dup(2)                        System Calls Manual                       dup(2)

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       dup, dup2, dup3 - <?><?><?><?><?><?><?> <?><?><?><?><?><?><?><?>
       <?><?><?><?><?><?><?><?><?> <?><?><?><?><?><?><?><?><?><?><?>

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       <?><?><?><?><?> C (libc, -lc)

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       #include <unistd.h>

       int dup(int oldfd);
       int dup2(int oldfd, int newfd);

       #define _GNU_SOURCE             /* <?><?>. feature_test_macros(7) */
       #include <fcntl.h>              /* <?><?><?><?><?><?><?><?><?><?><?> <?><?><?><?><?><?><?><?> O_* */
       #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.

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       <?><?><?><?><?><?><?><?><?> <?><?><?><?><?>
       <?><?><?><?><?><?><?><?><?><?><?> <?><?><?><?><?> (<?><?><?><?>
       close-on-exec). <?><?><?><?> close-on-exec (FD_CLOEXEC; <?><?>.
       fcntl(2)) <?> <?><?><?><?><?> <?><?><?><?><?><?><?><?><?><?><?>
       <?><?><?><?><?><?><?><?><?><?><?><?>.

   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()).

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       <?><?><?><?><?><?><?><?><?> <?><?><?><?><?><?><?><?><?><?><?> newfd
       <?><?><?><?><?><?><?><?><?><?><?> <?><?><?><?><?><?><?><?>. <?><?><?>
       <?><?><?><?><?>, <?><?><?> <?><?><?> <?><?><?><?><?><?><?>
       <?><?><?><?><?><?><?><?><?><?><?> <?><?><?><?><?><?><?><?> <?>
       <?><?><?><?><?><?><?> close(2) <?> dup() <?><?><?><?><?><?><?> <?><?>
       <?> <?><?><?><?><?><?><?><?><?><?><?><?><?><?><?><?>, <?> <?><?><?><?>
       <?><?><?><?> newfd <?><?><?> <?><?><?><?>
       <?><?><?><?><?><?><?><?><?><?><?><?> <?><?><?><?><?><?><?><?>
       <?><?><?><?><?> <?><?><?><?><?> <?><?><?><?><?> <?><?><?><?><?><?>.
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       <?><?><?><?><?><?><?><?><?><?> <?><?><?><?><?><?><?><?>
       <?><?><?><?><?><?><?><?><?><?>.

       <?><?><?><?><?> <?><?><?><?><?><?><?> <?><?><?><?><?><?><?><?><?>:

       o  <?><?><?><?> oldfd <?><?><?><?><?><?><?><?>
          <?><?><?><?><?><?><?><?><?><?><?><?> <?><?><?><?><?><?><?><?>
          <?><?><?><?><?><?><?><?><?><?><?><?>, <?><?> <?><?><?><?><?>
          <?><?><?><?><?><?><?><?><?><?><?> <?> <?><?><?><?><?><?><?>, <?>
          newfd <?><?> <?><?><?><?><?><?><?><?><?><?><?>.

       o  <?><?><?><?> oldfd <?><?><?><?><?><?><?><?>
          <?><?><?><?><?><?><?><?><?><?> <?><?><?><?><?><?><?><?>
          <?><?><?><?><?><?><?><?><?><?><?><?>, <?> <?><?><?><?><?> newfd
          <?><?><?><?><?><?><?><?><?> <?> oldfd, <?><?> dup2() <?><?>
          <?><?><?><?><?><?> <?><?><?><?><?><?> <?>
          <?><?><?><?><?><?><?><?><?><?> <?><?><?><?><?><?><?><?> newfd.

   dup3()
       dup3() <?><?><?><?><?> <?><?> dup2(). <?><?><?><?><?><?><?>
       <?><?><?><?><?><?><?><?><?><?><?> <?> <?><?><?><?><?><?><?><?><?>:

       o  <?><?><?><?><?><?><?><?><?><?> <?><?><?><?><?>
          <?><?><?><?><?><?><?><?><?><?><?><?><?>
          <?><?><?><?><?><?><?><?><?><?> <?><?><?><?> close-on-exec flag <?>
          <?><?><?><?><?><?> <?><?><?><?><?><?><?><?><?>
          <?><?><?><?><?><?><?><?><?><?><?>, <?><?><?><?><?><?> O_CLOEXEC <?>
          flags. <?><?><?><?><?> <?><?><?> <?><?><?><?><?> <?><?><?><?>
          <?><?><?><?><?> <?><?><?><?><?><?><?><?> <?> open(2).

       o  <?><?><?><?> oldfd <?><?><?><?><?> newfd, <?><?> dup3()
          <?><?><?><?><?><?> <?><?><?><?><?><?> EINVAL.

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

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       EBADF  <?><?><?><?><?><?><?><?> oldfd <?><?> <?><?><?><?><?><?><?><?>
              <?><?><?><?><?><?><?><?> <?><?><?><?><?><?><?><?>
              <?><?><?><?><?><?><?><?><?><?><?><?>.

