Can a pointer (address) ever be negative?

Question

I have a function that I would like to be able to return special values for failure and uninitialized (it returns a pointer on success).

Currently i

Answer 1

James answer is probably correct, but of course describes an implementation choice, not a choice that you can make.

Personally, I think addresses are "intuitively" unsigned. Finding a pointer that compares as less-than a null pointer would seem wrong. But ~0 and -1, for the same integer type, give the same value. If it's intuitively unsigned, ~0 may make a more intuitive special-case value - I use it for error-case unsigned ints quite a lot. It's not really different (zero is an int by default, so ~0 is -1 until you cast it) but it looks different.

Pointers on 32-bit systems can use all 32 bits BTW, though -1 or ~0 is an extremely unlikely pointer to occur for a genuine allocation in practice. There are also platform-specific rules - for example on 32-bit Windows, a process can only have a 2GB address space, and there's a lot of code around that encodes some kind of flag into the top bit of a pointer (e.g. for balancing flags in balanced binary trees).