How to store a 64 bit integer in 2 32 bit integers and convert back again

Question

Hi, basically exactly as the title says, I'm pretty sure its just a matter of some bitwise operations, I'm just not entirely sure of exactly what I should be doing, and all searches return back "64 bit vs 32 bit" etc etc thanks

Answer 1

pack:

u32 x, y;
u64 v = ((u64)x) << 32 | y;

unpack:

x = (u32)((v & 0xFFFFFFFF00000000) >> 32);
y = (u32)(v & 0xFFFFFFFF);

Answer 2

The basic method is as follows:

uint64_t int64;
uint32_t int32_1, int32_2;

int32_1 = int64 & 0xFFFFFFFF;
int32_2 = (int64 & (0xFFFFFFFF << 32) ) >> 32;

// ...

int64 = int32_1 | (int32_2 << 32);

Note that your integers must be unsigned; or the operations are undefined.

Answer 3

Or this, if you're not interested in what the two 32-bits numbers mean:

u32 x[2];
u64 z;
memcpy(x,&z,sizeof(z));
memcpy(&z,x,sizeof(z));

Answer 4

Use a union and get rid of the bit-operations:

<stdint.h> // for int32_t, int64_t

union {
  int64_t big;
  struct {
    int32_t x;
    int32_t y;
  };
};
assert(&y == &x + sizeof(x));

simple as that. big consists of both x and y.