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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.\" 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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.\" @(#)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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