1.\" Copyright (c) 1980, 1991 Regents of the University of California. 2.\" 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.\" from: @(#)vfork.2 6.3 (Berkeley) 3/10/91 33.\" $Id: vfork.2,v 1.4 1993/12/30 19:16:46 jtc Exp $ 34.\" 35.Dd March 10, 1991 36.Dt VFORK 2 37.Os BSD 4 38.Sh NAME 39.Nm vfork 40.Nd spawn new process in a virtual memory efficient way 41.Sh SYNOPSIS 42.Fd #include <unistd.h> 43.Ft pid_t 44.Fn vfork void 45.Sh DESCRIPTION 46.Fn Vfork 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. It is useful when the purpose of 50.Xr fork 2 51would have been to create a new system context for an 52.Xr execve . 53.Fn Vfork 54differs from 55.Xr fork 56in that the child borrows the parent's memory and thread of 57control until a call to 58.Xr execve 2 59or an exit (either by a call to 60.Xr exit 2 61or abnormally.) 62The parent process is suspended while the child is using its resources. 63.Pp 64.Fn Vfork 65returns 0 in the child's context and (later) the pid of the child in 66the parent's context. 67.Pp 68.Fn Vfork 69can normally be used just like 70.Xr fork . 71It does not work, however, to return while running in the childs context 72from the procedure that called 73.Fn vfork 74since the eventual return from 75.Fn vfork 76would then return to a no longer existent stack frame. 77Be careful, also, to call 78.Xr _exit 79rather than 80.Xr exit 81if you can't 82.Xr execve , 83since 84.Xr exit 85will flush and close standard I/O channels, and thereby mess up the 86parent processes standard I/O data structures. 87(Even with 88.Xr fork 89it is wrong to call 90.Xr exit 91since buffered data would then be flushed twice.) 92.Sh SEE ALSO 93.Xr fork 2 , 94.Xr execve 2 , 95.Xr sigvec 2 , 96.Xr wait 2 , 97.Sh DIAGNOSTICS 98Same as for 99.Xr fork . 100.Sh BUGS 101This system call will be eliminated when proper system sharing 102mechanisms are implemented. 103Users should not depend on the memory 104sharing semantics of 105.Xr vfork 106as it will, in that case, be made synonymous to 107.Xr fork . 108.Pp 109To avoid a possible deadlock situation, 110processes that are children in the middle 111of a 112.Fn vfork 113are never sent 114.Dv SIGTTOU 115or 116.Dv SIGTTIN 117signals; rather, 118output or 119.Xr ioctl 2 120calls 121are allowed 122and input attempts result in an end-of-file indication. 123.Sh HISTORY 124The 125.Fn vfork 126function call appeared in 127.Bx 3.0 . 128