Any precision loss when converting float64 to uint64 in C? assuming only the whole number part of the data is meaningful

I have a counter field from one TCP protocol which keeps track of samples sent out. It is defined as float64. I need to translate this protocol to a different one, in which the counter is defined as uint64.

Can I safely store the float64 value in the uint64 without losing any precision on the whole number part of the data? Assuming the fractional part can be ignored.

EDIT: Sorry if I didn't describe it clearly. There is no code. I'm just looking at two different protocol documentations to see if one can be translated to the other.

Please treat float64 as double, the documentation isn't well written and is pretty old.

I am assuming you are asking about 64 bit floating point values such as IEEE Binary64 as documented in https://en.wikipedia.org/wiki/Double-precision_floating-point_format .

Converting a double represented as a 64 bit IEEE floating point value to a uint64_t will not lose any precision on the integral part of the value, as long as the value itself is non negative and less than 2⁶⁴. But if the value is larger than 2⁵³, the representation as a double does not allow complete precision, so whatever computation led to the value probably was inaccurate anyway.

Note that the reverse is false, IEEE floats have less precision than 64 bit uint64_t, so close but different large values will convert to the same double values.

Note that a counter implemented as a 64 bit double is intrinsically limited by the precision of the floating point type. Incrementing a value larger than 2⁵³ by one is likely to have no effect at all. Using a floating point type to implement a packet counter seems a very bad idea. Using a uint64_t counter directly seem a safer bet. You only have to worry about wrap around at 2⁶⁴, a condition that you can check for in the unlikely case where you would actually expect to count that far.

If you cannot change the format, verify that the floating point value is within range for the conversion and store an appropriate value if it is not:

#include <stdint.h>

...

double v = get_64bit_value();
uint64_t result;

if (v < 0) {
    result = 0;
} else
if (v >= (double)UINT64_MAX) {
    result = UINT64_MAX;
} else {
    result = (uint64_t)v;
}