Question

how to do a lexicographic compare between two pointers with unknown byte size,
assuming both array size is equal,
eg implementing the following function to return -1 or 0 or 1 based on the comparecement and converting each byte to ascii

compare(const void *p1, const void *p2, int size)

EDIT: i added another argument with the size of bytes to compare, how can i cast each byte to char and compare between them ?

Answer 1

You’re probably looking for the strcmp(3) function.

Update

If your arrays are not of ASCII strings and may contain the 0x00 byte at any position without actually meaning anything, then you’ll need the length as Oli points out, so you can use the memcmp(3) function:

int memcmp(const void *s1, const void *s2, size_t n);

Update #2

Okay, now that we’ve established you also want the byte that differs, you’ll need your own function. Something like this (untested):

size_t cmp(const unsigned char *c1, const unsigned char *c2, ssize_t n) {
    ssize_t i;
    for (i=0; i<n; i++) {
        if (c1[i] != c2[i])
            return i;
    }
    -1;
}

This will let you find the byte values for whichever bytes differ. Note the -1 return in case of no difference. This limits you to comparing objects whose size fit entirely in a ssize_t, which is smaller than the size_t type — but since you’re comparing two of them, I figure each one gets half the process address space at most anyway.