7e6cabd06e
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
170 lines
4.1 KiB
Groff
170 lines
4.1 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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.\" @(#)dup.2 8.1 (Berkeley) 6/4/93
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.\" $FreeBSD$
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.\"
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.Dd June 1, 2013
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.Dt DUP 2
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.Os
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.Sh NAME
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.Nm dup ,
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.Nm dup2
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.Nd duplicate an existing file descriptor
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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 int
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.Fn dup "int oldd"
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.Ft int
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.Fn dup2 "int oldd" "int newd"
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.Sh DESCRIPTION
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The
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.Fn dup
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system call
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duplicates an existing object descriptor and returns its value to
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the calling process
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.Fa ( newd
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=
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.Fn dup oldd ) .
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The argument
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.Fa oldd
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is a small non-negative integer index in
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the per-process descriptor table.
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The new descriptor returned by the call
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is the lowest numbered descriptor
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currently not in use by the process.
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.Pp
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The object referenced by the descriptor does not distinguish
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between
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.Fa oldd
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and
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.Fa newd
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in any way.
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Thus if
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.Fa newd
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and
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.Fa oldd
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are duplicate references to an open
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file,
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.Xr read 2 ,
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.Xr write 2
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and
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.Xr lseek 2
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calls all move a single pointer into the file,
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and append mode, non-blocking I/O and asynchronous I/O options
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are shared between the references.
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If a separate pointer into the file is desired, a different
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object reference to the file must be obtained by issuing an
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additional
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.Xr open 2
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system call.
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The close-on-exec flag on the new file descriptor is unset.
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.Pp
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In
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.Fn dup2 ,
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the value of the new descriptor
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.Fa newd
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is specified.
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If this descriptor is already in use and
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.Fa oldd
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\*(Ne
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.Fa newd ,
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the descriptor is first deallocated as if the
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.Xr close 2
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system call had been used.
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If
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.Fa oldd
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is not a valid descriptor, then
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.Fa newd
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is not closed.
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If
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.Fa oldd
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==
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.Fa newd
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and
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.Fa oldd
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is a valid descriptor, then
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.Fn dup2
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is successful, and does nothing.
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.Sh RETURN VALUES
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These calls return the new file descriptor if successful;
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otherwise the value -1 is returned and
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the external variable
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.Va errno
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is set to indicate the cause of the error.
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.Sh ERRORS
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The
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.Fn dup
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system call fails if:
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.Bl -tag -width Er
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.It Bq Er EBADF
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The
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.Fa oldd
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argument
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is not a valid active descriptor
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.It Bq Er EMFILE
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Too many descriptors are active.
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.El
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.Pp
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The
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.Fn dup2
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system call fails if:
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.Bl -tag -width Er
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.It Bq Er EBADF
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The
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.Fa oldd
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argument is not a valid active descriptor or the
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.Fa newd
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argument is negative or exceeds the maximum allowable descriptor number
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.El
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.Sh SEE ALSO
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.Xr accept 2 ,
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.Xr close 2 ,
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.Xr fcntl 2 ,
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.Xr getdtablesize 2 ,
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.Xr open 2 ,
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.Xr pipe 2 ,
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.Xr socket 2 ,
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.Xr socketpair 2 ,
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.Xr dup3 3
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.Sh STANDARDS
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The
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.Fn dup
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and
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.Fn dup2
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system calls are expected to conform to
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.St -p1003.1-90 .
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.Sh HISTORY
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The
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.Fn dup
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and
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.Fn dup2
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functions appeared in
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.At v7 .
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