man-pages/man2/dup.2

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.\" and Copyright (C) 1993 Michael Haardt, Ian Jackson.
.\" and Copyright (C) 2005, 2008 Michael Kerrisk <mtk.manpages@gmail.com>
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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
.\" 2009-01-12, mtk, reordered text in DESCRIPTION and added some
.\"     details for dup2().
.\" 2008-10-09, mtk: add description of dup3()
.\"
.TH DUP 2 2008-10-09 "Linux" "Linux Programmer's Manual"
.SH NAME
dup, dup2, dup3 \- duplicate a file descriptor
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.sp
.BI "int dup(int " oldfd );
.BI "int dup2(int " oldfd ", int " newfd );
.sp
.B #define _GNU_SOURCE
.B #include <unistd.h>
.sp
.BI "int dup3(int " oldfd ", int " newfd ", int " flags );
.fi
.SH DESCRIPTION
The system calls create a copy of the file descriptor
.IR oldfd .

.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, but note the following:
.IP * 3
If
.I oldfd
is not a valid file descriptor, then the call fails, and
.I newfd
is not closed.
.IP *
If
.I oldfd
is a valid file descriptor, and
.I newfd
has the same value as
.IR oldfd ,
then
.BR dup2 ()
does nothing, and returns
.IR newfd .
.PP
After a successful return from one of these system calls,
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 dup3 ()
is the same as
.BR dup2 (),
except that:
.IP * 3
The caller can force the close-on-exec flag to be set
for the new file descriptor by specifying
.BR O_CLOEXEC
in
.IR flags .
See the description of the same flag in
.BR open (2)
for reasons why this may be useful.
.IP *
.\" FIXME . To confirm with Al Viro that this was intended, and its rationale
If
.IR oldfd
equals
.IR newfd ,
then
.BR dup3 ()
fails with the error
.BR EINVAL .
.SH "RETURN VALUE"
On success, these system calls
return the new descriptor.
On error, \-1 is returned, and
.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 ()
or
.BR dup3 ()
during a race condition with
.BR open (2)
and
.BR dup ().
.TP
.B EINTR
The
.BR dup2 ()
or    
.BR dup3 ()
call was interrupted by a signal; see
.BR signal (7).
.TP
.B EINVAL
.RB ( dup3 ())
.I flags
contain an invalid value.
.\" FIXME . To confirm with Al Viro that this was intended, and its rationale
Or,
.I oldfd
was equal to
.IR newfd .
.TP
.B EMFILE
The process already has the maximum number of file
descriptors open and tried to open a new one.
.SH VERSIONS
.BR dup3 ()
was added to Linux in version 2.6.27;
glibc support is available starting with
version 2.9.
.SH "CONFORMING TO"
.BR dup (),
.BR dup2 ():
SVr4, 4.3BSD, POSIX.1-2001.

.BR dup3 ()
is Linux-specific.
.\" SVr4 documents additional
.\" EINTR and ENOLINK error conditions.  POSIX.1 adds EINTR.
.\" The EBUSY return is Linux-specific.
.SH NOTES
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 ()
or
.BR dup3 ()
without closing
.I newfd
first.
.SH "SEE ALSO"
.BR close (2),
.BR fcntl (2),
.BR open (2)