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.\" from: @(#)fork.2 6.5 (Berkeley) 3/10/91 33.\" $Id: fork.2,v 1.2 1993/08/01 07:43:10 mycroft Exp $ 34.\" 35.Dd March 10, 1991 36.Dt FORK 2 37.Os BSD 4.0 38.Sh NAME 39.Nm fork 40.Nd create a new process 41.Sh SYNOPSIS 42.Fd #include <unistd.h> 43.Ft pid_t 44.Fn fork void 45.Sh DESCRIPTION 46.Fn Fork 47causes 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 64or 65.Xr write 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 processes resource utilizations 72are set to 0; see 73.Xr setrlimit 2 . 74.El 75.Sh RETURN VALUES 76Upon successful completion, 77.Fn fork 78returns a value 79of 0 to the child process and returns the process ID of the child 80process to the parent process. Otherwise, a value of -1 is returned 81to the parent process, no child process is created, and the global 82variable 83.Va errno 84is set to indicate the error. 85.Sh ERRORS 86.Fn Fork 87will fail and no child process will be created if: 88.Bl -tag -width [EAGAIN] 89.It Bq Er EAGAIN 90The system-imposed limit on the total 91number of processes under execution would be exceeded. 92This limit is configuration-dependent. 93.It Bq Er EAGAIN 94The system-imposed limit 95.Dv MAXUPRC 96.Pq Aq Pa sys/param.h 97on the total number of 98processes under execution by a single user would be exceeded. 99.It Bq Er ENOMEM 100There is insufficient swap space for the new process. 101.El 102.Sh SEE ALSO 103.Xr execve 2 , 104.Xr wait 2 105.Sh HISTORY 106A 107.Nm 108function call appeared in Version 6 AT&T UNIX. 109