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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); |
These system calls create a copy of the file descriptor
oldfd.
dup() uses the
lowest-numbered unused descriptor for the new descriptor.
dup2() makes newfd be the copy of oldfd, closing newfd first if necessary, but
note the following:
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.
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 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 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
(FD_CLOEXEC; see fcntl(2)) for the duplicate
descriptor is off.
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 descriptor.
On error, −1 is returned, and errno is set appropriately.
oldfd isn't
an open file descriptor, or newfd is out of the
allowed range for file descriptors.
(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. Or, oldfd was equal to
newfd.
The process already has the maximum number of file descriptors open and tried to open a new one.
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. A
careful programmer will not use dup2() or dup3() without closing newfd first.
This page is part of release 3.52 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man−pages/.
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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> %%%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 typoes. 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() |