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. Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" @(#)vfork.2 8.1 (Berkeley) 6/4/93 29.\" $FreeBSD: src/lib/libc/sys/vfork.2,v 1.12.2.6 2002/12/29 16:35:34 schweikh Exp $ 30.\" 31.Dd November 16, 2012 32.Dt VFORK 2 33.Os 34.Sh NAME 35.Nm vfork 36.Nd spawn new process in a virtual memory efficient way 37.Sh LIBRARY 38.Lb libc 39.Sh SYNOPSIS 40.In unistd.h 41.Ft pid_t 42.Fn vfork void 43.Sh DESCRIPTION 44.Fn Vfork 45can be used to create new processes without fully copying the address 46space of the old process, which is horrendously inefficient in a paged 47environment. It is useful when the purpose of 48.Xr fork 2 49would have been to create a new system context for an 50.Xr execve 2 . 51.Fn Vfork 52differs from 53.Xr fork 2 54in that the child borrows the parent's memory and thread of 55control until a call to 56.Xr execve 2 57or an exit (either by a call to 58.Xr _exit 2 59or abnormally). 60The parent process is suspended while the child is using its resources. 61.Pp 62.Fn Vfork 63returns 0 in the child's context and (later) the pid of the child in 64the parent's context. 65.Pp 66.Fn Vfork 67can normally be used just like 68.Xr fork 2 . 69It does not work, however, to return while running in the child's context 70from the procedure that called 71.Fn vfork 72since the eventual return from 73.Fn vfork 74would then return to a no longer existent stack frame. 75Be careful, also, to call 76.Xr _exit 2 77rather than 78.Xr exit 3 79if you can't 80.Xr execve 2 , 81since 82.Xr exit 3 83will flush and close standard I/O channels, and thereby mess up the 84parent processes standard I/O data structures. 85(Even with 86.Xr fork 2 87it is wrong to call 88.Xr exit 3 89since buffered data would then be flushed twice.) 90.Sh RETURN VALUES 91Same as for 92.Xr fork 2 . 93.Sh SEE ALSO 94.Xr execve 2 , 95.Xr fork 2 , 96.Xr rfork 2 , 97.Xr sigaction 2 , 98.Xr wait 2 , 99.Xr _exit 2 , 100.Xr exit 3 101.Sh HISTORY 102The 103.Fn vfork 104function call appeared in 105.Bx 2.9 . 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.Fn vfork 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