[Solved] Copy 6 byte array to long long integer variable

Question

What you want to accomplish is called de-serialisation or de-marshalling.

For values that wide, using a loop is a good idea, unless you really need the max. speed and your compiler does not vectorise loops:

uint8_t array[6];
...
uint64_t value = 0;

uint8_t *p = array;
for ( int i = (sizeof(array) - 1) * 8 ; i >= 0 ; i -= 8 )
    value |= (uint64_t)*p++ << i;

// left-align
value <<= 64 – (sizeof(array) * 8);

Note using stdint.h types and sizeof(uint8_t) cannot differ from1`. Only these are guaranteed to have the expected bit-widths. Also use unsigned integers when shifting values. Right shifting certain values is implementation defined, while left shifting invokes undefined behaviour.

Iff you need a signed value, just

int64_t final_value = (int64_t)value;

after the shifting. This is still implementation defined, but all modern implementations (and likely the older) just copy the value without modifications. A modern compiler likely will optimize this, so there is no penalty.

The declarations can be moved, of course. I just put them before where they are used for completeness.

Accepted Answer