When you allocate a block with malloc, the address of the block
never changes unless you use realloc to change its size. Thus,
you can safely store the address in various places, temporarily or
permanently, as you like. This is not safe when you use the relocating
memory allocator, because any and all relocatable blocks can move
whenever you allocate memory in any fashion. Even calling malloc
or realloc can move the relocatable blocks.
For each relocatable block, you must make a handle---a pointer object in memory, designated to store the address of that block. The relocating allocator knows where each block's handle is, and updates the address stored there whenever it moves the block, so that the handle always points to the block. Each time you access the contents of the block, you should fetch its address anew from the handle.
To call any of the relocating allocator functions from a signal handler is almost certainly incorrect, because the signal could happen at any time and relocate all the blocks. The only way to make this safe is to block the signal around any access to the contents of any relocatable block--not a convenient mode of operation. See section Signal Handling and Nonreentrant Functions.
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