1.\" $NetBSD: vfork.2,v 1.21 2002/10/01 18:10:46 wiz 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 39.Sh NAME 40.Nm vfork 41.Nd spawn new process in a virtual memory efficient way 42.Sh LIBRARY 43.Lb libc 44.Sh SYNOPSIS 45.Fd #include \*[Lt]unistd.h\*[Gt] 46.Ft pid_t 47.Fn vfork void 48.Sh DESCRIPTION 49The 50.Nm 51system call creates a new process that does not have a new 52virtual address space, but rather shares address space with the 53parent, thus avoiding potentially expensive copy-on-write operations 54normally associated with creating a new process. 55It is useful when the purpose of 56.Xr fork 2 57would have been to create a new system context for an 58.Xr execve 2 . 59The 60.Nm 61system call differs from 62.Xr fork 2 63in that the child borrows the parent's memory and thread of 64control until a call to 65.Xr execve 2 66or an exit (either by a call to 67.Xr _exit 2 68or abnormally). 69The parent process is suspended while the child is using its resources. 70.Pp 71The 72.Nm 73system call returns 0 in the child's context and (later) the pid 74of the child in the parent's context. 75.Pp 76The 77.Nm 78system call can normally be used just like 79.Xr fork 2 . 80It does not work, however, to return while running in the childs context 81from the procedure that called 82.Fn vfork 83since the eventual return from 84.Fn vfork 85would then return to a no longer existent stack frame. 86Be careful, also, to call 87.Xr _exit 2 88rather than 89.Xr exit 3 90if you can't 91.Xr execve 2 , 92since 93.Xr exit 3 94will flush and close standard I/O channels, and thereby mess up the 95standard I/O data structures 96in the parent process. 97(Even with 98.Xr fork 2 99it is wrong to call 100.Xr exit 3 101since buffered data would then be flushed twice.) 102.Sh RETURN VALUES 103Same as for 104.Xr fork 2 . 105.Sh ERRORS 106Same as for 107.Xr fork 2 . 108.Sh SEE ALSO 109.Xr execve 2 , 110.Xr fork 2 , 111.Xr sigaction 2 , 112.Xr wait 2 113.Sh HISTORY 114The 115.Fn vfork 116function call appeared in 117.Bx 3.0 . 118In 119.Bx 4.4 , 120the semantics were changed to only suspend the parent. 121The original semantics were reintroduced in 122.Nx 1.4 . 123.Sh BUGS 124.\" XXX We reintroduced the original semantics. 125.\" This system call will be eliminated when proper system sharing 126.\" mechanisms are implemented. 127Users should not depend on the memory sharing semantics of 128.Fn vfork 129as other ways of speeding up the fork process may be developed in 130the future. 131.Pp 132To avoid a possible deadlock situation, processes that are children 133in the middle of a 134.Fn vfork 135are never sent 136.Dv SIGTTOU 137or 138.Dv SIGTTIN 139signals; rather, output or 140.Xr ioctl 2 141calls are allowed and input attempts result in an end-of-file indication. 142