内存映射文件

一块内存里的内容, 对应磁盘上文件的内容, 对这块内存的修改会自动修改对应的文件上的内容;
内存修改回写文件的时机(C++):

• 内存不足
• 进程 crash
• 调用 msync 或者 munmap
• 不设置 MAP_NOSYNC 情况下 30s-60s(仅限FreeBSD)

public static long checksumMappedFile(String fileName){  
        CRC32 crc = new CRC32();  
        try {  
            FileInputStream in = new FileInputStream(fileName);  
            FileChannel channel = in.getChannel();  
            int length = (int) channel.size();  
            MappedByteBuffer buffer = channel.map(FileChannel.MapMode.READ_ONLY, 0, length);  

            for(int p = 0;p<length;p++){  
                int c = buffer.getInt(p);  
                crc.update(c);  
            }  
        } catch (FileNotFoundException e) {  
            e.printStackTrace();  
        } catch(IOException ioe){  
            ioe.printStackTrace();  
        }  
        return crc.getValue();  
    }

C++的里的 void mmap(void start,size_t length,int prot,int flags,int fd,off_t offset);
int munmap(void* start,size_t length);

#include <iostream>
#include <assert.h>

#include <Windows.h>
#include <WinBase.h>

#define BAD_POS 0xFFFFFFFF // returned by SetFilePointer and GetFileSize
#define SUCCESS 0
using namespace std;


typedef DWORD mmf_share_mode;
typedef DWORD mmf_access_mode;
typedef DWORD mmf_flags;


int main(){
    cout<<"create memorymapfile..."<<endl;
    const char* shared_name = "testMmf";
    const char* file_name = "d:\\testMmf.mmf";
    const DWORD mmf_size = 512*1024;
    //存取模式
    mmf_access_mode access_mode = (GENERIC_READ|GENERIC_WRITE);
    //共享模式
    mmf_share_mode share_mode = FILE_SHARE_READ | FILE_SHARE_WRITE;
    //文件属性
    mmf_flags flags = FILE_FLAG_SEQUENTIAL_SCAN;//|FILE_FLAG_WRITE_THROUGH|FILE_FLAG_NO_BUFFERING;
    DWORD error_code;

    //创建文件
    HANDLE mmHandle =
        CreateFile(file_name,
             access_mode, 
             share_mode, 
             NULL, 
             OPEN_ALWAYS,
             flags,
             NULL);

    if (mmHandle == INVALID_HANDLE_VALUE) {
        error_code = GetLastError();
        cout<<"创建mmf失败:"<<error_code<<endl;
    }else{
        DWORD high_size;
        DWORD file_size = GetFileSize(mmHandle, &high_size);
        if (file_size == BAD_POS && (error_code = GetLastError()) != SUCCESS) {
            CloseHandle(mmHandle);            
            cout<<"error："<<error_code<<endl;
        }
        cout<<"create mmf sucessfully"<<endl;

        //assert(file_size == 0);

        DWORD size_high = 0;
        //创建文件映射，如果要创建内存页面文件的映射，第一个参数设置为INVALID_HANDLE_VALUE
        HANDLE mmfm = CreateFileMapping(mmHandle,
            NULL,
            PAGE_READWRITE,
            size_high, 
            mmf_size, 
            shared_name);

        error_code = GetLastError();
        if(SUCCESS != error_code){
            cout<<"createFileMapping error"<<error_code<<endl;
        }else{
            if(mmfm == NULL){
                if(mmHandle != INVALID_HANDLE_VALUE){
                    CloseHandle(mmHandle);
                }
            }else{
                //char write_chars[] = "hello chars";
                //size_t position = 0;
                //DWORD written = 0;
                //const size_t write_chars_size = sizeof(write_chars);
                //WriteFile(mmHandle,write_chars,write_chars_size,&written,NULL);
                size_t view_size = 1024*256;
                DWORD view_access = FILE_MAP_ALL_ACCESS;

                //获得映射视图
                char* mmfm_base_address = (char*)MapViewOfFile(mmfm,view_access,0,0,view_size);
                if(mmfm_base_address == NULL){
                    error_code = GetLastError();
                    if(error_code != SUCCESS){
                        cout<<"error code "<<error_code<<endl;
                    }
                }else{
                    char write_chars[] = "hello chars";
                    const size_t write_chars_size = sizeof(write_chars);
                    //向内存映射视图中写数据
                    CopyMemory((PVOID)mmfm_base_address, write_chars, write_chars_size);

                    //memcpy(mmfm_base_address,write_chars,write_chars_size);

                    size_t position = 0;
                    char read_chars[write_chars_size];

                    //读数据
                    memcpy(read_chars,mmfm_base_address,write_chars_size);
                    cout<<"read chars "<<read_chars<<endl;

                    //卸载映射
                    UnmapViewOfFile(mmfm_base_address);
                    //关闭内存映射文件
                    CloseHandle(mmfm);
                    //关闭文件
                    CloseHandle(mmHandle);
                }
            }
        }
    }



    system("pause");
    exit(0);
    return EXIT_SUCCESS;
}