How to get the head of a pointer?

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http://stackoverflow.com – I was wondering, what is the best way to get the head pointer of the given memory address? In example, if p is allocated at the address 0 with a size of 32 bytes how can I get back to 0 from the address 12? In other words, get back to the "main address" from an address further into its data structure. Is that possible? Any help is welcome, thanks! (HowTos)