dup2

       #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-numbered unused 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 descrip-
       tors, the offset is also changed for the other.

       The two 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 descrip-
       tor number specified in newfd.  If the descriptor newfd was  previously
       open, it is silently closed before being reused.

       The  steps  of  closing  and reusing the file descriptor newfd are per-
       formed 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  descrip-
          tion of the same flag in open(2) for reasons why this may be useful.

       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  sig-
              nal(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  getr-
              limit(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 sin-
       gle-threaded and does not allocate  file  descriptors  in  signal  han-
       dlers--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) {

       http://www.kernel.org/doc/man-pages/.

Linux                             2015-12-28                            DUP(2)
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