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.\" 4. 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.\" @(#)fork.2 8.1 (Berkeley) 6/4/93 29.\" $FreeBSD$ 30.\" 31.Dd June 4, 1993 32.Dt FORK 2 33.Os 34.Sh NAME 35.Nm fork 36.Nd create a new process 37.Sh LIBRARY 38.Lb libc 39.Sh SYNOPSIS 40.In sys/types.h 41.In unistd.h 42.Ft pid_t 43.Fn fork void 44.Sh DESCRIPTION 45The 46.Fn fork 47system call causes creation of a new process. 48The new process (child process) is an exact copy of the 49calling process (parent process) except for the following: 50.Bl -bullet -offset indent 51.It 52The child process has a unique process ID. 53.It 54The child process has a different parent 55process ID (i.e., the process ID of the parent process). 56.It 57The child process has its own copy of the parent's descriptors. 58These descriptors reference the same underlying objects, so that, 59for instance, file pointers in file objects are shared between 60the child and the parent, so that an 61.Xr lseek 2 62on a descriptor in the child process can affect a subsequent 63.Xr read 2 64or 65.Xr write 2 66by the parent. 67This descriptor copying is also used by the shell to 68establish standard input and output for newly created processes 69as well as to set up pipes. 70.It 71The child process' resource utilizations 72are set to 0; see 73.Xr setrlimit 2 . 74.It 75All interval timers are cleared; see 76.Xr setitimer 2 . 77.El 78.Sh RETURN VALUES 79Upon successful completion, 80.Fn fork 81returns a value 82of 0 to the child process and returns the process ID of the child 83process to the parent process. 84Otherwise, a value of -1 is returned 85to the parent process, no child process is created, and the global 86variable 87.Va errno 88is set to indicate the error. 89.Sh ERRORS 90The 91.Fn fork 92system call will fail and no child process will be created if: 93.Bl -tag -width Er 94.It Bq Er EAGAIN 95The system-imposed limit on the total 96number of processes under execution would be exceeded. 97The limit is given by the 98.Xr sysctl 3 99MIB variable 100.Dv KERN_MAXPROC . 101(The limit is actually ten less than this 102except for the super user). 103.It Bq Er EAGAIN 104The user is not the super user, and 105the system-imposed limit 106on the total number of 107processes under execution by a single user would be exceeded. 108The limit is given by the 109.Xr sysctl 3 110MIB variable 111.Dv KERN_MAXPROCPERUID . 112.It Bq Er EAGAIN 113The user is not the super user, and 114the soft resource limit corresponding to the 115.Fa resource 116argument 117.Dv RLIMIT_NPROC 118would be exceeded (see 119.Xr getrlimit 2 ) . 120.It Bq Er ENOMEM 121There is insufficient swap space for the new process. 122.El 123.Sh SEE ALSO 124.Xr execve 2 , 125.Xr rfork 2 , 126.Xr setitimer 2 , 127.Xr setrlimit 2 , 128.Xr vfork 2 , 129.Xr wait 2 130.Sh HISTORY 131The 132.Fn fork 133function appeared in 134.At v6 . 135