1.\" $NetBSD: vfork.2,v 1.12 1998/06/07 04:56:21 enami 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 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 47The 48.Nm 49system call creates a new process that that does not have a new 50virtual address space, but rather shares address space with the 51parent, thus avoiding potentially expensive copy-on-write operations 52normally associated with creating a new process. 53It is useful when the purpose of 54.Xr fork 2 55would have been to create a new system context for an 56.Xr execve 2 . 57The 58.Nm 59system call differs from 60.Xr fork 2 61in that the child borrows the parent's memory and thread of 62control until a call to 63.Xr execve 2 64or an exit (either by a call to 65.Xr _exit 2 66or abnormally). 67The parent process is suspended while the child is using its resources. 68.Pp 69The 70.Nm 71system call returns 0 in the child's context and (later) the pid 72of the child in the parent's context. 73.Pp 74The 75.Nm 76system call can normally be used just like 77.Xr fork 2 . 78It does not work, however, to return while running in the childs context 79from the procedure that called 80.Fn vfork 81since the eventual return from 82.Fn vfork 83would then return to a no longer existent stack frame. 84Be careful, also, to call 85.Xr _exit 2 86rather than 87.Xr exit 3 88if you can't 89.Xr execve 2 , 90since 91.Xr exit 3 92will flush and close standard I/O channels, and thereby mess up the 93parent processes standard I/O data structures (even with 94.Xr fork 2 95it is wrong to call 96.Xr exit 3 97since buffered data would then be flushed twice). 98.Sh RETURN VALUES and ERRORS 99Same as for 100.Xr fork 2 . 101.Sh SEE ALSO 102.Xr fork 2 , 103.Xr execve 2 , 104.Xr sigaction 2 , 105.Xr wait 2 106.Sh BUGS 107.\" XXX We reintroduced the original semantics. 108.\" This system call will be eliminated when proper system sharing 109.\" mechanisms are implemented. 110Users should not depend on the memory sharing semantics of 111.Xr vfork 2 112as other ways of speeding up the fork process may be developed in 113the future. 114.Pp 115To avoid a possible deadlock situation, processes that are children 116in the middle of a 117.Fn vfork 118are never sent 119.Dv SIGTTOU 120or 121.Dv SIGTTIN 122signals; rather, output or 123.Xr ioctl 2 124calls are allowed and input attempts result in an end-of-file indication. 125.Sh HISTORY 126The 127.Fn vfork 128function call appeared in 129.Bx 3.0 . 130In 131.Bx 4.4 , 132the semantics were changed to only suspend the parent. 133The original semantics were reintroduced in 134.Nx 1.4 . 135