"Type" of symbolic constants?

Question

1. When is it appropriate to include a type conversion in a symbolic constant/macro, like this:

   #define MIN_BUF_SIZE ((size_t) 256)
   

   Is it a good way to make it behave more like a real variable, with type checking?

2. When is it appropriate to use the L or U (or LL) suffixes:

   #define NBULLETS 8U
   #define SEEK_TO 150L
   

Answer 1

You need to do it any time the default type isn't appropriate. That's it.

Answer 2

Typing a constant can be important at places where the automatic conversions are not applied, in particular functions with variable argument list

printf("my size is %zu\n", MIN_BUF_SIZE);

could easily crash when the width of int and size_t are different and you wouldn't do the cast.

But your macro leaves room for improvement. I'd do that as

#define MIN_BUF_SIZE ((size_t)+256U)

(see the little + sign, there?)

When given like that the macro still can be used in preprocessor expressions (with #if). This is because in the preprocessor the (size_t) evaluates to 0 and thus the result is an unsigned 256 there, too.