| 1 | .\" Copyright (c) 1980 Regents of the University of California.
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| 2 | .\" All rights reserved. The Berkeley software License Agreement
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| 3 | .\" specifies the terms and conditions for redistribution.
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| 4 | .\"
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| 5 | .\" @(#)dup.2 6.3 (Berkeley) 5/13/86
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| 6 | .\"
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| 7 | .TH DUP 2 "May 13, 1986"
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| 8 | .UC 4
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| 9 | .SH NAME
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| 10 | dup, dup2 \- duplicate a descriptor
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| 11 | .SH SYNOPSIS
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| 12 | .nf
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| 13 | .ft B
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| 14 | #include <unistd.h>
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| 15 |
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| 16 | int dup(int \fIoldd\fP)
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| 17 | int dup2(int \fIoldd\fP, int \fInewd\fP)
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| 18 | .SH DESCRIPTION
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| 19 | .B Dup
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| 20 | duplicates an existing descriptor.
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| 21 | The argument \fIoldd\fP is a small non-negative integer index in
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| 22 | the per-process descriptor table. The value must be less
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| 23 | than OPEN_MAX, the size of the table.
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| 24 | The new descriptor returned by the call, let's name it
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| 25 | .I newd,
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| 26 | is the lowest numbered descriptor that is
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| 27 | not currently in use by the process.
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| 28 | .PP
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| 29 | The object referenced by the descriptor does not distinguish
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| 30 | between references using \fIoldd\fP and \fInewd\fP in any way.
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| 31 | Thus if \fInewd\fP and \fIoldd\fP are duplicate references to an open
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| 32 | file,
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| 33 | .BR read (2),
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| 34 | .BR write (2)
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| 35 | and
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| 36 | .BR lseek (2)
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| 37 | calls all move a single pointer into the file,
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| 38 | and append mode, non-blocking I/O and asynchronous I/O options
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| 39 | are shared between the references.
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| 40 | If a separate pointer into the file is desired, a different
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| 41 | object reference to the file must be obtained by issuing an
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| 42 | additional
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| 43 | .BR open (2)
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| 44 | call.
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| 45 | The close-on-exec flag on the new file descriptor is unset.
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| 46 | .PP
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| 47 | In the second form of the call, the value of
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| 48 | .IR newd
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| 49 | desired is specified. If this descriptor is already
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| 50 | in use, the descriptor is first deallocated as if a
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| 51 | .BR close (2)
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| 52 | call had been done first.
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| 53 | .I Newd
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| 54 | is not closed if it equals
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| 55 | .IR oldd .
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| 56 | .SH "RETURN VALUE
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| 57 | The value \-1 is returned if an error occurs in either call.
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| 58 | The external variable
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| 59 | .B errno
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| 60 | indicates the cause of the error.
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| 61 | .SH "ERRORS
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| 62 | .B Dup
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| 63 | and
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| 64 | .B dup2
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| 65 | fail if:
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| 66 | .TP 15
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| 67 | [EBADF]
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| 68 | \fIOldd\fP or
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| 69 | \fInewd\fP is not a valid active descriptor
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| 70 | .TP 15
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| 71 | [EMFILE]
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| 72 | Too many descriptors are active.
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| 73 | .SH NOTES
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| 74 | .B Dup
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| 75 | and
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| 76 | .B dup2
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| 77 | are now implemented using the
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| 78 | .B F_DUPFD
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| 79 | function of
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| 80 | .BR fcntl (2),
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| 81 | although the old system call interfaces still exist to support old programs.
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| 82 | .SH "SEE ALSO"
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| 83 | .BR open (2),
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| 84 | .BR close (2),
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| 85 | .BR fcntl (2),
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| 86 | .BR pipe (2).
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