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.\" 4. 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.\" $FreeBSD$ 30.\" 31.Dd April 6, 2015 32.Dt VFORK 2 33.Os 34.Sh NAME 35.Nm vfork 36.Nd create a new process without copying the address space 37.Sh LIBRARY 38.Lb libc 39.Sh SYNOPSIS 40.In unistd.h 41.Ft pid_t 42.Fn vfork void 43.Sh DESCRIPTION 44The 45.Fn vfork 46system call 47can be used to create new processes without fully copying the address 48space of the old process, which is horrendously inefficient in a paged 49environment. 50It is useful when the purpose of 51.Xr fork 2 52would have been to create a new system context for an 53.Xr execve 2 . 54The 55.Fn vfork 56system call 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 67The 68.Fn vfork 69system call 70returns 0 in the child's context and (later) the pid of the child in 71the parent's context. 72.Pp 73The 74.Fn vfork 75system call 76can normally be used just like 77.Xr fork 2 . 78It does not work, however, to return while running in the child's context 79from the procedure that called 80.Fn vfork 81since the eventual return from 82.Fn vfork 83would then return to a no longer existent stack frame. 84Be careful, also, to call 85.Xr _exit 2 86rather than 87.Xr exit 3 88if you cannot 89.Xr execve 2 , 90since 91.Xr exit 3 92will flush and close standard I/O channels, and thereby mess up the 93parent processes standard I/O data structures. 94(Even with 95.Xr fork 2 96it is wrong to call 97.Xr exit 3 98since buffered data would then be flushed twice.) 99.Sh RETURN VALUES 100Same as for 101.Xr fork 2 . 102.Sh SEE ALSO 103.Xr execve 2 , 104.Xr _exit 2 , 105.Xr fork 2 , 106.Xr rfork 2 , 107.Xr sigaction 2 , 108.Xr wait 2 , 109.Xr exit 3 110.Sh HISTORY 111The 112.Fn vfork 113system call appeared in 114.Bx 3 . 115.Sh BUGS 116To avoid a possible deadlock situation, 117processes that are children in the middle 118of a 119.Fn vfork 120are never sent 121.Dv SIGTTOU 122or 123.Dv SIGTTIN 124signals; rather, 125output or 126.Xr ioctl 2 127calls 128are allowed 129and input attempts result in an end-of-file indication. 130