gno/usr.man/man2/dup.2

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.\"     @(#)dup.2	8.1 (Berkeley) 6/4/93
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.TH DUP 2 "16 January 1997" GNO "System Calls"
.SH NAME
.BR dup ,
.BR dup2
\- duplicate an existing file descriptor
.SH SYNOPSIS
.br
#include <unistd.h>
.sp 1
int
\fBdup\fR (int \fIoldfd\fR);
.br
int
\fBdup2\fR (int \fIoldfd\fR, int \fInewfd\fR);
.SH DESCRIPTION
.BR Dup 
duplicates an existing file descriptor and returns its value to
the calling process.
The argument
.I oldfd
is a small non-negative integer index in
the per-process descriptor table.  The value must be less
than the size of the table, which is returned by
.BR getdtablesize (2).
The new descriptor returned by the call
is the lowest numbered descriptor
currently not in use by the process.
.LP
The file referenced by the descriptor does not distinguish
between
.I oldfd
and
.I newfd
in any way.
Thus if
.I newfd
and
.I oldfd
are duplicate references to an open
file,
.BR read (2),
.BR write (2)
and
.BR lseek (2)
calls all move a single pointer into the file,
and append mode, non-blocking I/O and asynchronous I/O options
are shared between the references.
If a separate pointer into the file is desired, a different
reference to the file must be obtained by issuing an
additional
.BR open (2)
call.
The close-on-exec flag on the new file descriptor is unset.
.LP
In 
.BR dup2 ,
the value of the new descriptor
.I newfd
is specified.  If this descriptor is already
in use, the descriptor is first deallocated as if a
.BR close (2)
call had been done first.
.SH RETURN VALUES
The value -1 is returned if an error occurs in either call.
The external variable
.IR errno
indicates the cause of the error.
.SH ERRORS
.BR Dup 
and
.BR dup2 
fail if:
.RS
.IP \fBEBADF\fR
.I Oldfd
or
.I newfd
is not a valid active descriptor
.IP \fBEMFILE\fR
Too many descriptors are active.
.RE
.SH SEE ALSO
.BR accept (2),
.BR open (2),
.BR close (2),
.BR fcntl (2),
.BR pipe (2),
.BR socket (2),
.BR socketpair (2),
.BR getdtablesize (2)
.SH STANDARDS
.BR Dup 
and
.BR dup2 
are expected to conform
to IEEE Std 1003.1-1988 (POSIX).