bsd man page for dup

DUP(2)								System Calls 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 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 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)

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