1.\" Copyright (c) 1980, 1991, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 3. All advertising materials mentioning features or use of this software 13.\" must display the following acknowledgement: 14.\" This product includes software developed by the University of 15.\" California, Berkeley and its contributors. 16.\" 4. Neither the name of the University nor the names of its contributors 17.\" may be used to endorse or promote products derived from this software 18.\" without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.\" @(#)vfork.2 8.1 (Berkeley) 6/4/93 33.\" $FreeBSD: src/lib/libc/sys/vfork.2,v 1.12.2.6 2002/12/29 16:35:34 schweikh Exp $ 34.\" $DragonFly: src/lib/libc/sys/vfork.2,v 1.2 2003/06/17 04:26:47 dillon Exp $ 35.\" 36.Dd June 4, 1993 37.Dt VFORK 2 38.Os 39.Sh NAME 40.Nm vfork 41.Nd spawn new process in a virtual memory efficient way 42.Sh LIBRARY 43.Lb libc 44.Sh SYNOPSIS 45.In unistd.h 46.Ft int 47.Fn vfork void 48.Sh DESCRIPTION 49.Fn Vfork 50can be used to create new processes without fully copying the address 51space of the old process, which is horrendously inefficient in a paged 52environment. It is useful when the purpose of 53.Xr fork 2 54would have been to create a new system context for an 55.Xr execve 2 . 56.Fn Vfork 57differs from 58.Xr fork 2 59in that the child borrows the parent's memory and thread of 60control until a call to 61.Xr execve 2 62or an exit (either by a call to 63.Xr _exit 2 64or abnormally). 65The parent process is suspended while the child is using its resources. 66.Pp 67.Fn Vfork 68returns 0 in the child's context and (later) the pid of the child in 69the parent's context. 70.Pp 71.Fn Vfork 72can normally be used just like 73.Xr fork 2 . 74It does not work, however, to return while running in the child's context 75from the procedure that called 76.Fn vfork 77since the eventual return from 78.Fn vfork 79would then return to a no longer existent stack frame. 80Be careful, also, to call 81.Xr _exit 2 82rather than 83.Xr exit 3 84if you can't 85.Xr execve 2 , 86since 87.Xr exit 3 88will flush and close standard I/O channels, and thereby mess up the 89parent processes standard I/O data structures. 90(Even with 91.Xr fork 2 92it is wrong to call 93.Xr exit 3 94since buffered data would then be flushed twice.) 95.Sh SEE ALSO 96.Xr execve 2 , 97.Xr fork 2 , 98.Xr rfork 2 , 99.Xr sigvec 2 , 100.Xr wait 2 , 101.Xr _exit 2 , 102.Xr exit 3 103.Sh RETURN VALUES 104Same as for 105.Xr fork 2 . 106.Sh BUGS 107This system call will be eliminated when proper system sharing 108mechanisms are implemented. 109Users should not depend on the memory 110sharing semantics of 111.Xr vfork 2 112as it will, in that case, be made synonymous to 113.Xr fork 2 . 114.Pp 115To avoid a possible deadlock situation, 116processes that are children in the middle 117of a 118.Fn vfork 119are never sent 120.Dv SIGTTOU 121or 122.Dv SIGTTIN 123signals; rather, 124output or 125.Xr ioctl 2 126calls 127are allowed 128and input attempts result in an end-of-file indication. 129.Sh HISTORY 130The 131.Fn vfork 132function call appeared in 133.Bx 2.9 . 134