这个给你个函数，可以自己看看原理。

//↓↓↓↓↓↓↓↓↓↓↓↓↓↓   通用函数的实现   ↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓
int Str2Hex(int InLen char *InBuffer BYTE *OutArray)
{
	int  i = 0j = 0;
    char tmp;
	if(InLen%2) return 0;//数据不为偶数，有一位十六进制数少一位数据
    for( i=0j=0;i < InLen;)
    {
		if(isxdigit(InBuffer[i])==0) return 0;//判断字符是否是十六进制字符
		
        if( InBuffer[i] >= 0x30 && InBuffer[i] <= 0x39 )
        {
            tmp = (InBuffer[i] - 0x30)<<4;
        }
        else if( InBuffer[i] >= 0x41 && InBuffer[i] <= 0x46 )
        {
            tmp=(InBuffer[i] - 0x37)<<4;
        }
        else if( InBuffer[i] >= 0x61 && InBuffer[i] <= 0x66 )
        {
            tmp=(InBuffer[i] - 0x57)<<4; 	
        }
        i++;
		
        if( InBuffer[i] >= 0x30 && InBuffer[i] <= 0x39 )
        {
            tmp = tmp | (InBuffer[i] - 0x30);
        }
        else if( InBuffer[i] >= 0x41 && InBuffer[i] <= 0x46 )
        {
            tmp= tmp | (InBuffer[i] - 0x37);
        }
        else if( InBuffer[i] >= 0x61 && InBuffer[i] <= 0x66 )
        {
            tmp =tmp | (InBuffer[i] - 0x57);
        }
		
        OutArray[j++] = tmp;
        i++;
    }
   	return(j);
}