Seemingly useless parenthesis in #define macros (thing)

Very, very important, due to the fact that #define is really just preprocessed substitution.

Examine the following clearly wrong code:

	#include <stdio.h>
	#define HALF(X) X/2
	int main() {
		printf( "Half of 4 is %d\n", HALF(4+0) );
		return 0;
	}

Run it. It'll try to convince you that half of four is four! This is because the code is preprocessed as this:

	/* contents of stdio.h here */
	int main() {
		printf( "Half of 4 is %d\n", 4+0/2 );
		return 0;
	}

See? You need the parenthesis; the proper macro is:

	#define HALF(X) ((X)/2)

This will give you the (4+0)/2 you need. Also note surrounding parenthesis, which allows one to safely multiply by its value.