The zpl_u64 seems to be because they're trying to support older versions of the MSVC compiler[1]. As for why they're using a signed size parameter, I'm not really sure. One idiom they appear to use often is[2]:

    for (...; x--; y++)

where x is a size-type of some kind. They may be defaulting to signed size types to avoid errors in case somebody decides to change to above to:

    for (...; x-- >= 0; y++)

There's even a macro zpl_size_of:

    #define zpl_size_of(x) (zpl_isize)(sizeof(x))

But the real weirdness is in the other macros:

    #define cast(Type) (Type)

...just why? And

    #define ZPL_MULTILINE(...) #__VA_ARGS__

is supposed to be a way of having multiline literals in C. But, everything inside the "..." has to be valid C tokens. It might work as a string literal in many cases, but it's misleading at best and broken at worst.

I'm not sure what the advantage of the following is vs. just setting the bits using & and | which every C programmer should already be familiar with:

    #define ZPL_MASK_SET(var, set, mask) \
    do {                                 \
        if (set)                         \
        (var) |= (mask);                 \
        else                             \
        (var) &= ~(mask);                \
    } while (0)

There's surely no excuse for this:

    #define zpl_global static        // Global variables

While a lot of the library looks useful, stuff like the above makes it feel like it's written by somebody who knows C, but doesn't feel fully comfortable writing it.

[1]: https://github.com/zpl-c/zpl/blob/master/code/header/essenti...

[2]: https://github.com/zpl-c/zpl/blob/27c80bd5807da5238777bfcba8...