path: root/man2/dup.2

.\" This manpage is Copyright (C) 1992 Drew Eckhardt;
.\" and Copyright (C) 1993 Michael Haardt, Ian Jackson.
.\" and Copyright (C) 2005, 2008 Michael Kerrisk <mtk.manpages@gmail.com>
.\" and Copyright (C) 2014 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" Modified 1993-07-21, Rik Faith <faith@cs.unc.edu>
.\" Modified 1994-08-21, Michael Chastain <mec@shell.portal.com>:
.\"   Fixed typos.
.\" 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 2023-05-03 "Linux man-pages 6.05.01"
.SH NAME
dup, dup2, dup3 \- duplicate a file descriptor
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.PP
.BI "int dup(int " oldfd );
.BI "int dup2(int " oldfd ", int " newfd );
.PP
.BR "#define _GNU_SOURCE" "             /* See feature_test_macros(7) */"
.BR "#include <fcntl.h>" "              /* Definition of " O_* " constants */"
.B #include <unistd.h>
.PP
.BI "int dup3(int " oldfd ", int " newfd ", int " flags );
.fi
.SH DESCRIPTION
The
.BR dup ()
system call allocates a new file descriptor that refers to the same
open file description as the descriptor
.IR oldfd .
(For an explanation of open file descriptions, see
.BR open (2).)
The new file descriptor number is guaranteed to be the lowest-numbered
file descriptor that was unused in the calling process.
.PP
After a successful return,
the old and new file descriptors may be used interchangeably.
Since the two file descriptors refer to the same open file description,
they share file offset and file status flags;
for example, if the file offset is modified by using
.BR lseek (2)
on one of the file descriptors,
the offset is also changed for the other file descriptor.
.PP
The two file 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.
.\"
.SS dup2()
The
.BR dup2 ()
system call performs the same task as
.BR dup (),
but instead of using the lowest-numbered unused file descriptor,
it uses the file descriptor number specified in
.IR newfd .
In other words,
the file descriptor
.I newfd
is adjusted so that it now refers to the same open file description as
.IR oldfd .
.PP
If the file descriptor
.I newfd
was previously open, it is closed before being reused;
the close is performed silently
(i.e., any errors during the close are not reported by
.BR dup2 ()).
.PP
The steps of closing and reusing the file descriptor
.I newfd
are performed
.IR atomically .
This is important, because trying to implement equivalent functionality using
.BR close (2)
and
.BR dup ()
would be
subject to race conditions, whereby
.I newfd
might be reused between the two steps.
Such reuse could happen because the main program is interrupted
by a signal handler that allocates a file descriptor,
or because a parallel thread allocates a file descriptor.
.PP
Note the following points:
.IP \[bu] 3
If
.I oldfd
is not a valid file descriptor, then the call fails, and
.I newfd
is not closed.
.IP \[bu]
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 .
.\"
.SS dup3()
.BR dup3 ()
is the same as
.BR dup2 (),
except that:
.IP \[bu] 3
The caller can force the close-on-exec flag to be set
for the new file descriptor by specifying
.B O_CLOEXEC
in
.IR flags .
See the description of the same flag in
.BR open (2)
for reasons why this may be useful.
.IP \[bu]
.\" Ulrich Drepper, LKML, 2008-10-09:
.\"	We deliberately decided on this change.  Otherwise, what is the
.\"	result of dup3(fd, fd, O_CLOEXEC)?
If
.I oldfd
equals
.IR newfd ,
then
.BR dup3 ()
fails with the error
.BR EINVAL .
.SH RETURN VALUE
On success, these system calls
return the new file descriptor.
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EBADF
.I oldfd
isn't an open file descriptor.
.TP
.B EBADF
.I newfd
is out of the allowed range for file descriptors (see the discussion of
.B RLIMIT_NOFILE
in
.BR getrlimit (2)).
.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.
.TP
.B EINVAL
.RB ( dup3 ())
.I oldfd
was equal to
.IR newfd .
.TP
.B EMFILE
The per-process limit on the number of open file descriptors has been reached
(see the discussion of
.B RLIMIT_NOFILE
in
.BR getrlimit (2)).
.SH STANDARDS
.TP
.BR dup ()
.TQ
.BR dup2 ()
POSIX.1-2008.
.TP
.BR dup3 ()
Linux.
.SH HISTORY
.TP
.BR dup ()
.TQ
.BR dup2 ()
POSIX.1-2001, SVr4, 4.3BSD.
.\" SVr4 documents additional
.\" EINTR and ENOLINK error conditions.  POSIX.1 adds EINTR.
.\" The EBUSY return is Linux-specific.
.TP
.BR dup3 ()
Linux 2.6.27,
glibc 2.9.
.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 .
.PP
If
.I newfd
was open, any errors that would have been reported at
.BR close (2)
time are lost.
If this is of concern,
then\[em]unless the program is single-threaded and does not allocate
file descriptors in signal handlers\[em]the correct approach is
.I not
to close
.I newfd
before calling
.BR dup2 (),
because of the race condition described above.
Instead, code something like the following could be used:
.PP
.in +4n
.EX
/* Obtain a duplicate of \[aq]newfd\[aq] that can subsequently
   be used to check for close() errors; an EBADF error
   means that \[aq]newfd\[aq] was not open. */
\&
tmpfd = dup(newfd);
if (tmpfd == \-1 && errno != EBADF) {
    /* Handle unexpected dup() error. */
}
\&
/* Atomically duplicate \[aq]oldfd\[aq] on \[aq]newfd\[aq]. */
\&
if (dup2(oldfd, newfd) == \-1) {
    /* Handle dup2() error. */
}
\&
/* Now check for close() errors on the file originally
   referred to by \[aq]newfd\[aq]. */
\&
if (tmpfd != \-1) {
    if (close(tmpfd) == \-1) {
        /* Handle errors from close. */
    }
}
.EE
.in
.SH SEE ALSO
.BR close (2),
.BR fcntl (2),
.BR open (2),
.BR pidfd_getfd (2)