Correct format specifier to print pointer or address?

Question

Which format specifier should I be using to print the address of a variable? I am confused between the below lot.

%u - unsigned integer

%x

Answer 1

Use %p, for "pointer", and don't use anything else*. You aren't guaranteed by the standard that you are allowed to treat a pointer like any particular type of integer, so you'd actually get undefined behaviour with the integral formats. (For instance, %u expects an unsigned int, but what if void* has a different size or alignment requirement than unsigned int?)

*) [See Jonathan's fine answer!] Alternatively to %p, you can use pointer-specific macros from , added in C99.

All object pointers are implicitly convertible to void* in C, but in order to pass the pointer as a variadic argument, you have to cast it explicitly (since arbitrary object pointers are only convertible, but not identical to void pointers):

printf("x lives at %p.\n", (void*)&x);