	90	(Even with
	91	.Xr fork
	92	it is wrong to call
	93	.Xr exit
	94	since buffered data would then be flushed twice.)
	95	.Sh SEE ALSO
	96	.Xr execve 2 ,
	97	.Xr fork 2 ,
	98	.Xr sigaction 2 ,
	99	.Xr wait 2
	100	.Sh ERRORS
	101	The
	102	.Fn vfork
	103	system call will fail for any of the reasons described
	104	in the
	105	.Xr fork
	106	man page.
	107	In addition, it will fail if:
	108	.Bl -tag -width Er
	109	.\" ===========
	110	.It Bq Er EINVAL
	111	A system call other than
	112	.Xr _exit()
	113	or
	114	.Xr execve()
	115	(or libc functions that make no system calls other than those)
	116	is called following calling a
	117	.Fn vfork
	118	call.
	119	.El
	120	.Sh BUGS
	121	This system call will be eliminated when proper system sharing
	122	mechanisms are implemented.
	123	Users should not depend on the memory
	124	sharing semantics of
	125	.Xr vfork
	126	as it will, in that case, be made synonymous to
	127	.Xr fork .
	128	.Pp
	129	To avoid a possible deadlock situation,
	130	processes that are children in the middle
	131	of a
	132	.Fn vfork
	133	are never sent
	134	.Dv SIGTTOU
	135	or
	136	.Dv SIGTTIN
	137	signals; rather,
	138	output or
	139	.Xr ioctl 2
	140	calls
	141	are allowed
	142	and input attempts result in an end-of-file indication.
	143	.Sh HISTORY
	144	The
	145	.Fn vfork
	146	function call appeared in
	147	.Bx 3.0 .