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.\" 35.Dd November 16, 2012 36.Dt VFORK 2 37.Os 38.Sh NAME 39.Nm vfork 40.Nd spawn new process in a virtual memory efficient way 41.Sh LIBRARY 42.Lb libc 43.Sh SYNOPSIS 44.In unistd.h 45.Ft pid_t 46.Fn vfork void 47.Sh DESCRIPTION 48.Fn Vfork 49can be used to create new processes without fully copying the address 50space of the old process, which is horrendously inefficient in a paged 51environment. It is useful when the purpose of 52.Xr fork 2 53would have been to create a new system context for an 54.Xr execve 2 . 55.Fn Vfork 56differs from 57.Xr fork 2 58in that the child borrows the parent's memory and thread of 59control until a call to 60.Xr execve 2 61or an exit (either by a call to 62.Xr _exit 2 63or abnormally). 64The parent process is suspended while the child is using its resources. 65.Pp 66.Fn Vfork 67returns 0 in the child's context and (later) the pid of the child in 68the parent's context. 69.Pp 70.Fn Vfork 71can normally be used just like 72.Xr fork 2 . 73It does not work, however, to return while running in the child's context 74from the procedure that called 75.Fn vfork 76since the eventual return from 77.Fn vfork 78would then return to a no longer existent stack frame. 79Be careful, also, to call 80.Xr _exit 2 81rather than 82.Xr exit 3 83if you can't 84.Xr execve 2 , 85since 86.Xr exit 3 87will flush and close standard I/O channels, and thereby mess up the 88parent processes standard I/O data structures. 89(Even with 90.Xr fork 2 91it is wrong to call 92.Xr exit 3 93since buffered data would then be flushed twice.) 94.Sh RETURN VALUES 95Same as for 96.Xr fork 2 . 97.Sh SEE ALSO 98.Xr execve 2 , 99.Xr fork 2 , 100.Xr rfork 2 , 101.Xr sigaction 2 , 102.Xr wait 2 , 103.Xr _exit 2 , 104.Xr exit 3 105.Sh HISTORY 106The 107.Fn vfork 108function call appeared in 109.Bx 2.9 . 110.Sh BUGS 111This system call will be eliminated when proper system sharing 112mechanisms are implemented. 113Users should not depend on the memory 114sharing semantics of 115.Xr vfork 2 116as it will, in that case, be made synonymous to 117.Xr fork 2 . 118.Pp 119To avoid a possible deadlock situation, 120processes that are children in the middle 121of a 122.Fn vfork 123are never sent 124.Dv SIGTTOU 125or 126.Dv SIGTTIN 127signals; rather, 128output or 129.Xr ioctl 2 130calls 131are allowed 132and input attempts result in an end-of-file indication. 133