1.\" $NetBSD: vfork.2,v 1.6 1995/02/27 12:39:30 cgd 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 June 4, 1993 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 48can be used to create new processes without fully copying the address 49space of the old process, which is horrendously inefficient in a paged 50environment. It is useful when the purpose of 51.Xr fork 2 52would have been to create a new system context for an 53.Xr execve . 54.Fn Vfork 55differs from 56.Xr fork 57in that the child borrows the parent's memory and thread of 58control until a call to 59.Xr execve 2 60or an exit (either by a call to 61.Xr exit 2 62or abnormally.) 63The parent process is suspended while the child is using its resources. 64.Pp 65.Fn Vfork 66returns 0 in the child's context and (later) the pid of the child in 67the parent's context. 68.Pp 69.Fn Vfork 70can normally be used just like 71.Xr fork . 72It does not work, however, to return while running in the childs context 73from the procedure that called 74.Fn vfork 75since the eventual return from 76.Fn vfork 77would then return to a no longer existent stack frame. 78Be careful, also, to call 79.Xr _exit 80rather than 81.Xr exit 82if you can't 83.Xr execve , 84since 85.Xr exit 86will flush and close standard I/O channels, and thereby mess up the 87parent processes standard I/O data structures. 88(Even with 89.Xr fork 90it is wrong to call 91.Xr exit 92since buffered data would then be flushed twice.) 93.Sh SEE ALSO 94.Xr fork 2 , 95.Xr execve 2 , 96.Xr sigaction 2 , 97.Xr wait 2 , 98.Sh DIAGNOSTICS 99Same as for 100.Xr fork . 101.Sh BUGS 102This system call will be eliminated when proper system sharing 103mechanisms are implemented. 104Users should not depend on the memory 105sharing semantics of 106.Xr vfork 107as it will, in that case, be made synonymous to 108.Xr fork . 109.Pp 110To avoid a possible deadlock situation, 111processes that are children in the middle 112of a 113.Fn vfork 114are never sent 115.Dv SIGTTOU 116or 117.Dv SIGTTIN 118signals; rather, 119output or 120.Xr ioctl 2 121calls 122are allowed 123and input attempts result in an end-of-file indication. 124.Sh HISTORY 125The 126.Fn vfork 127function call appeared in 128.Bx 3.0 . 129