Why is memcmp(a, b, 4) only sometimes optimized to a uint32 comparison?

Question

Given this code:

#include 

int equal4(const char* a, const char* b)
{
    return memcmp(a, b, 4) == 0;
}

int less4(const char* a, const char* b         


        

Answer 1

Endianness is the problem here. Consider this input:

a = 01 00 00 03
b = 02 00 00 02

If you compare these two arrays by treating them as 32-bit integers, then you'll find that a is larger (because 0x03000001 > 0x02000002). On a big-endian machine, this test would probably work as expected.