This snippet is not correct: #include <limits.h> #if ULONG_MAX != -1UL #error "This code requires 2s' complement arithmetic" #endif The #error case wi

This snippet is not correct: #include <limits.h> #if ULONG_MAX != -1UL #error "This code requires 2s' complement arithmetic" #endif The #error case will never be called, because the behaviour of the unsigned types is completely defined in C - it doesn't matter whether the system uses 2s complement or not, the result of applying the negation operator to 1UL must always be ULONG_MAX.