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.\" This manpage is Copyright (C) 1992 Drew Eckhardt;
.\"                               1993 Michael Haardt, Ian Jackson.
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.\" Modified 1993-07-21, Rik Faith <faith@cs.unc.edu>
.\" Modified 1994-08-21, Michael Chastain <mec@shell.portal.com>:
.\"   Fixed typoes.
.\" Modified 1997-01-31, Eric S. Raymond <esr@thyrsus.com>
.\" Modified 2002-09-28, aeb
.\"
.TH DUP 2 1994-08-21 "Linux 1.1.46" "Linux Programmer's Manual"
.SH NAME
dup, dup2 \- duplicate a file descriptor
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.sp
.BI "int dup(int " oldfd );
.BI "int dup2(int " oldfd ", int " newfd );
.fi
.SH DESCRIPTION
.BR dup ()
and
.BR dup2 ()
create a copy of the file descriptor
.IR oldfd .

After a successful return from \fBdup\fR() or \fBdup2\fR(),
the old and new file descriptors may be used interchangeably.
They refer to the same open file description (see
.BR open (2))
and thus share file offset and file status flags;
for example, if the file offset is modified by using
.BR lseek (2)
on one of the descriptors, the offset is also changed for the other.

The two descriptors do not share file descriptor flags
(the close-on-exec flag).
The close-on-exec flag
.RB ( FD_CLOEXEC ;
see
.BR fcntl (2))
for the duplicate descriptor is off.

.BR dup ()
uses the lowest-numbered unused descriptor for the new descriptor.

.BR dup2 ()
.RI "makes " newfd " be the copy of " oldfd ", closing " newfd
first if necessary.
.SH "RETURN VALUE"
.BR dup ()
and
.BR dup2 ()
return the new descriptor, or \-1 if an error occurred (in which case,
.I errno
is set appropriately).
.SH ERRORS
.TP
.B EBADF
.I oldfd
isn't an open file descriptor, or
.I newfd
is out of the allowed range for file descriptors.
.TP
.B EBUSY
(Linux only) This may be returned by
.BR dup2 ()
during a race condition with
.BR open (2)
and
.BR dup ().
.TP
.B EINTR
The
.BR dup2 ()
call was interrupted by a signal.
.TP
.B EMFILE
The process already has the maximum number of file
descriptors open and tried to open a new one.
.SH WARNINGS
The error returned by
.BR dup2 ()
is different from that returned by
.BR fcntl( "..., " F_DUPFD ", ..." )
when
.I newfd
is out of range.
On some systems
.BR dup2 ()
also sometimes returns
.B EINVAL
like
.BR F_DUPFD .

If
.I newfd
was open, any errors that would have been reported at
.BR close (2)
time, are lost.
A careful programmer will not use
.BR dup2 ()
without closing
.I newfd
first.
.SH "CONFORMING TO"
SVr4, 4.3BSD, POSIX.1-2001.
.\" SVr4 documents additional
.\" EINTR and ENOLINK error conditions.  POSIX.1 adds EINTR.
.\" The EBUSY return is Linux specific.
.SH "SEE ALSO"
.BR close (2),
.BR fcntl (2),
.BR open (2)