#include <stdio.h>
unsigned char mychar[]={0x03,0xc0};
unsigned long StringToHex(char *str)
{
        char    *p;
        unsigned long val = 0;
        for (p = str; *p; ++p) {
                val <<= 4;
                switch (*p) {
                case '0':       val += 0;       break;
                case '1':       val += 1;       break;
                case '2':       val += 2;       break;
                case '3':       val += 3;       break;
                case '4':       val += 4;       break;
                case '5':       val += 5;       break;
                case '6':       val += 6;       break;
                case '7':       val += 7;       break;
                case '8':       val += 8;       break;
                case '9':       val += 9;       break;
                case 'A':
                case 'a':       val += 10;      break;
                case 'B':
                case 'b':       val += 11;      break;
                case 'C':
                case 'c':       val += 12;      break;
                case 'D':
                case 'd':       val += 13;      break;
                case 'E':
                case 'e':       val += 14;      break;
                case 'F':
                case 'f':       val += 15;      break;
                }
        }
        return val;
}

int main()
{
unsigned char tmpchar[255];
snprintf(tmpchar,sizeof(tmpchar),"0x%02x%02x",mychar[0],mychar[1]);
printf("%d\n",StringToHex(tmpchar));
}