185 lines
5.2 KiB
Groff
185 lines
5.2 KiB
Groff
.\" Copyright (c) 1980, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)fork.2 8.1 (Berkeley) 6/4/93
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.\" $FreeBSD$
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.\"
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.Dd April 20, 2021
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.Dt FORK 2
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.Os
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.Sh NAME
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.Nm fork
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.Nd create a new process
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In unistd.h
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.Ft pid_t
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.Fn fork void
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.Sh DESCRIPTION
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The
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.Fn fork
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system call causes creation of a new process.
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The new process (child process) is an exact copy of the
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calling process (parent process) except for the following:
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.Bl -bullet -offset indent
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.It
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The child process has a unique process ID.
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.It
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The child process has a different parent
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process ID (i.e., the process ID of the parent process).
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.It
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The child process has its own copy of the parent's descriptors,
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except for descriptors returned by
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.Xr kqueue 2 ,
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which are not inherited from the parent process.
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These descriptors reference the same underlying objects, so that,
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for instance, file pointers in file objects are shared between
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the child and the parent, so that an
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.Xr lseek 2
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on a descriptor in the child process can affect a subsequent
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.Xr read 2
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or
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.Xr write 2
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by the parent.
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This descriptor copying is also used by the shell to
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establish standard input and output for newly created processes
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as well as to set up pipes.
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.It
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The child process' resource utilizations
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are set to 0; see
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.Xr setrlimit 2 .
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.It
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All interval timers are cleared; see
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.Xr setitimer 2 .
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.It
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The child process has only one thread,
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corresponding to the calling thread in the parent process.
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If the process has more than one thread,
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locks and other resources held by the other threads are not released
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and therefore only async-signal-safe functions
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(see
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.Xr sigaction 2 )
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are guaranteed to work in the child process until a call to
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.Xr execve 2
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or a similar function.
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.El
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.Sh RETURN VALUES
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Upon successful completion,
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.Fn fork
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returns a value
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of 0 to the child process and returns the process ID of the child
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process to the parent process.
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Otherwise, a value of -1 is returned
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to the parent process, no child process is created, and the global
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variable
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.Va errno
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is set to indicate the error.
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.Sh EXAMPLES
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The following example shows a common pattern of how
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.Fn fork
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is used in practice.
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.Bd -literal -offset indent
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#include <err.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <unistd.h>
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int
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main(void)
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{
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pid_t pid;
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switch (pid = fork()) {
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case -1:
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err(1, "Failed to fork");
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case 0:
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printf("Hello from child process!\en");
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exit(0);
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default:
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break;
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}
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printf("Hello from parent process (child's PID: %d)!\en", pid);
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return (0);
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}
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.Ed
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.Pp
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The output of such a program is along the lines of:
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.Bd -literal -offset indent
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Hello from parent process (child's PID: 27804)!
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Hello from child process!
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.Ed
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.Sh ERRORS
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The
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.Fn fork
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system call will fail and no child process will be created if:
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.Bl -tag -width Er
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.It Bq Er EAGAIN
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The system-imposed limit on the total
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number of processes under execution would be exceeded.
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The limit is given by the
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.Xr sysctl 3
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MIB variable
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.Dv KERN_MAXPROC .
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(The limit is actually ten less than this
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except for the super user).
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.It Bq Er EAGAIN
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The user is not the super user, and
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the system-imposed limit
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on the total number of
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processes under execution by a single user would be exceeded.
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The limit is given by the
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.Xr sysctl 3
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MIB variable
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.Dv KERN_MAXPROCPERUID .
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.It Bq Er EAGAIN
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The user is not the super user, and
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the soft resource limit corresponding to the
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.Fa resource
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argument
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.Dv RLIMIT_NPROC
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would be exceeded (see
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.Xr getrlimit 2 ) .
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.It Bq Er ENOMEM
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There is insufficient swap space for the new process.
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.El
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.Sh SEE ALSO
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.Xr execve 2 ,
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.Xr rfork 2 ,
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.Xr setitimer 2 ,
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.Xr setrlimit 2 ,
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.Xr sigaction 2 ,
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.Xr vfork 2 ,
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.Xr wait 2
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.Sh HISTORY
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The
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.Fn fork
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function appeared in
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.At v1 .
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