C: Why isn't size_t a C keyword?

Question

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 1

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.

Answer 2

sizeof is a keyword because, despite it's name and usage, it is an operator like + or = or < rather than a function like printf() or atoi() or fgets(). A lot of people forget (or just don't know) that sizeof is actually an operator, and is always resolved at compile-time rather than at runtime.

The C language doesn't need size_t to be a usable, consistent language. That's just part of the standard library. The C language needs all operators. If, instead of +, C used the keyword plus to add numbers, you would make it an operator.

Besides, I do semi-implicit recasting of size_ts to unsigned ints (and regular ints, but Kernighan and Ritchie will someday smite me for this) all the time. You can assign the return type of a sizeof to an int if you like, but in my work I'm usually just passing it straight on to a malloc() or something.