double d = 1234.5678;
uint64_t bits;
memcpy(&bits, &d, sizeof bits);
printf("the representation of %g is %08"PRIX64"n", d, bits);

union { double d; uint64_t i; } u;
u.d = 1234.5678;
printf("the representation of %g is %08"PRIX64"n", d, u.i);

void *my_memcpy(void *dst, const void *src, size_t n) {
    unsigned char *d = dst;
    const unsigned char *s = src;
    for (size_t i = 0; i < n; i++) { d[i] = s[i]; }
    return dst;
}

double d = 1234.5678;
void *p = malloc(sizeof(double));
if (p != NULL) {
    uint64_t *pbits = memcpy(p, &d, sizeof(double));
    uint64_t bits = *pbits;
    printf("the representation of %g is %08"PRIX64"n", d, bits);
}

double d = 1234.5678;
void *p = malloc(sizeof(double));
assert(p);
uint64_t *pbits = memcpy(p, &d, sizeof(double));
uint64_t bits = *pbits;
printf("the representation of %g is %08"PRIX64"n", d, bits);

double d = 1234.5678;
uint64_t bits;
memcpy(&bits, &d, sizeof bits);
printf("the representation of %g is %08"PRIX64"n", d, bits);

double d = 1234.5678;
uint64_t bits;
memcpy(&bits, &d, sizeof bits);
printf("the representation of %g is %08"PRIX64"n", d, bits);

union { double d; uint64_t i; } u;
u.d = 1234.5678;
printf("the representation of %g is %08"PRIX64"n", d, u.i);

double d = 1234.5678;
unsigned char bits[sizeof(d)];
memcpy(&bits, &d, sizeof(bits));
printf("the representation of %g is ", d);
for(int i=0; i<sizeof(bits); i++) {
    printf("%02x", (unsigned int) bits[i]);
}
printf("n");