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.\" Copyright (c) 1980, 1991, 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
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.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" SUCH DAMAGE.
.\"
.\"	@(#)fork.2	8.1 (Berkeley) 6/4/93
.\" $FreeBSD$
.\"
.Dd April 20, 2021
.Dt FORK 2
.Os
.Sh NAME
.Nm fork
.Nd create a new process
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft pid_t
.Fn fork void
.Sh DESCRIPTION
The
.Fn fork
system call causes creation of a new process.
The new process (child process) is an exact copy of the
calling process (parent process) except for the following:
.Bl -bullet -offset indent
.It
The child process has a unique process ID.
.It
The child process has a different parent
process ID (i.e., the process ID of the parent process).
.It
The child process has its own copy of the parent's descriptors,
except for descriptors returned by
.Xr kqueue 2 ,
which are not inherited from the parent process.
These descriptors reference the same underlying objects, so that,
for instance, file pointers in file objects are shared between
the child and the parent, so that an
.Xr lseek 2
on a descriptor in the child process can affect a subsequent
.Xr read 2
or
.Xr write 2
by the parent.
This descriptor copying is also used by the shell to
establish standard input and output for newly created processes
as well as to set up pipes.
.It
The child process' resource utilizations
are set to 0; see
.Xr setrlimit 2 .
.It
All interval timers are cleared; see
.Xr setitimer 2 .
.It
The child process has only one thread,
corresponding to the calling thread in the parent process.
If the process has more than one thread,
locks and other resources held by the other threads are not released
and therefore only async-signal-safe functions
(see
.Xr sigaction 2 )
are guaranteed to work in the child process until a call to
.Xr execve 2
or a similar function.
.El
.Sh RETURN VALUES
Upon successful completion,
.Fn fork
returns a value
of 0 to the child process and returns the process ID of the child
process to the parent process.
Otherwise, a value of -1 is returned
to the parent process, no child process is created, and the global
variable
.Va errno
is set to indicate the error.
.Sh EXAMPLES
The following example shows a common pattern of how
.Fn fork
is used in practice.
.Bd -literal -offset indent
#include <err.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int
main(void)
{
	pid_t pid;

	switch (pid = fork()) {
	case -1:
		err(1, "Failed to fork");
	case 0:
		printf("Hello from child process!\en");
		exit(0);
	default:
		break;
	}

	printf("Hello from parent process (child's PID: %d)!\en", pid);

	return (0);
}
.Ed
.Pp
The output of such a program is along the lines of:
.Bd -literal -offset indent
Hello from parent process (child's PID: 27804)!
Hello from child process!
.Ed
.Sh ERRORS
The
.Fn fork
system call will fail and no child process will be created if:
.Bl -tag -width Er
.It Bq Er EAGAIN
The system-imposed limit on the total
number of processes under execution would be exceeded.
The limit is given by the
.Xr sysctl 3
MIB variable
.Dv KERN_MAXPROC .
(The limit is actually ten less than this
except for the super user).
.It Bq Er EAGAIN
The user is not the super user, and
the system-imposed limit
on the total number of
processes under execution by a single user would be exceeded.
The limit is given by the
.Xr sysctl 3
MIB variable
.Dv KERN_MAXPROCPERUID .
.It Bq Er EAGAIN
The user is not the super user, and
the soft resource limit corresponding to the
.Fa resource
argument
.Dv RLIMIT_NPROC
would be exceeded (see
.Xr getrlimit 2 ) .
.It Bq Er ENOMEM
There is insufficient swap space for the new process.
.El
.Sh SEE ALSO
.Xr execve 2 ,
.Xr rfork 2 ,
.Xr setitimer 2 ,
.Xr setrlimit 2 ,
.Xr sigaction 2 ,
.Xr vfork 2 ,
.Xr wait 2
.Sh HISTORY
The
.Fn fork
function appeared in
.At v1 .