How do I best use the const keyword in C?

Question

I am trying to get a sense of how I should use const in C code. First I didn't really bother using it, but then I saw a quite a few examples of const being used throughout. Should I make an effort and go back and religiously make suitable variables const? Or will I just be waisting my time?

I suppose it makes it easier to read which variables that are expected to change, especially in function calls, both for humans and the compiler. Am I missing any other important points?

Answer 1

const is typed, #define macros are not.

const is scoped by C block, #define applies to a file (or more strictly, a compilation unit).

const is most useful with parameter passing. If you see const used on a prototype with pointers, you know it is safe to pass your array or struct because the function will not alter it. No const and it can.

Look at the definition for such as strcpy() and you will see what I mean. Apply "const-ness" to function prototypes at the outset. Retro-fitting const is not so much difficult as "a lot of work" (but OK if you get paid by the hour).

Also consider:

const char *s = "Hello World";
char *s = "Hello World";

which is correct, and why?