进程间通信（3）：共享内存

管道通信在我之前的博客文章《进程间通信（1）：匿名管道》和《进程间通信（2）：命名管道》中进行了讲述，管道通信有其自身的缺点，管道的缓冲区是受限的，封装的网络操作，也会影响通信的速度，操作不当很容易阻塞。

共享内存操作一般被认为是最快的进程间通信方式，在系统内存中开辟一块内存区，分别映射到各个进程的虚拟地址空间中，任何一个进程操作了内存区都会反映到其他进程中。这种方式可以像访问自己的私有空间一样访问共享内存区。要说缺点，就是共享内存的数据同步比较困难，编程难度较大，不注意的话会造成数据的混乱。

结合例子进行讲解，例程如下。在服务器端通过CreateFileMapping函数创建文件映射，此处映射文件的名称作为共享内存区域的唯一标识，供其他进程索引。通过MapViewOfFile函数获得内存区域的首地址，然后就可以将数据拷贝到共享内存区域，通知给其他进程。显而易见，要首先启动服务器端程序。

文章中示例代码的完整工程文件可在http://download.csdn.net/download/ezhchai/9895833中下载

服务器端代码：

// SharedMemorySrv.cpp : 定义控制台应用程序的入口点。//#include "stdafx.h"#include <windows.h>#include <iostream>using namespace std;int main(){HANDLE hMapFile = NULL;char* pStr;// Create the file mapping object.    hMapFile = CreateFileMapping(INVALID_HANDLE_VALUE,   // Use paging file - shared memory    NULL,                   // Default security attributes    PAGE_READWRITE,         // Allow read and write access    0,                      // High-order DWORD of file mapping max size    256,// Low-order DWORD of file mapping max size    (LPCWSTR)"MapTrans"// Name of the file mapping object    );if (NULL == hMapFile){cout << "CreateFileMapping failed!　Error: " << GetLastError() << endl;return 1;}cout << "The file mapping MapTrans is created! " << endl;// Map a view of the file mapping into the address space of the current process.   pStr = (char*)MapViewOfFile(hMapFile,               // Handle of the map object    FILE_MAP_WRITE,// Read access    0,                      // High-order DWORD of the file offset     0,// Low-order DWORD of the file offset     256// The number of bytes to map to view  );if (NULL == pStr){cout << "MapViewOfFile failed!　Error: " << GetLastError() << endl;return 2;}cout << "The file view is mapped!" << endl;strcpy_s(pStr, 20, "ezhchai");//write data to the shared memorycout << "This message is written to the view: " << "ezhchai" << endl;getchar();UnmapViewOfFile(pStr);CloseHandle(hMapFile);return 0;}

客户端程序在服务器程序建立共享内存区域后，通过OpenFileMapping函数以共享内存文件名为标识，打开映射文件区域，同样通过MapViewOfFile函数获得内存区域的首地址，也就可以操作共享内存区域，实现进程间通信了。

客户端代码：

// SharedMemoryClt.cpp : 定义控制台应用程序的入口点。//  #include "stdafx.h"#include <windows.h>    #include <iostream>using namespace std;int main(){HANDLE hMapFile = NULL;char* pStr;// Try to open the named file mapping identified by the map name.    hMapFile = OpenFileMapping(FILE_MAP_READ,          // Read access    FALSE,                  // Do not inherit the name    (LPCWSTR)"MapTrans"// File mapping name     );if (NULL == hMapFile){cout << "OpenFileMapping failed!　Error: " << GetLastError() << endl;return 1;}cout << "The file mapping MapTrans is opened! " << endl;// Map a view of the file mapping into the address space of the current process.      pStr = (char*)MapViewOfFile(hMapFile,               // Handle of the map object    FILE_MAP_READ,          // Read access    0,                      // High-order DWORD of the file offset     0,// Low-order DWORD of the file offset    256// The number of bytes to map to view    );if (NULL == pStr){cout << "MapViewOfFile failed!　Error: " << GetLastError() << endl;return 2;}cout << "The file view is mapped!" << endl;puts(pStr);// Read sdata from the shared memorygetchar();UnmapViewOfFile(pStr);CloseHandle(hMapFile);return 0;}