Question

I am using a library that returns a structure with a time stamp that is represented by two (TimestampHi, TimestampLo) unsigned longs. I pretty much only need the timestamp printed out as %llu in printf.

What’s the easiest way to get the data from these two ints and correctly use it as a uint64_t?

Answer 1

Assuming that unsigned long long is a 64-bit type on your platform

assert(sizeof(unsigned long) * CHAR_BIT == 32);
assert(sizeof(unsigned long long) * CHAR_BIT == 64);
// Static asserts are more appropriate in cases like this

unsigned long long Timestamp = TimestampHi;
Timestamp <<= 32; Timestamp += TimestampLo; 

And then print the Timestamp value.

Same thing as an one-liner

unsigned long long Timestamp = ((unsigned long long) TimestampHi << 32) + TimestampLo; 

or simply

printf("%llu\n", ((unsigned long long) TimestampHi << 32) + TimestampLo);

If you wish to abstract your code from bit-based operations, the expression can be rewritten as

TimestampHi * ((unsigned long long) ULONG_MAX + 1) + TimestampLo