1.\" $NetBSD: vfork.2,v 1.10 1998/01/04 04:20:42 thorpej Exp $ 2.\" 3.\" Copyright (c) 1980, 1991, 1993 4.\" The Regents of the University of California. All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 3. All advertising materials mentioning features or use of this software 15.\" must display the following acknowledgement: 16.\" This product includes software developed by the University of 17.\" California, Berkeley and its contributors. 18.\" 4. Neither the name of the University nor the names of its contributors 19.\" may be used to endorse or promote products derived from this software 20.\" without specific prior written permission. 21.\" 22.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 23.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 24.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 25.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 26.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 27.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 28.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 29.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 30.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 31.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 32.\" SUCH DAMAGE. 33.\" 34.\" @(#)vfork.2 8.1 (Berkeley) 6/4/93 35.\" 36.Dd January 3, 1998 37.Dt VFORK 2 38.Os BSD 4 39.Sh NAME 40.Nm vfork 41.Nd spawn new process in a virtual memory efficient way 42.Sh SYNOPSIS 43.Fd #include <unistd.h> 44.Ft pid_t 45.Fn vfork void 46.Sh DESCRIPTION 47.Fn Vfork 48creates a new process that that does not have a new virtual address space, 49but rather shares address space with the parent, thus avoiding potentially 50expensive copy-on-write operations normally associated with creating a 51new process. 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 childs 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 , 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 SEE ALSO 95.Xr fork 2 , 96.Xr execve 2 , 97.Xr sigaction 2 , 98.Xr wait 2 99.Sh DIAGNOSTICS 100Same as for 101.Xr fork 2 . 102.Sh BUGS 103.\" XXX We reintroduced the original semantics. 104.\" This system call will be eliminated when proper system sharing 105.\" mechanisms are implemented. 106Users should not depend on the memory 107sharing semantics of 108.Xr vfork 109as other ways of speeding up the fork process may be developed in 110the future. 111.Pp 112To avoid a possible deadlock situation, 113processes that are children in the middle 114of a 115.Fn vfork 116are never sent 117.Dv SIGTTOU 118or 119.Dv SIGTTIN 120signals; rather, 121output or 122.Xr ioctl 2 123calls 124are allowed 125and input attempts result in an end-of-file indication. 126.Sh HISTORY 127The 128.Fn vfork 129function call appeared in 130.Bx 3.0 . 131In 132.Bx 4.4 , 133the semantics were changed to only suspend the parent. 134The original semantics were reintroduced in 135.Nx 1.4 . 136