dup(2) - Linux manual page (original) (raw)

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)](../man2/open.2.html).)  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)](../man2/lseek.2.html) 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)](../man2/fcntl.2.html)) 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)](../man2/close.2.html) 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)](../man2/open.2.html) for reasons why this
      may be useful.

   •  If _oldfd_ equals _newfd_, then **dup3**() fails with the error **EINVAL**.

RETURN VALUE top

   On success, these system calls return the new file descriptor.  On
   error, -1 is returned, and _[errno](../man3/errno.3.html)_ 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)](../man2/getrlimit.2.html)).

   **EBUSY** (Linux only) This may be returned by **dup2**() or **dup3**()
          during a race condition with [open(2)](../man2/open.2.html) and **dup**().

   **EINTR** The **dup2**() or **dup3**() call was interrupted by a signal; see
          [signal(7)](../man7/signal.7.html).

   **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)](../man2/getrlimit.2.html)).

   **ENOMEM** Insufficient kernel memory was available.

STANDARDS top

   **dup**()
   **dup2**() POSIX.1-2008.

   **dup3**() Linux.

HISTORY top

   **dup**()
   **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)](../man2/close.2.html) 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)](../man2/close.2.html), [fcntl(2)](../man2/fcntl.2.html), [open(2)](../man2/open.2.html), [pidfd_getfd(2)](../man2/pidfd%5Fgetfd.2.html)

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Linux man-pages 6.10 2024-11-01 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)