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. It 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.Fn vfork 59system call 60differs from 61.Xr fork 2 62in that the child borrows the parent's memory and thread of 63control until a call to 64.Xr execve 2 65or an exit (either by a call to 66.Xr _exit 2 67or abnormally). 68The parent process is suspended while the child is using its resources. 69.Pp 70The 71.Fn vfork 72system call 73returns 0 in the child's context and (later) the pid of the child in 74the parent's context. 75.Pp 76The 77.Fn vfork 78system call 79can normally be used just like 80.Xr fork 2 . 81It does not work, however, to return while running in the child's context 82from the procedure that called 83.Fn vfork 84since the eventual return from 85.Fn vfork 86would then return to a no longer existent stack frame. 87Be careful, also, to call 88.Xr _exit 2 89rather than 90.Xr exit 3 91if you can't 92.Xr execve 2 , 93since 94.Xr exit 3 95will flush and close standard I/O channels, and thereby mess up the 96parent processes standard I/O data structures. 97(Even with 98.Xr fork 2 99it is wrong to call 100.Xr exit 3 101since buffered data would then be flushed twice.) 102.Sh SEE ALSO 103.Xr execve 2 , 104.Xr fork 2 , 105.Xr rfork 2 , 106.Xr sigvec 2 , 107.Xr wait 2 , 108.Xr _exit 2 , 109.Xr exit 3 110.Sh RETURN VALUES 111Same as for 112.Xr fork 2 . 113.Sh BUGS 114This system call will be eliminated when proper system sharing 115mechanisms are implemented. 116Users should not depend on the memory 117sharing semantics of 118.Fn vfork 119as it will, in that case, be made synonymous to 120.Xr fork 2 . 121.Pp 122To avoid a possible deadlock situation, 123processes that are children in the middle 124of a 125.Fn vfork 126are never sent 127.Dv SIGTTOU 128or 129.Dv SIGTTIN 130signals; rather, 131output or 132.Xr ioctl 2 133calls 134are allowed 135and input attempts result in an end-of-file indication. 136.Sh HISTORY 137The 138.Fn vfork 139system call appeared in 140.Bx 2.9 . 141