1.\" $NetBSD: fork.2,v 1.6 1995/02/27 12:32:36 cgd Exp $ 2.\" 3.\" Copyright (c) 1980, 1991, 1993 4.\" The Regents of the University of California. All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 3. All advertising materials mentioning features or use of this software 15.\" must display the following acknowledgement: 16.\" This product includes software developed by the University of 17.\" California, Berkeley and its contributors. 18.\" 4. Neither the name of the University nor the names of its contributors 19.\" may be used to endorse or promote products derived from this software 20.\" without specific prior written permission. 21.\" 22.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 23.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 24.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 25.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 26.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 27.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 28.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 29.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 30.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 31.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 32.\" SUCH DAMAGE. 33.\" 34.\" @(#)fork.2 8.1 (Berkeley) 6/4/93 35.\" 36.Dd June 4, 1993 37.Dt FORK 2 38.Os BSD 4 39.Sh NAME 40.Nm fork 41.Nd create a new process 42.Sh SYNOPSIS 43.Fd #include <unistd.h> 44.Ft pid_t 45.Fo fork 46.Fa void 47.Fc 48.Sh DESCRIPTION 49.Fn fork 50causes creation of a new process. 51The new process (child process) is an exact copy of the 52calling process (parent process) except for the following: 53.Bl -bullet -offset indent 54.It 55The child process has a unique process ID. 56.It 57The child process has a different parent 58process ID (i.e., the process ID of the parent process). 59.It 60The child process has its own copy of the parent's descriptors. 61These descriptors reference the same underlying objects, so that, 62for instance, file pointers in file objects are shared between 63the child and the parent, so that an 64.Xr lseek 2 65on a descriptor in the child process can affect a subsequent 66.Xr read 67or 68.Xr write 69by the parent. 70This descriptor copying is also used by the shell to 71establish standard input and output for newly created processes 72as well as to set up pipes. 73.It 74The child processes resource utilizations 75are set to 0; see 76.Xr setrlimit 2 . 77.El 78.Sh RETURN VALUES 79Upon successful completion, 80.Fn fork 81returns a value of 0 to the child process 82and returns the process ID of the child process to the parent process. 83Otherwise, a value of -1 is returned to the parent process, 84no child process is created, and the global variable 85.Va errno 86is set to indicate the error. 87.Sh ERRORS 88.Fn fork 89will fail and no child process will be created if: 90.Bl -tag -width Er 91.\" ========== 92.It Bq Er EAGAIN 93The system-imposed limit 94on the total number of processes under execution 95would be exceeded. 96This limit is configuration-dependent. 97.\" ========== 98.It Bq Er EAGAIN 99The system-imposed limit 100.Dv MAXUPRC 101.Pq Aq Pa sys/param.h 102on the total number of 103processes under execution by a single user would be exceeded. 104.\" ========== 105.It Bq Er ENOMEM 106There is insufficient swap space for the new process. 107.El 108.Sh LEGACY SYNOPSIS 109.Fd #include <sys/types.h> 110.Fd #include <unistd.h> 111.Pp 112The include file 113.In sys/types.h 114is necessary. 115.Sh SEE ALSO 116.Xr execve 2 , 117.Xr sigaction 2 , 118.Xr wait 2 , 119.Xr compat 5 120.Sh HISTORY 121A 122.Fn fork 123function call appeared in 124.At v6 . 125.Sh CAVEATS 126There are limits to what you can do in the child process. 127To be totally safe you should restrict yourself to only 128executing async-signal safe operations until such time 129as one of the exec functions is called. All APIs, including 130global data symbols, in any framework or library should be 131assumed to be unsafe after a 132.Fn fork 133unless explicitly documented to be safe or async-signal 134safe. If you need to use these frameworks in the child 135process, you must exec. In this situation it is reasonable 136to exec yourself.