DUP(3P)                                             POSIX Programmer's Manual                                            DUP(3P)



PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may differ (con-
       sult the corresponding Linux manual page for details of Linux behavior), or the  interface  may  not  be  implemented  on
       Linux.

NAME
       dup, dup2 - duplicate an open file descriptor

SYNOPSIS
       #include <unistd.h>

       int dup(int fildes);
       int dup2(int fildes, int fildes2);


DESCRIPTION
       The dup() and dup2() functions provide an alternative interface to the service provided by fcntl() using the F_DUPFD com-
       mand. The call:


              fid = dup(fildes);

       shall be equivalent to:


              fid = fcntl(fildes, F_DUPFD, 0);

       The call:


              fid = dup2(fildes, fildes2);

       shall be equivalent to:


              close(fildes2);
              fid = fcntl(fildes, F_DUPFD, fildes2);

       except for the following:

        * If fildes2 is less than 0 or greater than or equal to {OPEN_MAX}, dup2() shall return -1 with errno set to [EBADF].

        * If fildes is a valid file descriptor and is equal to fildes2, dup2() shall return fildes2 without closing it.

        * If fildes is not a valid file descriptor, dup2() shall return -1 and shall not close fildes2.

        * The value returned shall be equal to the value of fildes2 upon successful completion, or -1 upon failure.

RETURN VALUE
       Upon successful completion a non-negative integer, namely the file descriptor, shall be returned; otherwise, -1 shall  be
       returned and errno set to indicate the error.

ERRORS
       The dup() function shall fail if:

       EBADF  The fildes argument is not a valid open file descriptor.

       EMFILE The number of file descriptors in use by this process would exceed {OPEN_MAX}.


       The dup2() function shall fail if:

       EBADF  The  fildes  argument  is  not a valid open file descriptor or the argument fildes2 is negative or greater than or
              equal to {OPEN_MAX}.

       EINTR  The dup2() function was interrupted by a signal.


       The following sections are informative.

EXAMPLES
   Redirecting Standard Output to a File
       The following example closes standard output for the current processes, re-assigns standard output to go to the file ref-
       erenced by pfd, and closes the original file descriptor to clean up.


              #include <unistd.h>
              ...
              int pfd;
              ...
              close(1);
              dup(pfd);
              close(pfd);
              ...

   Redirecting Error Messages
       The following example redirects messages from stderr to stdout.


              #include <unistd.h>
              ...
              dup2(1, 2);
              ...

APPLICATION USAGE
       None.

RATIONALE
       The  dup()  and  dup2() functions are redundant. Their services are also provided by the fcntl() function. They have been
       included in this volume of IEEE Std 1003.1-2001 primarily for historical reasons, since many  existing  applications  use
       them.

       While  the  brief  code  segment  shown is very similar in behavior to dup2(), a conforming implementation based on other
       functions defined in this volume of IEEE Std 1003.1-2001 is significantly more complex. Least  obvious  is  the  possible
       effect  of  a  signal-catching  function that could be invoked between steps and allocate or deallocate file descriptors.
       This could be avoided by blocking signals.

       The dup2() function is not marked obsolescent because it presents a type-safe version  of  functionality  provided  in  a
       type-unsafe version by fcntl(). It is used in the POSIX Ada binding.

       The dup2() function is not intended for use in critical regions as a synchronization mechanism.

       In  the  description  of  [EBADF], the case of fildes being out of range is covered by the given case of fildes not being
       valid. The descriptions for fildes and fildes2 are different because the only kind of invalidity  that  is  relevant  for
       fildes2 is whether it is out of range; that is, it does not matter whether fildes2 refers to an open file when the dup2()
       call is made.

FUTURE DIRECTIONS
       None.

SEE ALSO
       close(), fcntl(), open(), the Base Definitions volume of IEEE Std 1003.1-2001, <unistd.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003  Edition,  Standard  for
       Information  Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copy-
       right (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any
       discrepancy  between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group
       Standard  is  the  referee   document.   The   original   Standard   can   be   obtained   online   at   http://www.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                                       DUP(3P)