Snippet from Weechat's gui-completion.c. I feel like this could be rewritten with memcpy...

gui-completion.c

        if (pos_start <= pos_end)
        {
            completion->position_replace = pos_start;
            completion->base_word = malloc (pos_end - pos_start + 2);
            for (i = pos_start; i <= pos_end; i++)
            {
                completion->base_word[i - pos_start] = data[i];
            }
            completion->base_word[pos_end - pos_start + 1] = '\0';
        }