mmap方法用于存档

有时候一个游戏，像RPG这种的，经常在游戏过程中需要频繁存档，在性能不好的平台上容易出各种问题，比如游戏Crash，存档丢失等等，今天想到一个可选的解法，就是用MMAP。虽然我到现在也没有搞懂这个机制究竟是怎样让程序员得到了好处，但是，至少这是一个有可能解决问题的办法。

代码：

#include <sys/mman.h>
#include <fcntl.h>
#include <stdio.h>
#include <memory.h>
#include <unistd.h>
#include <stdlib.h>
#include <fstream>
                             
using namespace std;
                             
class Profile
{
// the data
public:
    int val1;
    int val2;
    int val3;
                             
public:
    Profile() 
    {
    // NOT to set the default value, seriously.
    }
    void* operator new(size_t size)
    {
        return OpenProfile(size);
    }
    void operator delete(void* p)
    {
                             
    }
    void UpdateProfile()
    {
        // Synchronize the content between memory and filesystem
        printf("msync() ret: %d\n", msync(this, sizeof(Profile), MS_SYNC));
    }
    void CloseProfile()
    {
        printf("munmap() ret: %d\n", munmap(this, sizeof(Profile)));
    }
                                 
private:
    static void* OpenProfile(size_t size)
    {
        void* ret = NULL;
        // open the file which name is "Profile".
        int fd = open("Profile", O_RDWR);
        // if there is not this file yet, create it.
        if(-1 == fd)
        {
            // Create the "Profile" file and write '00000..' in it, and the size of this file is sizeof(Profile).
            ofstream outfile("Profile", ofstream::out | ofstream::trunc | ofstream::binary);
            outfile.seekp(size - sizeof(char), ios::beg);
            outfile << '\0';
            outfile.close();
                             
            fd = open("Profile", O_RDWR);
            if(-1 == fd)
            {
                printf("cannot open \"file\"");
                exit(0);
            }
        }
        // Map the file
        ret = mmap(0, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
        if(MAP_FAILED == ret)
        {
            printf("cannot map file");
            exit(0);
        }
                                     
        return ret;
    }
};
                             
int main()
{
    // OPEN the Profile, and if there is no Profile now, will create one.
    Profile* ptr = new Profile();
                             
    // MODIFY the profile.
    ptr->val1 = 10;
    ptr->val2 = 20;
    ptr->val3 = 30;
                             
    // This code SAVE 
    ptr->UpdateProfile();
                             
    // This code CLOSE
    ptr->CloseProfile();
    delete ptr;
                                
    // READ the Profile:
    ptr = new Profile();//nothing else need to do, you could see the profile has already been read in.
    printf("val1 = %d, val2 = %d, val3 = %d\n", ptr->val1, ptr->val2, ptr->val3);
                             
    return 0;
}

上面这个存档类Profile，使用方法变简单了，只要new出来，就相当于进行了read存档的操作。在需要存档的时候可调用UpdateProfile，但实际上，由于mmap的机制，即使你没有调用UpdateProfile，存档实际上也有可能是保存了的。

不过我只知道mmap是把文件映射到内存中，这样你读写内存的时候实际上相当于读写了文件。对于mmap的原理和得到的实惠还不知晓。