DUP(2)		    UNIX Programmer's Manual		   DUP(2)


NAME
     dup, dup2 - duplicate a descriptor

SYNOPSIS
     newd = dup(oldd)
     int newd, oldd;

     dup2(oldd, newd)
     int oldd, newd;

DESCRIPTION
     Dup duplicates an existing object descriptor.  The argument
     oldd is a small non-negative integer index in the per-
     process descriptor table.	The value must be less than the
     size of the table, which is returned by getdtablesize(2).
     The new descriptor returned by the call, newd, is the lowest
     numbered descriptor that is not currently in use by the pro-
     cess.

     The object referenced by the descriptor does not distinguish
     between references using oldd and newd in any way.  Thus if
     newd and oldd are duplicate references to an open file,
     read(2), write(2) and lseek(2) calls all move a single
     pointer into the file, and append mode, non-blocking I/O and
     asynchronous I/O options are shared between the references.
     If a separate pointer into the file is desired, a different
     object reference to the file must be obtained by issuing an
     additional open(2) call.  The close-on-exec flag on the new
     file descriptor is unset.

     In the second form of the call, the value of newd desired is
     specified.  If this descriptor is already in use, the
     descriptor is first deallocated as if a close(2) call had
     been done first.

RETURN VALUE
     The value -1 is returned if an error occurs in either call.
     The external variable errno indicates the cause of the
     error.

ERRORS
     Dup and dup2 fail if:

     [EBADF]	    Oldd or newd is not a valid active descriptor

     [EMFILE]	    Too many descriptors are active.

SEE ALSO
     accept(2), open(2), close(2), fcntl(2), pipe(2), socket(2),
     socketpair(2), getdtablesize(2)


Printed 11/26/99	  May 13, 1986				1


 
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