Identify exactly where memcmp fails

Question

Title is pretty self-explanatory, I am comparing two memory blocks that SHOULD be identical but I'm getting a failure. I have no idea where the test fails. Finding that out would help me debug the issue. So is there a way to find exactly where memcmp fails?

Answer 1

You can use this:

void *mymemcmp(const void *ptr1, const void *ptr2, size_t num) {
    const unsigned char *s1 = (const unsigned char*)ptr1;
    const unsigned char *s2 = (const unsigned char*)ptr2;
    for(size_t index = 0; i<num; i++) {
        if(*s1 != *s2) return s1;
        s1++;
        s2++;
    }
    return NULL;
}    

It returns a pointer to the position of the first difference in the first argument. Then you can investigate it like this.

void *ao = mymecmp(a, b, n);
ptrdiff_t d = ao-(void*)a;
void *bo = d+(void*)b;

Now, ao and bo points to where the difference is.