86 lines
2.1 KiB
Groff
86 lines
2.1 KiB
Groff
.\" Copyright (c) 1980 Regents of the University of California.
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.\" All rights reserved. The Berkeley software License Agreement
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.\" specifies the terms and conditions for redistribution.
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.\"
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.\" @(#)dup.2 6.3 (Berkeley) 5/13/86
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.\"
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.TH DUP 2 "May 13, 1986"
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.UC 4
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.SH NAME
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dup, dup2 \- duplicate a descriptor
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.SH SYNOPSIS
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.nf
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.ft B
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#include <unistd.h>
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int dup(int \fIoldd\fP)
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int dup2(int \fIoldd\fP, int \fInewd\fP)
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.SH DESCRIPTION
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.B Dup
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duplicates an existing descriptor.
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The argument \fIoldd\fP 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 OPEN_MAX, the size of the table.
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The new descriptor returned by the call, let's name it
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.I newd,
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is the lowest numbered descriptor that is
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not currently 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 references using \fIoldd\fP and \fInewd\fP in any way.
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Thus if \fInewd\fP and \fIoldd\fP are duplicate references to an open
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file,
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.BR read (2),
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.BR write (2)
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and
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.BR 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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.BR 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 the second form of the call, the value of
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.IR newd
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desired is specified. If this descriptor is already
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in use, the descriptor is first deallocated as if a
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.BR close (2)
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call had been done first.
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.I Newd
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is not closed if it equals
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.IR oldd .
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.SH "RETURN VALUE
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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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.B errno
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indicates the cause of the error.
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.SH "ERRORS
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.B Dup
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and
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.B dup2
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fail if:
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.TP 15
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[EBADF]
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\fIOldd\fP or
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\fInewd\fP is not a valid active descriptor
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.TP 15
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[EMFILE]
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Too many descriptors are active.
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.SH NOTES
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.B Dup
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and
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.B dup2
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are now implemented using the
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.B F_DUPFD
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function of
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.BR fcntl (2),
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although the old system call interfaces still exist to support old programs.
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.SH "SEE ALSO"
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.BR open (2),
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.BR close (2),
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.BR fcntl (2),
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.BR pipe (2).
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