What is the difference between %zu and %lu in string formatting in C? %lu is used for unsigned long values and %zu is used for size_t values, but in practice, size_t is just an unsigned long. CppCheck complains about it, but both work for both types in my experience.
Is %zu just a standardized way of formatting size_t because size_t is commonly used, or is there more to it?
but in practice,
size_tis just anunsigned long
Not necessarily. There are systems with a 32 bit long and a 64 bit size_t. MSVC is one of them.
Given the following:
printf("long: %zu\n", sizeof(long));
printf("long long: %zu\n", sizeof(long long));
printf("size_t: %zu\n", sizeof(size_t));
Compiling under MSVC 2015 in x86 mode outputs:
long: 4
long long: 8
size_t: 4
While compiling in x64 mode outputs:
long: 4
long long: 8
size_t: 8
Having a separate size modifier for size_t ensures you're using the correct size.
z modifier is specifically for size_t, so it will be correct for whatever the size of size_t happens to be.
size_t is usually the same size as a pointer (but there are likely to be some exceptions - EDIT e.g. AS/400 has 128-bit pointers and 32-bit size_t!).
size_tcould beunsigned long longand then%luwould be wrong but%zuis still correct.size_tis 32-bits, in 64-bit it is 64 bits. Butlongis 32 bits in both.unsigned longis at least 32-bit.size_tis at least 16-bit.