215 lines
5.0 KiB
Groff
215 lines
5.0 KiB
Groff
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.\" $OpenBSD: dup.2,v 1.20 2018/06/25 16:06:27 visa Exp $
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.\" $NetBSD: dup.2,v 1.4 1995/02/27 12:32:21 cgd 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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.\" @(#)dup.2 8.1 (Berkeley) 6/4/93
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.\"
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.Dd $Mdocdate: June 25 2018 $
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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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.Nm dup3
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.Nd duplicate an existing file descriptor
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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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.In fcntl.h
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.In unistd.h
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.Ft int
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.Fn dup3 "int oldd" "int newd" "int flags"
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.Sh DESCRIPTION
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.Fn dup
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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 the per-process descriptor table.
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The value must be less than the size of the table, which is returned by
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.Xr getdtablesize 3 .
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The new descriptor returned by the call 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 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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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, it is first deallocated as if a
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.Xr close 2
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call had been done first.
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When
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.Fa newd
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equals
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.Fa oldd ,
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.Fn dup2
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just returns without affecting the close-on-exec flag.
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.Pp
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In
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.Fn dup3 ,
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both the value of the new descriptor and the close-on-exec flag on
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the new file descriptor are specified:
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.Fa newd
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specifies the value and the
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.Dv O_CLOEXEC
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bit in
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.Fa flags
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specifies the close-on-exec flag.
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Unlike
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.Fn dup2 ,
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if
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.Fa oldd
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and
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.Fa newd
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are equal then
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.Fn dup3
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fails.
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Otherwise, if
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.Fa flags
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is zero then
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.Fn dup3
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is identical to a call to
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.Fn dup2 .
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.Sh RETURN VALUES
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Upon successful completion, the value of the new descriptor is returned.
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The value \-1 is returned if an error occurs in either call.
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The external variable
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.Va errno
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indicates the cause of the error.
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.Sh ERRORS
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.Fn dup
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will fail if:
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.Bl -tag -width Er
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.It Bq Er EBADF
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.Fa oldd
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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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.Fn dup2
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and
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.Fn dup3
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will fail if:
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.Bl -tag -width Er
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.It Bq Er EBADF
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.Fa oldd
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is not a valid active descriptor or
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.Fa newd
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is negative or greater than or equal to the process's
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.Dv RLIMIT_NOFILE
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limit.
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.It Bq Er EBUSY
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A race condition with
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.Xr accept 2
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or
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.Xr open 2
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has been detected.
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.It Bq Er EINTR
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An interrupt was received.
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.It Bq Er EIO
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An I/O error occurred while writing to the file system.
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.El
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.Pp
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In addition,
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.Fn dup3
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will return the following error:
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.Bl -tag -width Er
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.It Bq Er EINVAL
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.Fa oldd
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is equal to
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.Fa newd
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or
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.Fa flags
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is invalid.
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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 getrlimit 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 getdtablesize 3
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.Sh STANDARDS
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.Fn dup
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and
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.Fn dup2
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conform to
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.St -p1003.1-2008 .
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The
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.Fn dup3
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function is expected to conform to a future revision of that standard.
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.Sh HISTORY
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The
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.Fn dup
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system call first appeared in
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.At v3 ,
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.Fn dup2
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in
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.At v7 ,
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and
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.Fn dup3
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in
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.Ox 5.7 .
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