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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 (consult 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 command. 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  suc-
          cessful 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 referenced 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 vol-
       ume 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 descrip-
       tors. 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 invalid-
       ity 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, Copyright (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.opengroup.org/unix/online.html .

IEEE/The Open Group                  2003                              DUP(3P)