1.\" $OpenBSD: fork.2,v 1.13 2003/06/02 20:18:39 millert Exp $ 2.\" $NetBSD: fork.2,v 1.6 1995/02/27 12:32:36 cgd Exp $ 3.\" 4.\" Copyright (c) 1980, 1991, 1993 5.\" The Regents of the University of California. All rights reserved. 6.\" 7.\" Redistribution and use in source and binary forms, with or without 8.\" modification, are permitted provided that the following conditions 9.\" are met: 10.\" 1. Redistributions of source code must retain the above copyright 11.\" notice, this list of conditions and the following disclaimer. 12.\" 2. Redistributions in binary form must reproduce the above copyright 13.\" notice, this list of conditions and the following disclaimer in the 14.\" documentation and/or other materials provided with the distribution. 15.\" 3. Neither the name of the University nor the names of its contributors 16.\" may be used to endorse or promote products derived from this software 17.\" without specific prior written permission. 18.\" 19.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 20.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 21.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 22.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 23.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 24.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 25.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 26.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 27.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 28.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 29.\" SUCH DAMAGE. 30.\" 31.\" @(#)fork.2 8.1 (Berkeley) 6/4/93 32.\" 33.Dd June 4, 1993 34.Dt FORK 2 35.Os 36.Sh NAME 37.Nm fork 38.Nd create a new process 39.Sh SYNOPSIS 40.Fd #include <sys/types.h> 41.Fd #include <unistd.h> 42.Ft pid_t 43.Fn fork void 44.Sh DESCRIPTION 45.Fn fork 46causes creation of a new process. 47The new process (child process) is an exact copy of the 48calling process (parent process) except for the following: 49.Bl -bullet -offset indent 50.It 51The child process has a unique process ID. 52.It 53The child process has a different parent 54process ID (i.e., the process ID of the parent process). 55.It 56The child process has its own copy of the parent's descriptors. 57These descriptors reference the same underlying objects, so that, 58for instance, file pointers in file objects are shared between 59the child and the parent, so that an 60.Xr lseek 2 61on a descriptor in the child process can affect a subsequent 62.Xr read 2 63or 64.Xr write 2 65by the parent. 66This descriptor copying is also used by the shell to 67establish standard input and output for newly created processes 68as well as to set up pipes. 69.It 70The child process' resource utilizations 71are set to 0; see 72.Xr setrlimit 2 . 73.It 74All interval timers are cleared; see 75.Xr setitimer 2 . 76.El 77.Pp 78In general, the child process should call 79.Xr _exit 2 80rather than 81.Xr exit 3 . 82Otherwise, any stdio buffers that exist both in the parent and child 83will be flushed twice. 84Similarly, 85.Xr _exit 2 86should be used to prevent 87.Xr atexit 3 88routines from being called twice (once in the parent and once in the child). 89.Sh RETURN VALUES 90Upon successful completion, 91.Fn fork 92returns a value 93of 0 to the child process and returns the process ID of the child 94process to the parent process. 95Otherwise, a value of \-1 is returned to the parent process, 96no child process is created, and the global variable 97.Va errno 98is set to indicate the error. 99.Sh ERRORS 100.Fn fork 101will fail and no child process will be created if: 102.Bl -tag -width [EAGAIN] 103.It Bq Er EAGAIN 104The system-imposed limit on the total 105number of processes under execution would be exceeded. 106This limit is configuration-dependent. 107.It Bq Er EAGAIN 108The limit 109.Dv RLIMIT_NPROC 110on the total number of processes under execution by the user ID 111would be exceeded. 112.It Bq Er ENOMEM 113There is insufficient swap space for the new process. 114.El 115.Sh SEE ALSO 116.Xr execve 2 , 117.Xr setrlimit 2 , 118.Xr wait 2 119.Sh STANDARDS 120The 121.Fn fork 122function conforms to 123.St -p1003.1-90 . 124.Sh HISTORY 125A 126.Fn fork 127function call appeared in 128.At v2 . 129