c - Can a pointer (address) ever be negative?

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 it returns NULL for failure, and -1 for uninitialized, and this seems to work... but I could be cheating the system. IIRC, addresses are always positive, are they not? (although since the compiler is allowing me to set an address to -1, this seems strange).

Another idea I had (in the event that -1 was risky) is to malloc a char @ the global scope, and use that address as a sentinel.

Answer

No, addresses aren't always positive - on x86_64, pointers are sign-extended and the address space is clustered symmetrically around 0 (though it is usual for the "negative" addresses to be kernel addresses).

However the point is mostly moot, since C only defines the meaning of < and > pointer comparisons between pointers that are to part of the same object, or one past the end of an array. Pointers to completely different objects cannot be meaningfully compared other than for exact equality, at least in standard C - if (p < NULL) has no well defined semantics.

You should create a dummy object with static storage duration and use its address as your unintialised value:

extern char uninit_sentinel;
#define UNINITIALISED ((void *)&uninit_sentinel)

It's guaranteed to have a single, unique address across your program.