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BSD 2.11 - man page for dup2 (bsd section 2)

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DUP(2)											   DUP(2)

       dup, dup2 - duplicate a descriptor

       newd = dup(oldd)
       int newd, oldd;

       dup2(oldd, newd)
       int oldd, newd;

       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 process.

       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  descrip-
       tor  is already in use, the descriptor is first deallocated as if a close(2) call had been
       done first.

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

       Dup and dup2 fail if:

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

       [EMFILE]       Too many descriptors are active.

       accept(2),  open(2),  close(2),	fcntl(2),  pipe(2),  socket(2), socketpair(2), getdtable-

4th Berkeley Distribution		   May 13, 1986 				   DUP(2)
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