dup, dup2, dup3 — duplicate a file descriptor
#include <unistd.h>
int
dup( |
int oldfd); |
int
dup2( |
int oldfd, |
int newfd); |
#define _GNU_SOURCE /* See feature_test_macros(7) */ #include <fcntl.h> /* Obtain O_* constant definitions */ #include <unistd.h>
int
dup3( |
int oldfd, |
| int newfd, | |
int flags); |
The dup() system call
creates a copy of the file descriptor oldfd, using the
lowest-numbered unused file descriptor for the new
descriptor.
After a successful return, the old and new file descriptors may be used interchangeably. They refer to the same open file description (see open(2)) and thus share file offset and file status flags; for example, if the file offset is modified by using lseek(2) on one of the file descriptors, the offset is also changed for the other.
The two file descriptors do not share file descriptor
flags (the close-on-exec flag). The close-on-exec flag
(FD_CLOEXEC; see fcntl(2)) for the duplicate
descriptor is off.
The dup2() system call
performs the same task as dup(), but instead of using the
lowest-numbered unused file descriptor, it uses the file
descriptor number specified in newfd. If the file descriptor
newfd was
previously open, it is silently closed before being
reused.
The steps of closing and reusing the file descriptor
newfd are performed
atomically. This
is important, because trying to implement equivalent
functionality using close(2) and dup() would be subject to race
conditions, whereby 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.
Note the following points:
If oldfd
is not a valid file descriptor, then the call fails,
and newfd is
not closed.
If oldfd
is a valid file descriptor, and newfd has the same
value as oldfd, then
dup2() does nothing,
and returns newfd.
dup3() is the same as
dup2(), except that:
The caller can force the close-on-exec flag to be
set for the new file descriptor by specifying
O_CLOEXEC in flags. See the
description of the same flag in open(2) for reasons
why this may be useful.
If oldfd
equals newfd,
then dup3() fails with
the error EINVAL.
On success, these system calls return the new file
descriptor. On error, −1 is returned, and errno is set appropriately.
oldfd isn't
an open file descriptor.
newfd is out
of the allowed range for file descriptors (see the
discussion of RLIMIT_NOFILE in getrlimit(2)).
(Linux only) This may be returned by dup2() or dup3() during a race condition with
open(2) and
dup().
The dup2() or
dup3() call was
interrupted by a signal; see signal(7).
(dup3()) flags contain an invalid
value.
(dup3()) oldfd was equal to
newfd.
The per-process limit on the number of open file
descriptors has been reached (see the discussion of
RLIMIT_NOFILE in
getrlimit(2)).
dup3() was added to Linux in
version 2.6.27; glibc support is available starting with
version 2.9.
The error returned by dup2()
is different from that returned by fcntl(...,F_DUPFD, ...) when newfd is out of range. On some
systems, dup2() also sometimes
returns EINVAL like
F_DUPFD.
If newfd was open,
any errors that would have been reported at close(2) time are lost. If
this is of concern, then—unless the program is
single-threaded and does not allocate file descriptors in
signal handlers—the correct approach is not to close newfd before calling
dup2(), because of the race
condition described above. Instead, code something like the
following could be used:
/* Obtain a duplicate of 'newfd' that can subsequently
be used to check for close() errors; an EBADF error
means that 'newfd' was not open. */
tmpfd = dup(newfd);
if (tmpfd == −1 && errno != EBADF) {
/* Handle unexpected dup() error */
}
/* Atomically duplicate 'oldfd' on 'newfd' */
if (dup2(oldfd, newfd) == −1) {
/* Handle dup2() error */
}
/* Now check for close() errors on the file originally
referred to by 'newfd' */
if (tmpfd != −1) {
if (close(tmpfd) == −1) {
/* Handle errors from close */
}
}
This page is part of release 4.07 of the Linux man-pages project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
|
This manpage is Copyright (C) 1992 Drew Eckhardt; and Copyright (C) 1993 Michael Haardt, Ian Jackson. and Copyright (C) 2005, 2008 Michael Kerrisk <mtk.manpagesgmail.com> and Copyright (C) 2014 Michael Kerrisk <mtk.manpagesgmail.com> %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this 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 Modified 1993-07-21, Rik Faith <faithcs.unc.edu> Modified 1994-08-21, Michael Chastain <mecshell.portal.com>: Fixed typos. Modified 1997-01-31, Eric S. Raymond <esrthyrsus.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() |