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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | SEE ALSO | COLOPHON |
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dup(2) System Calls Manual dup(2)
dup, dup2, dup3 - duplicate a file descriptor
Standard C library (libc, -lc)
#include <unistd.h>
int dup(int oldfd);
int dup2(int oldfd, int newfd);
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <fcntl.h> /* Definition of O_* constants */
#include <unistd.h>
int dup3(int oldfd, int newfd, int flags);
The dup() system call allocates a new file descriptor that refers
to the same open file description as the descriptor oldfd. (For
an explanation of open file descriptions, see open(2).) The new
file descriptor number is guaranteed to be the lowest-numbered
file descriptor that was unused in the calling process.
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 lseek(2) on one of the file descriptors, the offset is also
changed for the other file descriptor.
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.
dup2()
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. In other
words, the file descriptor newfd is adjusted so that it now
refers to the same open file description as oldfd.
If the file descriptor 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 dup2()).
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()
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 to indicate the error.
EBADF oldfd isn't an open file descriptor.
EBADF newfd is out of the allowed range for file descriptors
(see the discussion of RLIMIT_NOFILE in getrlimit(2)).
EBUSY (Linux only) This may be returned by dup2() or dup3()
during a race condition with open(2) and dup().
EINTR The dup2() or dup3() call was interrupted by a signal; see
signal(7).
EINVAL (dup3()) flags contain an invalid value.
EINVAL (dup3()) oldfd was equal to newfd.
EMFILE The per-process limit on the number of open file
descriptors has been reached (see the discussion of
RLIMIT_NOFILE in getrlimit(2)).
dup()
dup2() POSIX.1-2008.
dup3() Linux.
dup()
dup2() POSIX.1-2001, SVr4, 4.3BSD.
dup3() Linux 2.6.27, glibc 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. */
}
}
close(2), fcntl(2), open(2), pidfd_getfd(2)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
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2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.9.1 2024-05-02 dup(2)
Pages that refer to this page: bpf(2), fcntl(2), flock(2), getrlimit(2), kcmp(2), lseek(2), open(2), pidfd_getfd(2), syscalls(2), fileno(3), getdtablesize(3), posix_spawn(3), epoll(7), pipe(7), signal-safety(7), unix(7)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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