C++如何安全地并发访问同一文件 文件锁机制与同步方案

#include <iostream>
#include <fstream>
#include <fcntl.h>
#include <unistd.h>

bool lockFile(int fd) {
    struct flock fl;
    fl.l_type = F_WRLCK; // 写锁
    fl.l_whence = SEEK_SET;
    fl.l_start = 0;
    fl.l_len = 0; // 锁住整个文件
    fl.l_pid = getpid();

    if (fcntl(fd, F_SETLK, &fl) == -1) { // F_SETLK是非阻塞锁
        if (errno == EACCES || errno == EAGAIN) {
            std::cerr << "File is already locked." << std::endl;
            return false;
        } else {
            perror("fcntl");
            return false;
        }
    }
    return true;
}

bool unlockFile(int fd) {
    struct flock fl;
    fl.l_type = F_UNLCK; // 解锁
    fl.l_whence = SEEK_SET;
    fl.l_start = 0;
    fl.l_len = 0;
    fl.l_pid = getpid();

    if (fcntl(fd, F_SETLK, &fl) == -1) {
        perror("fcntl");
        return false;
    }
    return true;
}

int main() {
    int fd = open("example.txt", O_RDWR | O_CREAT, 0666);
    if (fd == -1) {
        perror("open");
        return 1;
    }

    if (lockFile(fd)) {
        std::cout << "File locked successfully." << std::endl;
        // 在这里进行文件操作
        sleep(10); // 模拟文件操作
        if (unlockFile(fd)) {
            std::cout << "File unlocked successfully." << std::endl;
        } else {
            std::cerr << "Failed to unlock file." << std::endl;
        }
    } else {
        std::cerr << "Failed to lock file." << std::endl;
    }

    close(fd);
    return 0;
}

#include <iostream>
#include <fstream>
#include <mutex>
#include <thread>

std::mutex fileMutex;

void writeFile(const std::string& data) {
    std::lock_guard<std::mutex> lock(fileMutex); // RAII风格的锁管理
    std::ofstream outfile("example.txt", std::ios::app);
    if (outfile.is_open()) {
        outfile << data << std::endl;
        outfile.close();
    } else {
        std::cerr << "Unable to open file." << std::endl;
    }
}

int main() {
    std::thread t1([]() { writeFile("Data from thread 1"); });
    std::thread t2([]() { writeFile("Data from thread 2"); });

    t1.join();
    t2.join();

    return 0;
}