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

Answer 1

Some headers from the C standard are defined for a freestanding environment, i.e. fit for use e.g. in an operating system kernel. They do not define any functions, merely defines and typedefs.

They are float.h, iso646.h, limits.h, stdarg.h, stdbool.h, stddef.h and stdint.h.

When working on an operating system, it isn't a bad idea to start with these headers. Having them available makes many things easier in your kernel. Especially stdint.h will become handy (uint32_t et al.).