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Using malloc() when exploiting the C library

If heap support is required for bare machine C, _init_alloc() must be called first to supply initial heap bounds, and __rt_heap_extend() must be provided even if it only returns failure. Without __rt_heap_extend(), certain library functionality is included that causes problems when you are writing bare machine C.

Prototypes for both _init_alloc() and __rt_heap_extend() are in rt_heap.h.