The C standard guarantees that size_t
is a type that can hold any array index. This means that, logically, size_t
should be able to hold any pointer type. I've read on some sites that I found on the Googles that this is legal and/or should always work:
void *v = malloc(10);
size_t s = (size_t) v;
So then in C99, the standard introduced the intptr_t
and uintptr_t
types, which are signed and unsigned types guaranteed to be able to hold pointers:
uintptr_t p = (size_t) v;
So what is the difference between using size_t
and uintptr_t
? Both are unsigned, and both should be able to hold any pointer type, so they seem functionally identical. Is there any real compelling reason to use uintptr_t
(or better yet, a void *
) rather than a size_t
, other than clarity? In an opaque structure, where the field will be handled only by internal functions, is there any reason not to do this?
By the same token, ptrdiff_t
has been a signed type capable of holding pointer differences, and therefore capable of holding most any pointer, so how is it distinct from intptr_t
?
Aren't all of these types basically serving trivially different versions of the same function? If not, why? What can't I do with one of them that I can't do with another? If so, why did C99 add two essentially superfluous types to the language?
I'm willing to disregard function pointers, as they don't apply to the current problem, but feel free to mention them, as I have a sneaking suspicion they will be central to the "correct" answer.
size_t
is a type that can hold any array index. This means that, logically, size_t should be able to hold any pointer type
Not necessarily! Hark back to the days of segmented 16-bit architectures for example: an array might be limited to a single segment (so a 16-bit size_t
would do) BUT you could have multiple segments (so a 32-bit intptr_t
type would be needed to pick the segment as well as the offset within it). I know these things sound weird in these days of uniformly addressable unsegmented architectures, but the standard MUST cater for a wider variety than "what's normal in 2009", you know!-)