小程序 - c字符串表示大整数

#define max(a, b) ((a) > (b) ? (a) : (b))

static char* reverse(char *str)
{
    char *l, *r, c;
    for (l = str, r = str + strlen(str) - 1; l < r; l++, r--) {
        c = *l;
        *l = *r;
        *r = c;
    }
    return str;
}

/*
 * Return the result of a big int as string when succeeded; otherwise
 * a NULL is returned. The caller needs to take care of the returned
 * heap memory; caller needs to ensure the inputs are valid big int.
 */
char* addBigInt(char *i1, char *i2) 
{
    char *p1, *p2, *p3, *res;
    int len1, len2, val, carry;
    
    if (!i1 || !*i1)
        return strdup(i2);
    if (!i2 || !*i2)
        return strdup(i1);
    
    len1 = strlen(i1);
    len2 = strlen(i2);
    if ((res = (char*)malloc(max(len1, len2) + 2)) == NULL)
        return NULL;
    for (p1 = i1 + len1 - 1, p2 = i2 + len2 - 1, p3 = res, val = 0; 
        p1 != i1 - 1 || p2 != i2 - 1 || val; 
　　　　 *p3++ = val % 10 + '0', val /= 10) {
        if (p1 != i1 - 1)
            val += *(p1--) - '0';
        if (p2 != i2 - 1)
            val += *(p2--) - '0';
    }
    *p3 = '\0';
    return reverse(res);
}