How to turn a double to a 64-bit integer bit by bit using unions

Question

I would like to store all bits of a double in a 64-bit integer in C.

I.e. the most significant bit should be the sign, after that the exponent, then the fraction.

I would like to use a union.

Answer 1

The safest way to do this is to use memcpy.

double x = 123.456;
uint64_t y;
memcpy(&y, &x, sizeof y);

If you don't have memcpy at your disposal, you can use a union:

union {
    double d;
    uint64_t i;
} u;

double x = 123.456;
uint64_t y;

u.d = x;
y = u.i;