in c programming i need custom atoi code

i want to subtract two 64 bit hex numbers(in c programming), 0x0000888888888888 -1st hex value - 0x00000000839DDB7A -2nd hex value = 0x0000888804EAAD0E =(need the above result in hex)

The 1st hex value is in a unsigned long long int variable. But the 2nd hex value is in a unsigned char array. So i want the value in this char array as hex value in another unsigned long long int variable,then i can subtract those two. I have tried ATOI function,but it can convert only ascii number to its integer,it wont convert ascii character such as 'A' to hex value 'A'.

Am using the final result as key for the DES operation. Please help me to get this...Am stuck in this part through my big project from Harman technologies..Thank you in advance.