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 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 descriptors, 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 descriptor 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 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 descriptor.  On error, -1 is  returned,  and
       errno is set appropriately.

ERRORS
       EBADF  oldfd  isn't an open file descriptor, or newfd is out of the allowed range for file
              descriptors.

       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.  Or, oldfd was equal to newfd.

       EMFILE The  process  already  has the maximum number of file descriptors open and tried to
              open a new one.

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(): SVr4, 4.3BSD, POSIX.1-2001.

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



Linux                                       2014-07-08                                     DUP(2)