C++中内存映射文件怎么用？大文件处理技术详解

#include <iostream>
#include <fcntl.h>
#include <sys/mman.h>
#include <unistd.h>
#include <sys/stat.h>

int main() {
    const char* filepath = "example.txt";
    int fd = open(filepath, O_RDWR | O_CREAT, 0666); // 打开文件，可读写，如果不存在则创建
    if (fd == -1) {
        perror("open");
        return 1;
    }

    // 假设文件大小为100字节
    size_t filesize = 100;
    ftruncate(fd, filesize); // 调整文件大小

    // 将文件映射到内存
    void* map_ptr = mmap(nullptr, filesize, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
    if (map_ptr == MAP_FAILED) {
        perror("mmap");
        close(fd);
        return 1;
    }

    // 现在你可以像访问内存一样访问文件内容
    char* data = static_cast<char*>(map_ptr);
    for (size_t i = 0; i < filesize; ++i) {
        data[i] = 'A' + (i % 26); // 写入一些数据
    }

    // 确保将内存中的修改写回磁盘
    if (msync(map_ptr, filesize, MS_SYNC) == -1) {
        perror("msync");
    }

    // 解除映射
    if (munmap(map_ptr, filesize) == -1) {
        perror("munmap");
    }

    close(fd);
    return 0;
}

#include <iostream>
#include <windows.h>

int main() {
    const char* filepath = "example.txt";
    HANDLE hFile = CreateFile(
        filepath,
        GENERIC_READ | GENERIC_WRITE,
        0,
        NULL,
        CREATE_ALWAYS,
        FILE_ATTRIBUTE_NORMAL,
        NULL);

    if (hFile == INVALID_HANDLE_VALUE) {
        std::cerr << "CreateFile failed: " << GetLastError() << std::endl;
        return 1;
    }

    // 假设文件大小为100字节
    size_t filesize = 100;
    LARGE_INTEGER fileSize;
    fileSize.QuadPart = filesize;

    HANDLE hFileMapping = CreateFileMapping(
        hFile,
        NULL,
        PAGE_READWRITE,
        fileSize.HighPart,
        fileSize.LowPart,
        NULL);

    if (hFileMapping == NULL) {
        std::cerr << "CreateFileMapping failed: " << GetLastError() << std::endl;
        CloseHandle(hFile);
        return 1;
    }

    LPVOID map_ptr = MapViewOfFile(
        hFileMapping,
        FILE_MAP_ALL_ACCESS,
        0,
        0,
        filesize);

    if (map_ptr == NULL) {
        std::cerr << "MapViewOfFile failed: " << GetLastError() << std::endl;
        CloseHandle(hFileMapping);
        CloseHandle(hFile);
        return 1;
    }

    // 现在你可以像访问内存一样访问文件内容
    char* data = static_cast<char*>(map_ptr);
    for (size_t i = 0; i < filesize; ++i) {
        data[i] = 'A' + (i % 26); // 写入一些数据
    }

    // 确保将内存中的修改写回磁盘
    FlushViewOfFile(map_ptr, filesize);

    // 解除映射
    UnmapViewOfFile(map_ptr);
    CloseHandle(hFileMapping);
    CloseHandle(hFile);

    return 0;
}