0bbe34c35d
There is nothing about pthread_atfork(3) or extensions like calling malloc(3) in the child process as this may be unreliable or broken.
146 lines
4.4 KiB
Groff
146 lines
4.4 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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.\" 4. 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 May 31, 2013
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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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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 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 v6 .
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