207 lines
4.8 KiB
Groff
207 lines
4.8 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. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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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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.\" @(#)dup.2 8.1 (Berkeley) 6/4/93
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.\" $FreeBSD$
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.\"
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.Dd June 4, 1993
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.Dt DUP 2
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.Os BSD 4
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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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.Fd #include <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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.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
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the per-process descriptor table. The value must be less
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than the size of the table, which is returned by
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.Xr getdtablesize 2 .
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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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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. If this descriptor is already in use and
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.Fa oldd
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!=
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.Fa newd ,
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the descriptor is first deallocated as if a
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.Xr close 2
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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 IMPLEMENTATION NOTES
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.Pp
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In the non-threaded library
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.Fn dup
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is implemented as the
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.Va dup
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syscall.
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.Pp
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In the threaded library, the
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.Va dup
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syscall is assembled to
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.Fn _thread_sys_dup
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and
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.Fn dup
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is implemented as a function which locks
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.Fa oldd
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for read and write, then calls
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.Fn _thread_sys_dup .
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Before returning,
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.Fn dup
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unlocks
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.Fa oldd .
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.Pp
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In the non-threaded library
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.Fn dup2
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is implemented as the
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.Va dup2
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syscall.
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.Pp
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In the threaded library, the
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.Va dup2
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syscall is assembled to
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.Fn _thread_sys_dup2
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and
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.Fn dup2
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is implemented as a function which locks both
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.Fa oldd
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and
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.Fa newd
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for read and write, then calls
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.Fn _thread_sys_dup2 .
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Before returning,
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.Fn dup2
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unlocks
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.Fa oldd .
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and
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.Fa newd .
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.Sh RETURN VALUES
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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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and
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.Fn dup2
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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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or
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.Fa newd
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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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.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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.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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function calls are expected to conform 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 dup
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and a
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.Fn dup2
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function call appeared in
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.At v7 .
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