“#define __SIZE_T”和“typedef unsigned int size_t”

Question

I've come across the following declaration of "size_t" a few times now, but don't really know what this means:

#ifndef __SIZE_T
#define __SIZE_T
typedef unsigned int size_t;
#endif

What exactly is going on here? What is #define __SIZE_T doing? Isn't the typedef alone enough to get the job done?

Answer 1

Identifiers and macros with double underscore or one underscore followed by a capital letter are reserved for the compiler. So __SIZE_T is most likely something internal in the specific compiler. (I think for example gcc/glibc internally uses something called __SIZE_TYPE__ .)

The #ifndef... #define is simply conditional compilation (sometimes called sloppily called "compiler switch", which is also a term used for compiler arguments). It checks if __SIZE_T has already been defined or not. This has to be a separate macro, since you can't do pre-processor checks on types. Something like #ifndef size_t won't work if size_t is a type.

So this seems to be a compiler's internal definition of the standard type size_t for a given compiler and target. Not something the application programmer need to care or worry about.

Typically you would find size_t inside stddef.h , same thing goes with NULL . However, the standard (7.19 and 7.21) states that stdio.h should also define these two. Hence the need for the macro, in case both stddef.h and stdio.h are included in the same program.