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.Dd May 22, 2016 29.Dt VFORK 2 30.Os 31.Sh NAME 32.Nm vfork 33.Nd create a new process without copying the address space 34.Sh LIBRARY 35.Lb libc 36.Sh SYNOPSIS 37.In unistd.h 38.Ft pid_t 39.Fn vfork void 40.Sh DESCRIPTION 41.Bf -symbolic 42Since this function is hard to use correctly from application software, 43it is recommended to use 44.Xr posix_spawn 3 45or 46.Xr fork 2 47instead. 48.Ef 49.Pp 50The 51.Fn vfork 52system call 53can be used to create new processes without fully copying the address 54space of the old process, which is inefficient in a paged 55environment. 56It is useful when the purpose of 57.Xr fork 2 58would have been to create a new system context for an 59.Xr execve 2 . 60The 61.Fn vfork 62system call 63differs from 64.Xr fork 2 65in that the child borrows the parent process's address space and the 66calling thread's stack 67until a call to 68.Xr execve 2 69or an exit (either by a call to 70.Xr _exit 2 71or abnormally). 72The calling thread is suspended while the child is using its resources. 73Other threads continue to run. 74.Pp 75The 76.Fn vfork 77system call 78returns 0 in the child's context and (later) the pid of the child in 79the parent's context. 80.Pp 81Many problems can occur when replacing 82.Xr fork 2 83with 84.Fn vfork . 85For example, it does not work to return while running in the child's context 86from the procedure that called 87.Fn vfork 88since the eventual return from 89.Fn vfork 90would then return to a no longer existent stack frame. 91Also, changing process state which is partially implemented in user space 92such as signal handlers with 93.Xr libthr 3 94will corrupt the parent's state. 95.Pp 96Be careful, also, to call 97.Xr _exit 2 98rather than 99.Xr exit 3 100if you cannot 101.Xr execve 2 , 102since 103.Xr exit 3 104will flush and close standard I/O channels, and thereby mess up the 105parent processes standard I/O data structures. 106(Even with 107.Xr fork 2 108it is wrong to call 109.Xr exit 3 110since buffered data would then be flushed twice.) 111.Sh RETURN VALUES 112Same as for 113.Xr fork 2 . 114.Sh SEE ALSO 115.Xr _exit 2 , 116.Xr execve 2 , 117.Xr fork 2 , 118.Xr rfork 2 , 119.Xr sigaction 2 , 120.Xr wait 2 , 121.Xr exit 3 , 122.Xr posix_spawn 3 123.Sh HISTORY 124The 125.Fn vfork 126system call appeared in 127.Bx 3 . 128.Sh BUGS 129To avoid a possible deadlock situation, 130processes that are children in the middle 131of a 132.Fn vfork 133are never sent 134.Dv SIGTTOU 135or 136.Dv SIGTTIN 137signals; rather, 138output or 139.Xr ioctl 2 140calls 141are allowed 142and input attempts result in an end-of-file indication. 143