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.
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() 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)).
dup3() was added in Linux 2.6.27; glibc support is available since glibc
2.9.
dup(),
dup2(): POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
dup3() is Linux-specific.
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)