NAME

       dup, dup2, dup3 - duplicate a file descriptor

LIBRARY

       Standard C library (libc, -lc)

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)).

STANDARDS

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

       dup3() Linux.

HISTORY

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

       dup3() Linux 2.6.27, glibc 2.9.

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)