fbbd9655e5
Renumber cluase 4 to 3, per what everybody else did when BSD granted them permission to remove clause 3. My insistance on keeping the same numbering for legal reasons is too pedantic, so give up on that point. Submitted by: Jan Schaumann <jschauma@stevens.edu> Pull Request: https://github.com/freebsd/freebsd/pull/96
146 lines
3.9 KiB
Groff
146 lines
3.9 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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.\" @(#)vfork.2 8.1 (Berkeley) 6/4/93
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.\" $FreeBSD$
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.\"
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.Dd May 22, 2016
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.Dt VFORK 2
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.Os
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.Sh NAME
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.Nm vfork
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.Nd create a new process without copying the address space
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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 vfork void
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.Sh DESCRIPTION
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.Bf -symbolic
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Since this function is hard to use correctly from application software,
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it is recommended to use
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.Xr posix_spawn 3
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or
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.Xr fork 2
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instead.
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.Ef
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.Pp
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The
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.Fn vfork
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system call
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can be used to create new processes without fully copying the address
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space of the old process, which is inefficient in a paged
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environment.
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It is useful when the purpose of
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.Xr fork 2
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would have been to create a new system context for an
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.Xr execve 2 .
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The
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.Fn vfork
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system call
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differs from
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.Xr fork 2
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in that the child borrows the parent process's address space and the
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calling thread's stack
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until a call to
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.Xr execve 2
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or an exit (either by a call to
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.Xr _exit 2
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or abnormally).
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The calling thread is suspended while the child is using its resources.
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Other threads continue to run.
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.Pp
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The
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.Fn vfork
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system call
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returns 0 in the child's context and (later) the pid of the child in
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the parent's context.
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.Pp
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Many problems can occur when replacing
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.Xr fork 2
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with
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.Fn vfork .
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For example, it does not work to return while running in the child's context
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from the procedure that called
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.Fn vfork
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since the eventual return from
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.Fn vfork
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would then return to a no longer existent stack frame.
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Also, changing process state which is partially implemented in user space
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such as signal handlers with
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.Xr libthr 3
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will corrupt the parent's state.
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.Pp
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Be careful, also, to 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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if you cannot
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.Xr execve 2 ,
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since
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.Xr exit 3
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will flush and close standard I/O channels, and thereby mess up the
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parent processes standard I/O data structures.
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(Even with
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.Xr fork 2
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it is wrong to call
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.Xr exit 3
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since buffered data would then be flushed twice.)
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.Sh RETURN VALUES
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Same as for
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.Xr fork 2 .
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.Sh SEE ALSO
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.Xr _exit 2 ,
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.Xr execve 2 ,
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.Xr fork 2 ,
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.Xr rfork 2 ,
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.Xr sigaction 2 ,
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.Xr wait 2 ,
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.Xr exit 3 ,
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.Xr posix_spawn 3
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.Sh HISTORY
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The
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.Fn vfork
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system call appeared in
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.Bx 3 .
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.Sh BUGS
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To avoid a possible deadlock situation,
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processes that are children in the middle
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of a
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.Fn vfork
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are never sent
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.Dv SIGTTOU
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or
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.Dv SIGTTIN
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signals; rather,
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output or
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.Xr ioctl 2
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calls
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are allowed
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and input attempts result in an end-of-file indication.
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