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To read from a file view, a process dereferences the pointer returned by the MapViewOfFile function:

The process also uses the pointer returned by MapViewOfFile to write to the file view:

 

The FlushViewOfFile function copies the specified number of bytes of the file view to the physical file, without waiting for the cached write operation to occur:

 

  Each process uses the UnmapViewOfFile function to invalidate the pointer to the mapped memory. This destroys the file view in the address space of the process. If any pages of the file view have changed since the file view was mapped, UnmapViewOfFile also copies the changed pages to the file on disk.

  When each process finishes using the file-mapping object and has umapped all views, it must close the file mapping object's handle by calling CloseHandle.

  CloseHandle(hMapFile); 

Creating Named Shared Memory:

  The first process calls the CreateFileMapping function to create a file-mapping object and give it the name MyFileMappingObject. By using the PAGE_READWRITE flag, the processes will have read/write permission to the memory through any file views that are created.

  The process then uses the file-mapping object handle returned by CreateFileMapping in the call to MapViewOfFile to create a view of the file in the process's address space. The MapViewOfFile function returns a pointer to the file view.

  The second process calls the OpenFileMapping function with the name MyFileMappingObject to use the same file-mapping object as the first process. Like the first process, the second process uses the MapViewOfFile function to obtain a pointer to the file view.

 

  } 

#include <iostream>
#include <time.h>

using namespace std;

// generate random numbers
class randomNumber
{
 public:
  // initialize the random number generator
  randomNumber(long s = 0);

  // return a 32-bit random integer m, 1 <= m <= 2^31-2
  long random();

  // return a 32-bit random integer m, 0 <= m <= n-1,
  // where n <= 2^31-1
  long random(long n);

  // return a real number x, 0 <= x < 1
  double frandom();

 private:
  static const long A;
  static const long M;
  static const long Q;
  static const long R;

  long seed;
};

const long randomNumber::A = 48271;
const long randomNumber::M = 2147483647;
const long randomNumber::Q = M / A;
const long randomNumber::R = M % A;

randomNumber::randomNumber(long s)
{
 if (s < 0)
  s = 0;

 if (s == 0)
 {
  // get time of day in seconds since 12:00 AM,
  // January 1, 1970
  long t_time = time(NULL);

  // mix-up bits by squaring
  t_time *= t_time;
  // result can overflow. handle cases
  // > 0, < 0, = 0
  if (t_time > 0)
   s = t_time ^ 0x5EECE66DL;
  else if (t_time < 0)
   s = (t_time & 0x7fffffff) ^ 0x5EECE66DL;
  else
   s = 0x5EECE66DL;
 }

 seed = s;
}

long randomNumber::random()
{
 long tmpSeed = A * ( seed % Q ) - R * ( seed / Q );

 if( tmpSeed >= 0 )
   seed = tmpSeed;
 else
   seed = tmpSeed + M;

 return seed;
}

long randomNumber::random(long n)
{
 double fraction = double(random())/double(M);

 return int(fraction * n);
}

double randomNumber::frandom()
{
 return double(random())/double(M);
}

 

构造函数:randomNumber(long s=0)

   设定随机数发生的种子.使用默认值0时,系统时间为初始化种子;否则,用户提供发生器的种子.

 

double frandom(): 返回实数x, 0.0<=x<1.0

 

long random():返回long型随机整数m, 0<=m<exp(2,31)-1;

 

long random(long n):返回随机整数m, 0<=m<n;

---

应用程序:掷骰子游戏  先掷两个骰子,根据总点数确定赢家?

通过声明一个随机数对象die进行掷骰子游戏的模拟,这个随机数使用自动种子,表达式:

(1+die.random(6)) + (1+die.random(6)). 注意,这里最好使用一个随机数来获取两次随机值.若用两个随机数对象来分别获取,则获取的两个随机数可能一样.因为默认初始化都使用系统时间,由于速度原因,可能将同样的时间设定为随机种子.