Using define in a c header file

Question

I am just starting to learn C so hopefully this isn't a dumb question. The problem I am having is regarding header files and using #define for constants. I have read that I should use the following to prevent my header from being compiled more than once.

#ifndef NAME_OF_FILE
#define NAME_OF_FILE
.
. //my header file content
.
#endif

I want to add a constant, and I believe I would also use a #define such as,

#ifndef NAME_OF_FILE
#define NAME_OF_FILE

#define NUM_CONST 5 //the constant I want in my header file
.
.
.
#endif

How does C know that #define NAME_OF_FILE is referring to the .h file while #define NUM_CONST 5 is just a constant? Is it because of the value at the end of NUM_CONST? Or do I have this all completely wrong?

Answer 1

There is no essential difference between the two defines. #define NAME_OF_FILE and #define NUM_CONST 5 are the same sort of thing. They define a plaintext replacement of tokens (in the first case, the replacement is nothing).

For example you could put subsequent code:

printf("%d\n", NAME_OF_FILE NUM_CONST NAME_OF_FILE);

and after the above substitutions the code would turn in to:

printf("%d\n", 5);

which is correct. There is no "magic". The compilation step that performs these substitutions is usually known as "preprocessing".

NAME_OF_FILE does not refer to the .h file, per se, but the use of the ifndef directive achieves the goal of not compiling the same code twice.