dup(2) — Linux manual page


dup(2)                     System Calls Manual                    dup(2)

NAME         top

       dup, dup2, dup3 - duplicate a file descriptor

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #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);

DESCRIPTION         top

       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

       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

RETURN VALUE         top

       On success, these system calls return the new file descriptor.
       On error, -1 is returned, and errno is set to indicate the error.

ERRORS         top

       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

       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)).

STANDARDS         top

       dup2() POSIX.1-2008.

       dup3() Linux.

HISTORY         top

       dup2() POSIX.1-2001, SVr4, 4.3BSD.

       dup3() Linux 2.6.27, glibc 2.9.

NOTES         top

       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. */

SEE ALSO         top

       close(2), fcntl(2), open(2), pidfd_getfd(2)

Linux man-pages (unreleased)     (date)                           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)