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DUP(2)                                                                  Linux Programmer's Manual                                                                 DUP(2)

NAME
       dup, dup2, dup3 - duplicate a file descriptor

SYNOPSIS
       #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
       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  de‐
       scriptor 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 spec‐
       ified 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 us‐
       ing 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  pro‐
       gram 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) for reasons why this may be useful.

       *  If oldfd equals newfd, then dup3() fails with the error EINVAL.

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

ERRORS
       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)).

VERSIONS
       dup3() was added to Linux in version 2.6.27; glibc support is available starting with version 2.9.

CONFORMING TO
       dup(), dup2(): POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.

       dup3() is Linux-specific.

NOTES
       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 re‐
       turns 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
       close(2), fcntl(2), open(2), pidfd_getfd(2)

Linux                                                                          2021-03-22                                                                         DUP(2)