usr/src/lib/libc/sys/vfork.2

.\" Copyright (c) 1980 Regents of the University of California.
.\" All rights reserved.  The Berkeley software License Agreement
.\" specifies the terms and conditions for redistribution.
.\"
.\"	@(#)vfork.2	6.2 (Berkeley) %G%
.\"
.TH VFORK 2 ""
.UC 4
.SH NAME
vfork \- spawn new process in a virtual memory efficient way
.SH SYNOPSIS
.B pid = vfork()
.br
.B int pid;
.SH DESCRIPTION
.I 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
.IR fork (2)
would have been to create a new system context for an
.IR execve .
.I Vfork
differs from
.I fork
in that the child borrows the parent's memory and thread of
control until a call to
.IR execve (2)
or an exit (either by a call to
.IR exit (2)
or abnormally.)
The parent process is suspended while the child is using its resources.
.PP
.I Vfork
returns 0 in the child's context and (later) the pid of the child in
the parent's context.
.PP
.I Vfork
can normally be used just like
.I fork.
It does not work, however, to return while running in the childs context
from the procedure that called
.I vfork
since the eventual return from
.I vfork
would then return to a no longer existent stack frame.
Be careful, also, to call
.I _exit
rather than
.I exit
if you can't
.IR execve ,
since
.I exit
will flush and close standard I/O channels, and thereby mess up the
parent processes standard I/O data structures.
(Even with
.I fork
it is wrong to call
.I exit
since buffered data would then be flushed twice.)
.SH SEE ALSO
fork(2), execve(2), sigvec(2), wait(2),
.SH DIAGNOSTICS
Same as for
.IR fork .
.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
.I vfork
as it will, in that case, be made synonymous to
.IR fork .
.PP
To avoid a possible deadlock situation,
processes that are children in the middle
of a
.I vfork
are never sent SIGTTOU or SIGTTIN signals; rather,
output or
.IR ioctl s
are allowed
and input attempts result in an end-of-file indication.
Commit	Line	Data
	1	.\" Copyright (c) 1980 Regents of the University of California.
	2	.\" All rights reserved. The Berkeley software License Agreement
	3	.\" specifies the terms and conditions for redistribution.
	4	.\"
	5	.\" @(#)vfork.2 6.2 (Berkeley) %G%
	6	.\"
	7	.TH VFORK 2 ""
	8	.UC 4
	9	.SH NAME
	10	vfork \- spawn new process in a virtual memory efficient way
	11	.SH SYNOPSIS
	12	.B pid = vfork()
	13	.br
	14	.B int pid;
	15	.SH DESCRIPTION
	16	.I Vfork
	17	can be used to create new processes without fully copying the address
	18	space of the old process, which is horrendously inefficient in a paged
	19	environment. It is useful when the purpose of
	20	.IR fork (2)
	21	would have been to create a new system context for an
	22	.IR execve .
	23	.I Vfork
	24	differs from
	25	.I fork
	26	in that the child borrows the parent's memory and thread of
	27	control until a call to
	28	.IR execve (2)
	29	or an exit (either by a call to
	30	.IR exit (2)
	31	or abnormally.)
	32	The parent process is suspended while the child is using its resources.
	33	.PP
	34	.I Vfork
	35	returns 0 in the child's context and (later) the pid of the child in
	36	the parent's context.
	37	.PP
	38	.I Vfork
	39	can normally be used just like
	40	.I fork.
	41	It does not work, however, to return while running in the childs context
	42	from the procedure that called
	43	.I vfork
	44	since the eventual return from
	45	.I vfork
	46	would then return to a no longer existent stack frame.
	47	Be careful, also, to call
	48	.I _exit
	49	rather than
	50	.I exit
	51	if you can't
	52	.IR execve ,
	53	since
	54	.I exit
	55	will flush and close standard I/O channels, and thereby mess up the
	56	parent processes standard I/O data structures.
	57	(Even with
	58	.I fork
	59	it is wrong to call
	60	.I exit
	61	since buffered data would then be flushed twice.)
	62	.SH SEE ALSO
	63	fork(2), execve(2), sigvec(2), wait(2),
	64	.SH DIAGNOSTICS
	65	Same as for
	66	.IR fork .
	67	.SH BUGS
	68	This system call will be eliminated when proper system sharing
	69	mechanisms are implemented.
	70	Users should not depend on the memory
	71	sharing semantics of
	72	.I vfork
	73	as it will, in that case, be made synonymous to
	74	.IR fork .
	75	.PP
	76	To avoid a possible deadlock situation,
	77	processes that are children in the middle
	78	of a
	79	.I vfork
	80	are never sent SIGTTOU or SIGTTIN signals; rather,
	81	output or
	82	.IR ioctl s
	83	are allowed
	84	and input attempts result in an end-of-file indication.