1.\" Copyright (c) 1980, 1991 Regents of the University of California. 2.\" 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 6.3 (Berkeley) 3/10/91 33.\" 34.Dd March 10, 1991 35.Dt VFORK 2 36.Os BSD 4 37.Sh NAME 38.Nm vfork 39.Nd spawn new process in a virtual memory efficient way 40.Sh SYNOPSIS 41.Fd #include <unistd.h> 42.Ft int 43.Fn vfork void 44.Sh DESCRIPTION 45.Fn Vfork 46can be used to create new processes without fully copying the address 47space of the old process, which is horrendously inefficient in a paged 48environment. It is useful when the purpose of 49.Xr fork 2 50would have been to create a new system context for an 51.Xr execve . 52.Fn Vfork 53differs from 54.Xr fork 55in that the child borrows the parent's memory and thread of 56control until a call to 57.Xr execve 2 58or an exit (either by a call to 59.Xr exit 2 60or abnormally.) 61The parent process is suspended while the child is using its resources. 62.Pp 63.Fn Vfork 64returns 0 in the child's context and (later) the pid of the child in 65the parent's context. 66.Pp 67.Fn Vfork 68can normally be used just like 69.Xr fork . 70It does not work, however, to return while running in the childs context 71from the procedure that called 72.Fn vfork 73since the eventual return from 74.Fn vfork 75would then return to a no longer existent stack frame. 76Be careful, also, to call 77.Xr _exit 78rather than 79.Xr exit 80if you can't 81.Xr execve , 82since 83.Xr exit 84will flush and close standard I/O channels, and thereby mess up the 85parent processes standard I/O data structures. 86(Even with 87.Xr fork 88it is wrong to call 89.Xr exit 90since buffered data would then be flushed twice.) 91.Sh SEE ALSO 92.Xr fork 2 , 93.Xr execve 2 , 94.Xr sigvec 2 , 95.Xr wait 2 , 96.Sh DIAGNOSTICS 97Same as for 98.Xr fork . 99.Sh BUGS 100This system call will be eliminated when proper system sharing 101mechanisms are implemented. 102Users should not depend on the memory 103sharing semantics of 104.Xr vfork 105as it will, in that case, be made synonymous to 106.Xr fork . 107.Pp 108To avoid a possible deadlock situation, 109processes that are children in the middle 110of a 111.Fn vfork 112are never sent 113.Dv SIGTTOU 114or 115.Dv SIGTTIN 116signals; rather, 117output or 118.Xr ioctl 2 119calls 120are allowed 121and input attempts result in an end-of-file indication. 122.Sh HISTORY 123The 124.Nm 125function call appeared in 126.Bx 3.0 . 127