Question

#if sizeof(int) != 4
/* do something */

Using sizeof inside #if doesn’t work while inside #define it works, why?

#define size(x) sizeof(x)/sizeof(x[0]) /*works*/

Answer 1

Nothing is evil – everything can be misused, or in your case misunderstood. The sizeof operator is a compiler feature, but compiler features are not available to the preprocessor (which runs before the compiler gets involved), and so cannot be used in #if preprocessor directives.

However, when you say:

#define size(x) sizeof(x)/sizeof(x[0])

and use it:

size(a)

the preprocessor performs a textual substitution that is handed to the compiler:

sizeof(a)/sizeof(a[0])