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>              /* Obtain O_* constant definitions */
       #include <unistd.h>

       int dup3(int oldfd, int newfd, int flags);

DESCRIPTION
       The  dup()  system call creates a copy of the file descriptor oldfd, using the lowest-num-
       bered unused file descriptor for the new descriptor.

       After a successful return, the old and new file descriptors may be  used  interchangeably.
       They  refer to the same open file description (see open(2)) and thus 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.

       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.
       If the file descriptor newfd was previously open,  it  is  silently  closed  before  being
       reused.

       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  han-
       dler  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 appropriately.

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 ver-
       sion 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)

COLOPHON
       This page is part of release 4.15 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                                       2017-09-15                                     DUP(2)