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.\" 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.\" @(#)vfork.2 8.1 (Berkeley) 6/4/93 33.\" $FreeBSD$ 34.\" 35.Dd June 4, 1993 36.Dt VFORK 2 37.Os 38.Sh NAME 39.Nm vfork 40.Nd spawn new process in a virtual memory efficient way 41.Sh LIBRARY 42.Lb libc 43.Sh SYNOPSIS 44.In unistd.h 45.Ft pid_t 46.Fn vfork void 47.Sh DESCRIPTION 48The 49.Fn vfork 50system call 51can be used to create new processes without fully copying the address 52space of the old process, which is horrendously inefficient in a paged 53environment. 54It is useful when the purpose of 55.Xr fork 2 56would have been to create a new system context for an 57.Xr execve 2 . 58The 59.Fn vfork 60system call 61differs 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.Fn vfork 73system call 74returns 0 in the child's context and (later) the pid of the child in 75the parent's context. 76.Pp 77The 78.Fn vfork 79system call 80can normally be used just like 81.Xr fork 2 . 82It does not work, however, to return while running in the child's context 83from the procedure that called 84.Fn vfork 85since the eventual return from 86.Fn vfork 87would then return to a no longer existent stack frame. 88Be careful, also, to call 89.Xr _exit 2 90rather than 91.Xr exit 3 92if you cannot 93.Xr execve 2 , 94since 95.Xr exit 3 96will flush and close standard I/O channels, and thereby mess up the 97parent processes standard I/O data structures. 98(Even with 99.Xr fork 2 100it is wrong to call 101.Xr exit 3 102since buffered data would then be flushed twice.) 103.Sh RETURN VALUES 104Same as for 105.Xr fork 2 . 106.Sh SEE ALSO 107.Xr execve 2 , 108.Xr _exit 2 , 109.Xr fork 2 , 110.Xr rfork 2 , 111.Xr sigvec 2 , 112.Xr wait 2 , 113.Xr exit 3 114.Sh HISTORY 115The 116.Fn vfork 117system call appeared in 118.Bx 2.9 . 119.Sh BUGS 120This system call will be eliminated when proper system sharing 121mechanisms are implemented. 122Users should not depend on the memory 123sharing semantics of 124.Fn vfork 125as it will, in that case, be made synonymous to 126.Xr fork 2 . 127.Pp 128To avoid a possible deadlock situation, 129processes that are children in the middle 130of a 131.Fn vfork 132are never sent 133.Dv SIGTTOU 134or 135.Dv SIGTTIN 136signals; rather, 137output or 138.Xr ioctl 2 139calls 140are allowed 141and input attempts result in an end-of-file indication. 142