RT</tt> and <TT>DATAEND</tt> for root pointers.
On some platforms, this can be defined to a constant address,
though experience has shown that to be risky.  Ideally the linker will
define a symbol (e.g. <TT>_data</tt> whose address is the beginning
of the data segment.  Sometimes the value can be computed using
the <TT>GC_SysVGetDataStart</tt> function.  Not used if either
the next macro is defined, or if dynamic loading is supported, and the
dynamic loading support defines a function
<TT>GC_register_main_s