将字符串"0x20"之类的转为数值

static int _atoul(const char *str, unsigned char *pvalue)
{
    unsigned int result=0;

    while (*str)
    {
        if (isdigit((int)*str))
        {
            //对于unsigned int为32位的编译环境来说，result最大的值为4294967296
            if ((result<429496729) || ((result==429496729) && (*str<'6')))
            {
                //'0'的ASCII码为48
                result = result*10 + (*str)-48;
            }
            else
            {
                *pvalue = result;
                return -1;
            }
        }
        else
        {
            *pvalue=result;
            return -1;
        }
        str++;
    }
    *pvalue=result;
    return 0;
}


#define ASC2NUM(ch) (ch - '0')
#define HEXASC2NUM(ch) (ch - 'A' + 10)

static int  _atoulx(const char *str, unsigned char *pvalue)
{
    unsigned int   result=0;
    unsigned char  ch;

    while (*str)
    {
        ch=toupper(*str);//字符串转为大写表示
        if (isdigit(ch) || ((ch >= 'A') && (ch <= 'F' )))
        {
            //对于unsigned int为32位的编译环境来说，result最大的值为4294967296，十六进制表示为0x10000000
            if (result < 0x10000000)
            {
                result = (result << 4) + ((ch<='9')?(ASC2NUM(ch)):(HEXASC2NUM(ch)));
            }
            else
            {
                *pvalue=result;
                return -1;
            }
        }
        else
        {
            *pvalue=result;
            return -1;
        }
        str++;
    }

    *pvalue=result;
    return 0;
}

/*used for convert hex value from string to int*/
//第二个参数pvalue的类型为unsigned char *?为什么不是unsigned int *
static int str_to_num(const char *str, unsigned char *pvalue)
{
    if ( *str == '0' && (*(str+1) == 'x' || *(str+1) == 'X') ){
        if (*(str+2) == '\0'){
            return -1;
        }
        else{
            //十六进制转换
            return _atoulx(str+2, pvalue);
        }
    }
    else {
        //字符串转十进制
        return _atoul(str,pvalue);
    }
}