MFC 进程之间通讯之内存映射文件

内存映射文件，是由一个文件到一块内存的映射。Win32提供了允许应用程序把文件映射到一个进程的函数 (CreateFileMapping)。内存映射文件与虚拟内存有些类似，通过内存映射文件可以保留一个地址空间的区域，同时将物理存储器提交给此区域，内存文件映射的物理存储器来自一个已经存在于磁盘上的文件，而且在对该文件进行操作之前必须首先对文件进行映射。使用内存映射文件处理存储于磁盘上的文件时，将不必再对文件执行I/O操作，使得内存映射文件在处理大数据量的文件时能起到相当重要的作用。

代码操作：

新建一个基于对话框的MFC工程，名字为：ShareMemoryTest

在该工程添加一个类：CShareMemory便于以后可以重用

ShareMemory.h:

#pragma once
class CShareMemory
{
    HANDLE m_hFileHandle;
    HANDLE m_hFileMapHanlde;
    char * m_pMapOfFile;

public:
    CShareMemory(void);
    ~CShareMemory(void);
    char * Create(CString strFileName, DWORD dwFileSize);
    BOOL Delete();
    char * Open(CString strFileName);
};

ShareMemory.cpp

#include "StdAfx.h"
#include "ShareMemory.h"


CShareMemory::CShareMemory(void)
{
    m_hFileHandle = NULL;
    m_hFileMapHanlde = NULL;
    m_pMapOfFile = NULL;
}


CShareMemory::~CShareMemory(void)
{
    Delete();
}

/*
    Function: create map file and return point to operation
    Param:    
            strFileName[in]: local harddisk file name 
            dwFileSize[in]: file map size
    return: 
            char*: the point to read/write file
*/
char* CShareMemory::Create(CString strFileName, DWORD dwFileSize)
{
    m_hFileHandle = ::CreateFile(strFileName, GENERIC_READ | GENERIC_WRITE, 0, NULL, OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL);

    if(NULL == m_hFileHandle)
    {
        return NULL;
    }

    m_hFileMapHanlde = ::CreateFileMapping(m_hFileHandle, NULL, PAGE_READWRITE, 0, dwFileSize, strFileName);

    if(NULL == m_hFileMapHanlde)
    {
        int errNo = GetLastError();

        ::CloseHandle(m_hFileHandle);
        return NULL;
    }


    char * pMapOfFile = (char *)::MapViewOfFile(m_hFileMapHanlde, FILE_MAP_ALL_ACCESS, 0, 0, 0);    //the entire file is mapped. 

    if(NULL == pMapOfFile)
    {
        ::CloseHandle(m_hFileMapHanlde);
        return NULL;
    }

    m_pMapOfFile = pMapOfFile;
    memset(m_pMapOfFile, 0, dwFileSize);


    return pMapOfFile;
}

/*
    Function: delete handle and release memory
    Param: NULL
    return: 
            TRUE: delete success
            FALSE: delete failed
*/
BOOL CShareMemory::Delete()
{
    if(NULL != m_hFileHandle)
    {
        if(!::CloseHandle(m_hFileHandle))
        {
            return FALSE;
        }
    }
    
    if(NULL != m_hFileMapHanlde)
    {
        if(!::CloseHandle(m_hFileMapHanlde))
        {
            return FALSE;
        }
    }
    
    if(NULL != m_pMapOfFile)
    {
        if(!::UnmapViewOfFile(m_pMapOfFile))
        {
            return FALSE;
        }
    }

    return TRUE;
}

/*
    Function:    open a exist file mapping
    Param:    
            strFileName: is need to same to Create
    return: 
            char*: the point to read/write file
*/
char* CShareMemory::Open(CString strFileName)    //strFileName is need to equal to Create
{
    HANDLE hExistFileMapHandle = ::OpenFileMapping(FILE_MAP_ALL_ACCESS, FALSE, strFileName);

    if(NULL == hExistFileMapHandle)
    {
        return NULL;
    }

    char * pMapOfFile = (char *)::MapViewOfFile(hExistFileMapHandle, FILE_MAP_ALL_ACCESS, 0, 0, 0);    //the entire file is mapped. 

    if(NULL == pMapOfFile)
    {
        ::CloseHandle(m_hFileMapHanlde);
        return NULL;
    }

    m_pMapOfFile = pMapOfFile;
    
    return pMapOfFile;
}

在ShareMemoryTestDlg.cpp中，【ok】按钮的响应函数，添加如下代码：

void CShareMemoryTestDlg::OnBnClickedOk()
{
    // TODO: Add your control notification handler code here
    //CDialogEx::OnOK();

    CString strShareMemory = "hello my first mapping file!\n";

    CShareMemory ShareMemory;

    char *pShareMemory = ShareMemory.Create("D:\ShareMemoryTestFile.txt", strShareMemory.GetLength());

    if(pShareMemory)
    {
        sprintf(pShareMemory, strShareMemory);

        AfxMessageBox("write over");
    }
    
}

以上第一个进程的写操作执行完毕，下面再创建另一个基于对话框的MFC的工程，名字为：ShareMemoryTest2

同样在ShareMemoryTestDlg.cpp中，【ok】按钮的响应函数，添加如下代码：

void CShareMemoryTest2Dlg::OnBnClickedOk()
{
    // TODO: Add your control notification handler code here
    //CDialogEx::OnOK();

    CShareMemory ShareMemory;

    char *pShareMemory = ShareMemory.Open("D:\ShareMemoryTestFile.txt");

    if(pShareMemory)
    {
        AfxMessageBox(pShareMemory);

    }

    
}