.\" 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>
.\"
-.\" %%%LICENSE_START(verbatim)
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-.\" manual provided the copyright notice and this permission notice are
-.\" preserved on all copies.
-.\"
-.\" Permission is granted to copy and distribute modified versions of this
-.\" manual under the conditions for verbatim copying, provided that the
-.\" entire resulting derived work is distributed under the terms of a
-.\" permission notice identical to this one.
-.\"
-.\" Since the Linux kernel and libraries are constantly changing, this
-.\" manual page may be incorrect or out-of-date. The author(s) assume no
-.\" responsibility for errors or omissions, or for damages resulting from
-.\" the use of the information contained herein. The author(s) may not
-.\" have taken the same level of care in the production of this manual,
-.\" which is licensed free of charge, as they might when working
-.\" professionally.
-.\"
-.\" Formatted or processed versions of this manual, if unaccompanied by
-.\" the source, must acknowledge the copyright and authors of this work.
-.\" %%%LICENSE_END
+.\" 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 typoes.
+.\" 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 2012-02-14 "Linux" "Linux Programmer's Manual"
+.TH dup 2 (date) "Linux man-pages (unreleased)"
.SH NAME
dup, dup2, dup3 \- duplicate a file descriptor
+.SH LIBRARY
+Standard C library
+.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <unistd.h>
-.sp
+.PP
.BI "int dup(int " oldfd );
.BI "int dup2(int " oldfd ", int " newfd );
-.sp
+.PP
.BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */"
-.BR "#include <fcntl.h>" " /* Obtain O_* constant definitions */
+.BR "#include <fcntl.h>" " /* Definition of " O_* " constants */"
.B #include <unistd.h>
-.sp
+.PP
.BI "int dup3(int " oldfd ", int " newfd ", int " flags );
.fi
.SH DESCRIPTION
-These system calls create a copy of the file descriptor
-.IR oldfd .
-
+The
.BR dup ()
-uses the lowest-numbered unused descriptor for the new descriptor.
-
+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 ()
-.RI "makes " newfd " be the copy of " oldfd ", closing " newfd
-first if necessary, but note the following:
-.IP * 3
+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 *
+.IP \(bu
If
.I oldfd
is a valid file descriptor, and
.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.
-
+.\"
+.SS dup3()
.BR dup3 ()
is the same as
.BR dup2 (),
except that:
-.IP * 3
+.IP \(bu 3
The caller can force the close-on-exec flag to be set
for the new file descriptor by specifying
-.BR O_CLOEXEC
+.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 *
-.\" FIXME . To confirm with Al Viro that this was intended, and its rationale
+.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
-.IR oldfd
+.I oldfd
equals
.IR newfd ,
then
.BR EINVAL .
.SH RETURN VALUE
On success, these system calls
-return the new descriptor.
+return the new file descriptor.
On error, \-1 is returned, and
.I errno
-is set appropriately.
+is set to indicate the error.
.SH ERRORS
.TP
.B EBADF
.I oldfd
-isn't an open file descriptor, or
+isn't an open file descriptor.
+.TP
+.B EBADF
.I newfd
-is out of the allowed range for file descriptors.
+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
.RB ( dup3 ())
.I flags
contain an invalid value.
-.\" FIXME . To confirm with Al Viro that this was intended, and its rationale
-Or,
+.TP
+.B EINVAL
+.RB ( dup3 ())
.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.
+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 VERSIONS
.BR dup3 ()
-was added to Linux in version 2.6.27;
-glibc support is available starting with
-version 2.9.
-.SH CONFORMING TO
+was added in Linux 2.6.27;
+glibc support is available since glibc 2.9.
+.SH STANDARDS
.BR dup (),
.BR dup2 ():
-SVr4, 4.3BSD, POSIX.1-2001.
-
+POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
+.PP
.BR dup3 ()
is Linux-specific.
.\" SVr4 documents additional
when
.I newfd
is out of range.
-On some systems
+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.
-A careful programmer will not use
-.BR dup2 ()
-or
-.BR dup3 ()
-without closing
+If this is of concern,
+then\(emunless the program is single-threaded and does not allocate
+file descriptors in signal handlers\(emthe correct approach is
+.I not
+to close
.I newfd
-first.
+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 \(aqnewfd\(aq that can subsequently
+ be used to check for close() errors; an EBADF error
+ means that \(aqnewfd\(aq was not open. */
+
+tmpfd = dup(newfd);
+if (tmpfd == \-1 && errno != EBADF) {
+ /* Handle unexpected dup() error. */
+}
+
+/* Atomically duplicate \(aqoldfd\(aq on \(aqnewfd\(aq. */
+
+if (dup2(oldfd, newfd) == \-1) {
+ /* Handle dup2() error. */
+}
+
+/* Now check for close() errors on the file originally
+ referred to by \(aqnewfd\(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 open (2),
+.BR pidfd_getfd (2)
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