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. 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 December 1, 2017 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, 57except for descriptors returned by 58.Xr kqueue 2 , 59which are not inherited from the parent process. 60These descriptors reference the same underlying objects, so that, 61for instance, file pointers in file objects are shared between 62the child and the parent, so that an 63.Xr lseek 2 64on a descriptor in the child process can affect a subsequent 65.Xr read 2 66or 67.Xr write 2 68by the parent. 69This descriptor copying is also used by the shell to 70establish standard input and output for newly created processes 71as well as to set up pipes. 72.It 73The child process' resource utilizations 74are set to 0; see 75.Xr setrlimit 2 . 76.It 77All interval timers are cleared; see 78.Xr setitimer 2 . 79.It 80The child process has only one thread, 81corresponding to the calling thread in the parent process. 82If the process has more than one thread, 83locks and other resources held by the other threads are not released 84and therefore only async-signal-safe functions 85(see 86.Xr sigaction 2 ) 87are guaranteed to work in the child process until a call to 88.Xr execve 2 89or a similar function. 90.El 91.Sh RETURN VALUES 92Upon successful completion, 93.Fn fork 94returns a value 95of 0 to the child process and returns the process ID of the child 96process to the parent process. 97Otherwise, a value of -1 is returned 98to the parent process, no child process is created, and the global 99variable 100.Va errno 101is set to indicate the error. 102.Sh ERRORS 103The 104.Fn fork 105system call will fail and no child process will be created if: 106.Bl -tag -width Er 107.It Bq Er EAGAIN 108The system-imposed limit on the total 109number of processes under execution would be exceeded. 110The limit is given by the 111.Xr sysctl 3 112MIB variable 113.Dv KERN_MAXPROC . 114(The limit is actually ten less than this 115except for the super user). 116.It Bq Er EAGAIN 117The user is not the super user, and 118the system-imposed limit 119on the total number of 120processes under execution by a single user would be exceeded. 121The limit is given by the 122.Xr sysctl 3 123MIB variable 124.Dv KERN_MAXPROCPERUID . 125.It Bq Er EAGAIN 126The user is not the super user, and 127the soft resource limit corresponding to the 128.Fa resource 129argument 130.Dv RLIMIT_NPROC 131would be exceeded (see 132.Xr getrlimit 2 ) . 133.It Bq Er ENOMEM 134There is insufficient swap space for the new process. 135.El 136.Sh SEE ALSO 137.Xr execve 2 , 138.Xr rfork 2 , 139.Xr setitimer 2 , 140.Xr setrlimit 2 , 141.Xr sigaction 2 , 142.Xr vfork 2 , 143.Xr wait 2 144.Sh HISTORY 145The 146.Fn fork 147function appeared in 148.At v1 . 149