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Function names starting with "str" followed by a lowercase letter are reserved. So technically "str" itself and "str_" are not.

Doesn’t it stop you modifying them inside the function?

I'm not sure about C but at least in C++ having const on the prototype is meaningless as you can still have the arguments as non-const in the actual definition. Considering that C is usally less strict with these things I'd expect that to be the case there too.

String isn't a value type.

Yes it is, it's a small struct.