How to transfer UINT64 to 2 DWORDS?

Question

Is there efficient way to do this?

Answer 1

That's something you could be using a union for:

union {
    UINT64 ui64;
    struct {
        DWORD d0;
        DWORD d1;
    } y;
} un;

un.ui64 = 27;
// Use un.y.d0 and un.y.d1

An example (under Linix so using different types):

#include <stdio.h>
union {
    long ui64;
    struct {
        int d0;
        int d1;
    } y;
} un;
int main (void) {
    un.ui64 = 27;
    printf ("%d %d\n", un.y.d0, un.y.d1);
    return 0;
}

This produces:

27 0

Answer 2

Thought I would provide an example using LARGE_INTEGER FOR the windows platform. If I have a variable called "value" that is 64 bit, then I do:

LARGE_INTEGER li;
li.QuadPart = value;
DWORD low = li.LowPart;
DWORD high = li.HighPart;

Yes, this copies it, but I like the readability of it.

Answer 3

Keep in mind that 64-bit integers have alignment restrictions at least as great as 32-bit integers on all platforms. Therefore, it's perfectly safe to cast a pointer to a 64-bit integer as a pointer to a 32-bit.

ULONGLONG largeInt;
printf( "%u %u\n", ((DWORD *)&largeInt)[ 0 ], ((DWORD *)&largeInt)[ 1 ] );

Obviously, Pax's solution is a lot cleaner, but this is technically more efficient since it doesn't require any data copying.