string search across multiple buffers algorithm

I used a simple implementation (online).

/** match granularity is bytes, i.e. compare byte by byte. * @param mem the text to be searched * @param mem_len the text length * @param pattern the word sought * @param pattern_length the pattern length * @param pattern_index the continuing index from the last partial/whole match or 0 * @return the past-the-end index after the text is looked through or the stop index if partial match occurs * @example size_t mem_idx, index = 0; // if matched, current matching range in mem is [mem_idx-(index-old_index), mem_idx-1]. *  mem_idx = memmatch("ABC", 3, "FGH", 3, &index); // NotFound: mem_idx = 3; index = 0; *  mem_idx = memmatch("EFG", 3, "FGH", 3, &index); // Continue: mem_idx = 3; index = 2; mem[1,2]=pat[0,1] *  mem_idx = memmatch("HIJ", 3, "FGH", 3, &index); // Complete: mem_idx = 1; index = 3; mem[0,0]=pat[2,2] */size_t memmatch(const void* mem, size_t mem_len, const void* pattern, size_t pattern_length, size_t* pattern_index) {    assert(*pattern_index < pattern_length);    size_t idx = 0; // do for loop on `mem`    register size_t index = *pattern_index;    for (; idx < mem_len;) {        if (*((const char*)mem + idx++) == *(const char*)pattern + index) {            ++index;            if (pattern_length == index) {                break; // ++idx;            }        } else if (index) {            index = 0; // reset        }    }    *pattern_index = index;    return idx;}#ifdef MEMMATCH_EXAMPLEvoid memmatch_example0() {    size_t mem_idx, idx, index = 0;    mem_idx = memmatch("ABC", 3, "FGH", 3, (idx = index, &index)); // NotFound    std::cout << "mem_idx stops at " << mem_idx << ", pat_idx stops at " << index << " since " << idx << '\n';    mem_idx = memmatch("EFG", 3, "FGH", 3, (idx = index, &index)); // Continue    std::cout << "mem_idx stops at " << mem_idx << ", pat_idx stops at " << index << " since " << idx << '\n';    mem_idx = memmatch("HIJ", 3, "FGH", 3, (idx = index, &index)); // Complete    std::cout << "mem_idx stops at " << mem_idx << ", pat_idx stops at " << index << " since " << idx << '\n';}#endif