C: Why isn't size_t a C keyword?
sizeof is a C keyword. It returns the size in a type named size_t. However, size_t is not a keyword, but is defined primarily in stddef.h and probably other C standard header files too.
Consider a scenario where you want to create a C program which does not include any C standard headers or libraries. (Like for example, if you are creating an OS kernel.) Now, in such code, sizeof can be used (it is a C keyword, so it is a part of the language), but the type that it returns (size_t) is not available!
Does not this signify some kind of a problem in the C standard specification? Can you clarify this?
Answer
It does not literally return a value of type size_t since size_t is not a concrete type in itself, but rather a typedef to an unspecified built-in type. Typedef identifiers (such as size_t) are completely equivalent to their respective underlying types (and are converted thereto at compile time). If size_t is defined as an unsigned int on your platform, then sizeof returns an unsigned int when it is compiled on your system. size_t is just a handy way of maintaining portability and only needs to be included in stddef.h if you are using it explicitly by name.