C++使用共享内存实现进程间通信
文件映射是一种实现进程间单向或双向通信的机制。它允许两个或多个本地进程间相互通信。为了共享文件或内存，所有的进程必须使用相同的文件映射的名字或是句柄。
为了实现共享文件，第一个进程先调用CreateFile方法。接下来调用CreateFileMapping方法来创建一个文件映射对象。并为文件映射指明一个句柄和名称。由于事件，信号，互斥对象和文件映射等这些内核对象都共享同一个名字空间，所以如果这个名字和其他一个对象的名称重名的话那么将创建失败。
为了实现共享内存，进程应首先调用CreateFileMapping函数然后在hFile参数中传入INVALID_HANDLE_VALUE宏来替代句柄。相应的文件映射对象会从系统的分页文件中获得一段内存。如果hFile参数的值是INVALID_HANDLE_VALUE，那么你在调用CreateFileMapping时必须给共享内存指定一个大小值。
使用共享内存或文件的进程必须使用MapViewOfFile函数或MapViewOfFileEx函数来创建一个文件视图。
下面我们创建一个名称为"Local\SampleMap"的文件映射对象，并将一个字符串写入到文件映射中。
我们将创建两个程序，一个是服务程序，一个是客户程序。服务程序负责创建文件映射。
服务程序命名为CppFileMappingServer，它的执行过程是
1.创建一个特定大小的文件映射对象，名称为“Local\SampleMap”
2.将这个对象的文件视图映射到进程的地址空间，然后向视图中写入字符串。
接下来执行客户程序CppFileMappingClient，它首先打开这个名称为“Local\SampleMap”的文件映射对象。然后把相同的文件映射视图映射到自己的地址空间中。然后从视图中读取服务进程所写入的数据。
Server完整源码：
#pragma region Includes#include <stdio.h>#include <windows.h>#pragma endregion#define MAP_PREFIX L"Local\\"#define MAP_NAME L"SampleMap"#define FULL_MAP_NAME MAP_PREFIX MAP_NAME // Max size of the file mapping object.#define MAP_SIZE 65536 // File offset where the view is to begin.#define VIEW_OFFSET 0 // The number of bytes of a file mapping to map to the view. All bytes of the // view must be within the maximum size of the file mapping object (MAP_SIZE). // If VIEW_SIZE is 0, the mapping extends from the offset (VIEW_OFFSET) to // the end of the file mapping.#define VIEW_SIZE 1024 // Unicode string message to be written to the mapped view. Its size in byte // must be less than the view size (VIEW_SIZE).#define MESSAGE L"Message from the first process." int wmain(int argc, wchar_t* argv[]){ HANDLE hMapFile = NULL; PVOID pView = NULL; // 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 MAP_SIZE, // Low-order DWORD of file mapping max size FULL_MAP_NAME // Name of the file mapping object ); if (hMapFile == NULL) { wprintf(L"CreateFileMapping failed w/err 0x%08lx\n", GetLastError()); goto Cleanup; } wprintf(L"The file mapping (%s) is created\n", FULL_MAP_NAME); // Map a view of the file mapping into the address space of the current // process. pView = MapViewOfFile( hMapFile, // Handle of the map object FILE_MAP_ALL_ACCESS, // Read and write access 0, // High-order DWORD of the file offset VIEW_OFFSET, // Low-order DWORD of the file offset VIEW_SIZE // The number of bytes to map to view ); if (pView == NULL) { wprintf(L"MapViewOfFile failed w/err 0x%08lx\n", GetLastError()); goto Cleanup; } wprintf(L"The file view is mapped\n"); // Prepare a message to be written to the view. PWSTR pszMessage = MESSAGE; DWORD cbMessage = (wcslen(pszMessage) + 1) * sizeof(*pszMessage); // Write the message to the view. memcpy_s(pView, VIEW_SIZE, pszMessage, cbMessage); wprintf(L"This message is written to the view:\n\"%s\"\n", pszMessage); // Wait to clean up resources and stop the process. wprintf(L"Press ENTER to clean up resources and quit"); getchar(); Cleanup: if (hMapFile) { if (pView) { // Unmap the file view. UnmapViewOfFile(pView); pView = NULL; } // Close the file mapping object. CloseHandle(hMapFile); hMapFile = NULL; } return 0;}
Client完整源码
#pragma region Includes#include <stdio.h>#include <windows.h>#pragma endregion#define MAP_PREFIX L"Local\\"#define MAP_NAME L"SampleMap"#define FULL_MAP_NAME MAP_PREFIX MAP_NAME // File offset where the view is to begin.#define VIEW_OFFSET 0 // The number of bytes of a file mapping to map to the view. All bytes of the // view must be within the maximum size of the file mapping object. If // VIEW_SIZE is 0, the mapping extends from the offset (VIEW_OFFSET) to the // end of the file mapping.#define VIEW_SIZE 1024 int wmain(int argc, wchar_t* argv[]){ HANDLE hMapFile = NULL; PVOID pView = NULL; // 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 FULL_MAP_NAME // File mapping name ); if (hMapFile == NULL) { wprintf(L"OpenFileMapping failed w/err 0x%08lx\n", GetLastError()); goto Cleanup; } wprintf(L"The file mapping (%s) is opened\n", FULL_MAP_NAME); // Map a view of the file mapping into the address space of the current // process. pView = MapViewOfFile( hMapFile, // Handle of the map object FILE_MAP_READ, // Read access 0, // High-order DWORD of the file offset VIEW_OFFSET, // Low-order DWORD of the file offset VIEW_SIZE // The number of bytes to map to view ); if (pView == NULL) { wprintf(L"MapViewOfFile failed w/err 0x%08lx\n", GetLastError()); goto Cleanup; } wprintf(L"The file view is mapped\n"); // Read and display the content in view. wprintf(L"Read from the file mapping:\n\"%s\"\n", (PWSTR)pView); // Wait to clean up resources and stop the process. wprintf(L"Press ENTER to clean up resources and quit"); getchar(); Cleanup: if (hMapFile) { if (pView) { // Unmap the file view. UnmapViewOfFile(pView); pView = NULL; } // Close the file mapping object. CloseHandle(hMapFile); hMapFile = NULL; } return 0;}
运行效果：
Server
Client