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 May 31, 2013 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 unistd.h 41.Ft pid_t 42.Fn fork void 43.Sh DESCRIPTION 44The 45.Fn fork 46system call causes 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.It 77The child process has only one thread, 78corresponding to the calling thread in the parent process. 79If the process has more than one thread, 80locks and other resources held by the other threads are not released 81and therefore only async-signal-safe functions 82(see 83.Xr sigaction 2 ) 84are guaranteed to work in the child process until a call to 85.Xr execve 2 86or a similar function. 87.El 88.Sh RETURN VALUES 89Upon successful completion, 90.Fn fork 91returns a value 92of 0 to the child process and returns the process ID of the child 93process to the parent process. 94Otherwise, a value of -1 is returned 95to the parent process, no child process is created, and the global 96variable 97.Va errno 98is set to indicate the error. 99.Sh ERRORS 100The 101.Fn fork 102system call will fail and no child process will be created if: 103.Bl -tag -width Er 104.It Bq Er EAGAIN 105The system-imposed limit on the total 106number of processes under execution would be exceeded. 107The limit is given by the 108.Xr sysctl 3 109MIB variable 110.Dv KERN_MAXPROC . 111(The limit is actually ten less than this 112except for the super user). 113.It Bq Er EAGAIN 114The user is not the super user, and 115the system-imposed limit 116on the total number of 117processes under execution by a single user would be exceeded. 118The limit is given by the 119.Xr sysctl 3 120MIB variable 121.Dv KERN_MAXPROCPERUID . 122.It Bq Er EAGAIN 123The user is not the super user, and 124the soft resource limit corresponding to the 125.Fa resource 126argument 127.Dv RLIMIT_NPROC 128would be exceeded (see 129.Xr getrlimit 2 ) . 130.It Bq Er ENOMEM 131There is insufficient swap space for the new process. 132.El 133.Sh SEE ALSO 134.Xr execve 2 , 135.Xr rfork 2 , 136.Xr setitimer 2 , 137.Xr setrlimit 2 , 138.Xr sigaction 2 , 139.Xr vfork 2 , 140.Xr wait 2 141.Sh HISTORY 142The 143.Fn fork 144function appeared in 145.At v6 . 146