139 lines
4.4 KiB
Groff
139 lines
4.4 KiB
Groff
.\" $NetBSD: fork.2,v 1.22 2004/06/25 15:29:25 wiz Exp $
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.\"
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.\" 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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.\"
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.Dd June 10, 2004
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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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.Fn fork
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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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.El
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.Pp
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In general, the child process should call
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.Xr _exit 2
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rather than
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.Xr exit 3 .
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Otherwise, any stdio buffers that exist both in the parent and child
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will be flushed twice.
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Similarly,
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.Xr _exit 2
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should be used to prevent
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.Xr atexit 3
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routines from being called twice (once in the parent and once in the child).
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.Pp
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In case of a threaded program, only the thread calling
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.Fn fork
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is still running in the child processes.
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.Pp
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Child processes of a threaded program have additional restrictions,
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a child must only call functions that are async-signal-safe.
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Very few functions are asynchronously safe and applications should
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make sure they call
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.Xr exec 3
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as soon as possible.
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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 to the parent process, no
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child process is created, and the global 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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.Fn fork
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will fail and no child process will be created if:
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.Bl -tag -width [EAGAIN]
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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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This limit is configuration-dependent.
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.It Bq Er EAGAIN
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The limit
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.Dv RLIMIT_NPROC
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on the total number of
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processes under execution by this user id would be exceeded.
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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 setrlimit 2 ,
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.Xr vfork 2 ,
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.Xr wait 2 ,
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.Xr pthread_atfork 3
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.Sh STANDARDS
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The
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.Fn fork
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function conforms to
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.St -p1003.1-90 .
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.Sh HISTORY
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A
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.Fn fork
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system call appeared in
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.At v6 .
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