如何正确地乘以两个长多头？

#include <stdio.h>
#include <stdlib.h>
#include <limits.h>
typedef unsigned int uint32;
typedef unsigned long long uint64;
int main(int argc, char* argv[] )
{

  uint64 a = (uint64)ULONG_MAX;
  printf("%llun", a);
  uint64 b = (uint64)ULONG_MAX;  
  printf("%llun", b);  
  uint64 c = (uint64)(a*b);

  printf("%llun", c);  // prints 1. that would be to lower 32 bits of the results. the upper half is 0xFFFFFFFE

  printf("%llun", ULLONG_MAX);
  system("pause");
}

4294967295
4294967295
1
4294967295

  printf("%in", sizeof(unsigned long));
  printf("%in", sizeof(unsigned long long)); 

4
8

  printf("%lun", c & 0xFFFFFFFF);
  printf("%lun", (c >> 32));