Cool Compile time check

If only we could evaluate a condition at compile time instead of link or run time. Well there is a way ...
Picked up this nifty trick from the linux kernel code.

#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2 *!!(condition)]))

Awesome piece of code ....It's a beauty...

Concept : Array of negative size is illegal. Remember array of size 0 is possible
Therefore if a condition evaluates to true we have :-
void(sizeof(NEGATIVE ARRAY) which would trigger a compile time error

If the condition is false, we have :-

void (sizeof(char[1 - 2 * !!(0))])
==> void(sizeof(char[1-2* !1])
====>void(sizeof(char[1-0])
======>void(sizeof(char[1])
========> void(const number) ====> This has no impact on C code although it is a valid expression.
you could have 1; anywhere in the code but to be "gcc warning free" it is better to have (void) 1 .


Here is a simple example, don't worry about the logic as I just needed a condition and do NOT mean to say that size of int should be smaller than that of a char :)

#define ADY_BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2 * !!(condition)]))

int main(int argc, char *argv [])
{
   /* will not compile */
   ADY_BUILD_BUG_ON(sizeof(char) <  sizeof(int));
}

and here is what we get :--

nemesis@nemesis-laptop:~/test_code$ gcc build_bug_on_example.c
build_bug_on_example.c: In function ‘main’:
build_bug_on_example.c:9: error: size of array ‘type name’ is negative