dup2

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

COLOPHON
       This page is part of release 5.13 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.




Linux                              2021-03-22                             DUP(2)