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. Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" @(#)vfork.2 8.1 (Berkeley) 6/4/93 29.\" $FreeBSD$ 30.\" 31.Dd May 22, 2016 32.Dt VFORK 2 33.Os 34.Sh NAME 35.Nm vfork 36.Nd create a new process without copying the address space 37.Sh LIBRARY 38.Lb libc 39.Sh SYNOPSIS 40.In unistd.h 41.Ft pid_t 42.Fn vfork void 43.Sh DESCRIPTION 44.Bf -symbolic 45Since this function is hard to use correctly from application software, 46it is recommended to use 47.Xr posix_spawn 3 48or 49.Xr fork 2 50instead. 51.Ef 52.Pp 53The 54.Fn vfork 55system call 56can be used to create new processes without fully copying the address 57space of the old process, which is inefficient in a paged 58environment. 59It is useful when the purpose of 60.Xr fork 2 61would have been to create a new system context for an 62.Xr execve 2 . 63The 64.Fn vfork 65system call 66differs from 67.Xr fork 2 68in that the child borrows the parent process's address space and the 69calling thread's stack 70until a call to 71.Xr execve 2 72or an exit (either by a call to 73.Xr _exit 2 74or abnormally). 75The calling thread is suspended while the child is using its resources. 76Other threads continue to run. 77.Pp 78The 79.Fn vfork 80system call 81returns 0 in the child's context and (later) the pid of the child in 82the parent's context. 83.Pp 84Many problems can occur when replacing 85.Xr fork 2 86with 87.Fn vfork . 88For example, it does not work to return while running in the child's context 89from the procedure that called 90.Fn vfork 91since the eventual return from 92.Fn vfork 93would then return to a no longer existent stack frame. 94Also, changing process state which is partially implemented in user space 95such as signal handlers with 96.Xr libthr 3 97will corrupt the parent's state. 98.Pp 99Be careful, also, to call 100.Xr _exit 2 101rather than 102.Xr exit 3 103if you cannot 104.Xr execve 2 , 105since 106.Xr exit 3 107will flush and close standard I/O channels, and thereby mess up the 108parent processes standard I/O data structures. 109(Even with 110.Xr fork 2 111it is wrong to call 112.Xr exit 3 113since buffered data would then be flushed twice.) 114.Sh RETURN VALUES 115Same as for 116.Xr fork 2 . 117.Sh SEE ALSO 118.Xr _exit 2 , 119.Xr execve 2 , 120.Xr fork 2 , 121.Xr rfork 2 , 122.Xr sigaction 2 , 123.Xr wait 2 , 124.Xr exit 3 , 125.Xr posix_spawn 3 126.Sh HISTORY 127The 128.Fn vfork 129system call appeared in 130.Bx 3 . 131.Sh BUGS 132To avoid a possible deadlock situation, 133processes that are children in the middle 134of a 135.Fn vfork 136are never sent 137.Dv SIGTTOU 138or 139.Dv SIGTTIN 140signals; rather, 141output or 142.Xr ioctl 2 143calls 144are allowed 145and input attempts result in an end-of-file indication. 146