       EBADF  <?><?><?><?><?><?><?><?> newfd <?><?><?><?><?><?><?><?><?>
              <?><?><?> <?><?><?><?><?><?><?><?><?><?><?>
              <?><?><?><?><?><?><?><?><?> <?><?><?><?><?><?><?><?>
              <?><?><?><?><?><?><?><?><?><?><?><?> (<?><?><?><?><?><?><?><?>
              <?><?><?><?><?><?><?><?> RLIMIT_NOFILE <?> getrlimit(2)).

       EBUSY  (<?><?><?><?><?><?> <?> Linux) <?><?><?><?><?>
              <?><?><?><?><?><?><?><?><?> <?> dup2() <?><?><?> dup3()
              <?><?><?> <?><?><?><?><?><?><?><?><?><?><?><?><?>
              <?><?><?><?><?><?><?><?><?><?><?><?><?><?><?><?>
              <?><?><?><?><?><?><?> open(2) <?> dup().

       EINTR  <?><?><?><?><?> dup2() <?><?><?> dup3() <?><?><?>
              <?><?><?><?><?><?><?> <?><?><?><?><?>-<?><?><?><?>
              <?><?><?><?><?><?><?><?>. <?><?><?><?><?><?><?><?> signal(7).

       EINVAL (dup3()) flags <?><?><?><?><?><?><?><?>
              <?><?><?><?><?><?><?><?><?><?><?><?> <?><?><?><?><?><?><?><?>.

       EINVAL (dup3()) oldfd <?><?><?><?> <?><?><?><?><?> newfd.

       EMFILE <?><?><?><?> <?><?><?><?><?><?><?><?><?><?>
              <?><?><?><?><?><?><?><?><?><?><?> <?><?>
              <?><?><?><?><?><?><?><?><?><?> <?><?><?><?><?><?><?><?>
              <?><?><?><?><?><?><?><?> <?><?><?><?><?><?><?><?><?><?><?><?>
              <?><?> <?><?><?><?><?><?><?> (<?><?><?><?><?><?><?><?>
              <?><?><?><?><?><?><?><?> RLIMIT_NOFILE <?> getrlimit(2)).

       ENOMEM <?><?><?><?><?><?><?><?><?><?><?><?><?>
              <?><?><?><?><?><?><?><?><?><?> <?><?><?><?><?><?> <?><?><?><?>.

<?><?><?><?><?><?><?><?><?>
       POSIX.1-2024.

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       dup()
       dup2() 4.3BSD, SVr4, POSIX.1-1988.

       dup3() POSIX.1-2024.  Linux 2.6.27, glibc 2.9.

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       <?><?><?><?><?><?>, <?><?><?><?><?><?><?>
       <?><?><?><?><?><?><?><?><?><?> dup2(), <?><?><?><?><?><?><?><?><?><?>
       <?><?> <?><?><?>, <?><?><?> <?><?><?><?><?><?><?><?><?><?> fcntl(<?>,
       F_DUPFD, <?>), <?><?><?><?><?> newfd <?><?><?><?><?><?><?><?><?>
       <?><?><?> <?><?><?><?><?><?><?><?><?><?> <?><?><?><?><?><?><?><?>.
       <?><?> <?><?><?><?><?><?><?><?><?> <?><?><?><?><?><?><?><?> dup2()
       <?><?><?><?><?> <?><?><?><?><?><?> <?><?><?><?><?><?><?><?><?><?>
       EINVAL -- <?><?><?> 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)

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       <?><?><?><?><?><?><?><?> <?><?><?><?><?><?><?><?><?><?><?>
       <?><?><?><?><?><?><?><?><?><?>(<?>) Yuri Kozlov <yuray@komyakino.ru>
       <?> <?><?><?><?> <?><?><?><?><?><?> <pavia00@gmail.com>

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       <?><?><?><?><?><?><?><?> <?><?><?><?><?><?><?><?><?><?><?><?><?>
       <?><?><?><?><?><?><?><?> GNU (GNU General Public License - GPL,
       <https://www.gnu.org/licenses/gpl-3.0.html> <?><?><?><?><?><?> 3
       <?><?><?> <?><?><?><?><?> <?><?><?><?><?><?><?>) <?>
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       <?><?><?><?><?>, <?><?> <?><?><?> <?><?><?><?><?>-<?><?><?><?>
       <?><?><?><?><?><?><?><?>.

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       <?><?><?><?><?><?><?><?><?><?><?><?>(<?><?>) <?><?> <?><?><?>(<?><?>)
       <?><?><?><?><?><?>(<?><?>) <?><?><?><?><?><?><?><?><?><?><?>
       <?><?><?><?><?> <?><?><?> <?><?> <?><?><?><?><?><?> <?><?><?><?><?><?>
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       <?><?><?><?><?><?><?><?><?><?><?><?>
       <<debian-l10n-russian@lists.debian.org>>.

<?><?><?><?><?><?><?><?><?><?> <?><?><?><?><?><?><?><?> Linux 6.17
                       8 <?><?><?><?><?><?><?> 2026 <?>.                dup(2)