How to convert unsigned char* to unsigned long long int?

Question

Does anyone know of a library that provides a function that performs this logic or perhaps a method to perform this logic?

I'm trying to convert:

unsigned char test[] = "\x00\x00\x56\x4b\x7c\x8a\xc5\xde";

to:

94882212005342 / 0x0000564b7c8ac5de

I'm aware I could loop over each individual byte in test and utilize sprintf to convert each byte to string and concatenate them into a buffer with strcat and convert the buffer string to unsigned long long via strtoull. However I'm looking for something more comprehensive and simple. Is there such a way?

Answer 1

It's just maths.

unsigned char test[] = "\x00\x00\x56\x4b\x7c\x8a\xc5\xde";
unsigned long long num = 
     (unsigned long long)test[0] << 56 |
     (unsigned long long)test[1] << 48 |
     (unsigned long long)test[2] << 40 |
     (unsigned long long)test[3] << 32 |
     (unsigned long long)test[4] << 24 |
     (unsigned long long)test[5] << 16 |
     (unsigned long long)test[6] <<  8 |
     (unsigned long long)test[7] <<  0;

Remember to cast to type wide enough before shifting.

You have 8 values:

 { 0x00, 0x00, 0x56, 0x4b, 0x7c, 0x8a, 0xc5, 0xde }

which in the decimal is:

 0 0 86 75 124 138 197 222

and you want to have:

 94882212005342

which is:

  94882212005342 = 0*2^56 + 0*2^48 + 86*2^40 + 75*2^32 + 124*2^24 + 138*2^16 + 197*2^8 +  222*2^0

It's a mathematical operation. You could write ex test[0] * 72057594037927936ull but that's less readable then test[0] << 56.