C语言将16进制字符串转16进制数值

1、字母转小写 

 
int to_lower(int m)  
{  
    if (m >= 'A' && m <= 'Z')  
    {  
        return m + 'a' - 'A';  
    }  
    else  
    {  
        return m;  
    }  
} 

2、假如char a[2] ={'2','1'};

     %d   htoi(a) =33          u8(htoi(a)) = 21

//将十六进制的字符串转换成对应的整数
int htoi(char s[])  
{  
    int i = 0;  
    int n = 0;  
    if (s[0] == '0' && (s[1]=='x' || s[1]=='X')){  
        i = 2;  
    }  
    else{  
        i = 0;  
    }  
    for (; (s[i] >= '0' && s[i] <= '9') || (s[i] >= 'a' && s[i] <= 'z') || (s[i] >='A' && s[i] <= 'Z');++i)  
    {  
        if (to_lower(s[i]) > '9')  
        {  
            n = 16 * n + (10 + to_lower(s[i]) - 'a');  
        }  
        else  
        {  
            n = 16 * n + (to_lower(s[i]) - '0');  
        }  
    }  
    return n;  
}