1.1. How should I decide which integer type to use?
Programmer: Hmmm, I think these three should be shorts and these 10 can fit in chars.
Modern compiler: Fuck that, you're all 32-bits. I ain't got time for unaligned memory access...
Programmer: Hmmm, I think these three should be shorts and these 10 can fit in chars.
Modern compiler: Fuck that, you're all 32-bits. I ain't got time for unaligned memory access...
I don't disagree that size_t is an appropriate type for indexes, but I don't think indexes are literally the reason it was created.
Now, sizeof does measure the size of things, but, one of the obvious sizes you can measure is an array†, and that's definitely also the maximum index value for the array, so I do think it's fair to say that's (part of) literally why size_t exists.
† One of C's treacherous footguns. In the scope where the array was defined it's an array, and sizeof(array) tells you how big that array is. But, passed as a parameter it becomes a pointer and sizeof(resulting_pointer) is the size of the pointer, not the array :(
size_t is a typedef (alias) for an implementation-defined unsigned integer type.
It could have been worded differently with the same meaning. For example, the core language (section 6 of the standard) could have said that sizeof yields a result of an implementation-defined unsigned integer type without referring to "size_t". The library section (section 7) already says that size_t "is the unsigned integer type of the result of the sizeof operator". Personally I think that referring to size_t in the language section adds clarity, even if it's slightly redundant.
In a given implementation, a compiler might arrange for sizeof to yield a result of type unsigned long, for example. The corresponding <stddef.h> header must then define size_t as unsigned long for the implementation to be correct.
Make sure to be careful and realize that sizeof my_array returns the number of bytes that my_array uses, not the number of elements. An array of 10 ints likely has sizeof == 40, while indexing past 9 would be undefined behavior.
Then again, being wrong about something seems faster than asking a question, and this little thread has already unearthed interesting answers, so maybe you have the right of it!
Unaligned memory access, types generally have an alignment which is the same as their length and structs have the alignment of their largest member because many architectures get quite cross with unaligned access e.g. trying to load a 32b integer from memory to register while it's not aligned to 32b, this is where compilers will pad structures to ensure everything is aligned properly by default, and where (in C/C++ anyway) you want to take care of your struct layout to avoid unnecessary padding. That doesn't prevent "these three should be shorts and these 10 can fit in chars." at all, but if you intersperse them the structure will increase in size due to padding.
Then there's C being specified in terms of lowest capabilities so while "char" and "short" are at least 8 and at least 16 bits… they can also be larger (POSIX does specify that char must be exactly 8 bits but non-POSIX platforms don't require that). That matches everything being 32b. It's a somewhat common occurrences for DSPs to have char of 16 or 32 bits, it really has nothing to do with the compiler (except insofar as the compiler does what the architecture description specifies).
Then there's the preferred data size, which is probably a function of the architectural ALUs than the native registers e.g. if an architecture only works on 32b datum then computations on 8 bit data will require loading 8 bits, zeroing the upper 24, performing the computation in 32b, copying whichever bit is concerned to the overflow flag, then zeroing that. Whereas performing the computation on a 32b-native type would require loading 32b, performing the computation, done (that was never a concern on x86/64 but IIRC older revisions of ARM could only natively work in 32b, and not all possible arithmetic operations got expanded to 8/16b at the same time).
And finally there's the cache line alignment. There are actually two opposite issues with cacheline alignments: you usually want your structure to not span cachelines (as that gets more expensive) but sometimes you want your structures be always start at the start of a cacheline (and possibly take the entirety of the cacheline) to avoid false sharing issues: if two unrelated structures are on the same cache line and they're used from different cores, the cores will need a lot more synchronisation than if they were on different cache lines.