I'm implementing a heap allocator (malloc), and I need to choose a magic number to check if a given pointer point to a data structure I allocated. It seems obvious to me that no magic number can be considered completely safe (if the number is checked, I can be sure a point to one of my data structure), but maybe I missed something, so... if someone can help and bring me the number of my dreams, I'd really appreciate.
Thx in advance.
It depends on what you're doing this for. If you're doing it to try and catch programming mistakes (e.g., you want to make sure you don't accidentally mix up my_malloc/my_free and malloc/free), then just pick a random value. Sure, sometimes it'll fail to detect such a case, but that really doesn't matter. It shouldn't ever happen. So, here:
#define MAGIC_32BIT 0x77A5844CU
#define MAGIC_64BIT 0xD221A6BE96E04673UL
If correctness depends on this, then you really ought to do this another way. For example, by keeping track of which addresses you've allocated in a hash or tree or, in special cases, a bitmap.
If you're actually implementing malloc/free (e.g., writing your own C library), then keep in mind that freeing something that wasn't malloced (except NULL) is undefined behavior by the standard, so your code doesn't need to worry what happens.