C – When to Prefer uintptr_t Over intptr_t

c++pointersvoid-pointers

Given the requirement that I need to store the value of a "generic" pointer in a struct and have no interest in the pointed-at memory itself, I find it more semantically correct to store it as an intptr_t than a void*. The question is whether a uintptr_t is better suited or not, and when one is preferred over the other in general?

Best Answer

It is mostly a stylistic argument (an optimizing compiler would probably generate the same, or very similar, code). However, pointer compares may be a tricky issue.

Remember than in purely standard C pointer compare is roughly meaningful only for pointers to the same aggregate data. You are probably not allowed to compare two results from malloc, e.g. to keep a sorted array of pointers.

I would keep them as void*, or else as uintptr_t. The signed intptr_t has the inconvenience to seggregate negative and positive numbers, and where they are coming from significant application pointers, this is probably not welcome.

Notice that a void* cannot be dereferenced: as an uintptr_t, you have to cast it to do something useful with the data pointed by the address; however void* pointers can be passed to routines like memset

PS. I am assuming an ordinary processor (e.g. some x86, PowerPC, ARM, ...) with a flat virtual address space. You could find exotic processors -some DSPs perhaps- with very significant differences (and perhaps on which intptr_t is not always meaningful; remember that on the 1990s Cray Y-MP supercomputers sizeof(long*) != sizeof(char*); at that time C99 did not exist, and I am not sure its <stdint.h> could be meaningful on such machines)

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