1.1 str_cut function.c

char *
str_cut(char str[], int slice_from, int slice_to)
{
        if (str[0] == '\0')
                return NULL;

        char *buffer;
        size_t str_len, buffer_len;

        if (slice_to < 0 && slice_from > slice_to) {
                str_len = strlen(str);
                if (abs(slice_to) > str_len - 1)
                        return NULL;

                if (abs(slice_from) > str_len)
                        slice_from = (-1) * str_len;

                buffer_len = slice_to - slice_from;
                str += (str_len + slice_from);

        } else if (slice_from >= 0 && slice_to > slice_from) {
                str_len = strlen(str);

                if (slice_from > str_len - 1)
                        return NULL;
                buffer_len = slice_to - slice_from;
                str += slice_from;

        } else
                return NULL;

        buffer = calloc(buffer_len, sizeof(char));
        strncpy(buffer, str, buffer_len);
        return buffer;
}