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)