How C can bite you, an example

Consider the function foo:

int foo() { if (error) return -1; else return 12; }

and consider its use here:

int n;
...
if ((n = foo()) >= sizeof(bar)) { memcpy(dst, src, n - sizeof(bar)); }

Seems simple and safe right? Well, not so much.  The 'C' standard states that when you have types that aren't of the same rank, a conversion must happen. So, since n is an int, and sizeof(foo) is a size_t (unsigned long), a weird conversion happens when you hit an error in foo. It returns -1, but that -1 is converted to MAX_ULONG, so the conversion is true, and we copy way too much memory smashing everything in it's path.

When a coworker asked me about this, I replied "And verily, thou shalt not compare an int to a size_t when thine int can be that which represents a plurality of deficit as well as a plurality of surplus." because it is easily clearer than the C standard...