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.

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.

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)).

POSIX.1-2008.
Linux.

POSIX.1-2001, SVr4, 4.3BSD.
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)

2024-05-02 Linux man-pages 6.9.1