内存映射文件的方法

内存映射文件的方法

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Windows采用MapViewOfFile系统api，Linux则采用mmap相关函数。
之前在做大数据查询计算的时候，经常会出现内存不足的情况，malloc无法分配内存了。
那时候就经常在想能不能使用硬盘来充当内存，就像swap交换那样，硬盘是非常大的，如
果能使用硬盘来当内存也许速度会慢一点，但是内存问题就不需要再担心了。之后研究了
一段时间发现，内存映射文件依然是将整个文件映射到内存里，所以：

内存映射文件是无法利用文件来节省内存的!!

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Windows Demo：

#include <stdio.h>
#include <windows.h>

#define MMAP_ALLOCATOR_SIZE 1024

int main (int argc, char *argv[]) {
    HANDLE dumpFileDescriptor = CreateFile("1.txt", GENERIC_READ | GENERIC_WRITE,
        FILE_SHARE_READ | FILE_SHARE_WRITE, NULL, CREATE_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL);
    printf("dumpFileDescriptor=%d\n", dumpFileDescriptor);
    HANDLE fileMappingObject = CreateFileMapping(dumpFileDescriptor,
                      NULL,
                      PAGE_READWRITE,
                      0,
                      MMAP_ALLOCATOR_SIZE,
                      NULL);
    printf("fileMappingObject=%d\n", fileMappingObject);

    char* mappedFileAddress = (char *)MapViewOfFile(fileMappingObject,
                      FILE_MAP_ALL_ACCESS,
                      0,
                      0,
                      MMAP_ALLOCATOR_SIZE);

    printf("mappedFileAddress=%d\n", mappedFileAddress);
    memset(mappedFileAddress, 0, MMAP_ALLOCATOR_SIZE);
    sprintf(mappedFileAddress, "10086");
    printf("mappedFileAddress=%s\n", mappedFileAddress);

    FlushViewOfFile(mappedFileAddress, MMAP_ALLOCATOR_SIZE);


    UnmapViewOfFile(mappedFileAddress);
    CloseHandle(fileMappingObject);
    CloseHandle(dumpFileDescriptor);
    unlink("1.txt");
    return 1;
}

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Linux Demo:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/stat.h>
#include <unistd.h>

#define MMAP_ALLOCATOR_SIZE 65535

int main(int argc, char *argv[])
{
    struct stat statbuf;
    int fd;
    char *buf;
    int i;

    fd = open("1.txt", O_CREAT|O_RDWR, 0755);
    if (fd == -1) {
        printf("fail to open\n");
        exit(1);
    }

    ftruncate(fd, MMAP_ALLOCATOR_SIZE);

    buf = (char *)mmap(NULL, MMAP_ALLOCATOR_SIZE, PROT_READ|PROT_WRITE, MAP_PRIVATE, fd, 0);
    if (buf == MAP_FAILED) {
        printf("fail to open\n");
        exit(1);
    }

    memset(buf, 0, MMAP_ALLOCATOR_SIZE);
    sprintf(buf, "10086");
    msync(buf, MMAP_ALLOCATOR_SIZE, MS_SYNC);
    printf("buf=%s\n", buf);

    if (munmap(buf, MMAP_ALLOCATOR_SIZE) == -1) {
        printf("fail to munmap\n");
        exit(1);
    }

    close(fd);
    return 1;
}

相关连接:
https://blog.csdn.net/qq_33611327/article/details/81738195