boost::interprocess::file_lock 与 std::ostream 一起使用时的错误行为

Question

我正在尝试使用 file_lock for restricting multiple instances of the same program running at the same time (Implementing something mentioned in this answer)。我在 Linux.

上使用 1.66 版本的 boost

在锁定文件之前，我确保该文件存在（通过使用 std::ofstream 和 std::ios::app 打开它）。我注意到一件事，如果我们关闭流，那么 file_lock 会自动解锁，因此允许同一程序的多个实例同时运行。

下面的程序无法运行，因为file_lock是自动发布的。

int main(int argc, char *argv[])
{
    namespace bipc = boost::interprocess;
    if (argc < 2)
        return 0;

    std::string path = argv[1];
    std::string lock_path = "/var/lock/" + path + ".lock";

    std::ofstream stream(lock_path, std::ios::app);

    bipc::file_lock lock(lock_path.c_str());

    if (!lock.try_lock())
        throw std::runtime_error("Multiple instance");

    std::cout << "Running" << std::endl;
    stream.close();
    while (true)
        std::this_thread::sleep_for(std::chrono::seconds(1));
    return 0;
}

但是，下面的两个程序有效。

看起来如果我们在尝试获取 file_lock 时打开了一个文件，那么我们需要保持该文件打开直到我们想要持有锁。如果我们关闭文件，那么锁会自动释放。我不确定这是否是一个错误。有人可以帮助我了解这种行为的原因吗？

int main(int argc, char *argv[])
{
    namespace bipc = boost::interprocess;
    if (argc < 2)
        return 0;

    std::string path = argv[1];
    std::string lock_path = "/var/lock/" + path + ".lock";

    std::ofstream stream(lock_path, std::ios::app);
    bipc::file_lock lock(lock_path.c_str());

    if (!lock.try_lock())
        throw std::runtime_error("Multiple instance");

    std::cout << "Running" << std::endl;
    while (true)
        std::this_thread::sleep_for(std::chrono::seconds(1));
    return 0;
}

和

int main(int argc, char *argv[])
{
    namespace bipc = boost::interprocess;
    if (argc < 2)
        return 0;

    std::string path = argv[1];
    std::string lock_path = "/var/lock/" + path + ".lock";

    {
        std::ofstream stream(lock_path, std::ios::app);
    }
    bipc::file_lock lock(lock_path.c_str());

    if (!lock.try_lock())
        throw std::runtime_error("Multiple instance");

    std::cout << "Running" << std::endl;
    while (true)
        std::this_thread::sleep_for(std::chrono::seconds(1));
    return 0;
}

Answer 1

我找到原因了。这是因为 boost::interprocess::file_lock 是使用 Classic POSIX File-locks 实现的。

此 link 解释了 POSIX 锁的问题，因此也解释了 boost file_locks 的问题。

More troublingly, the standard states that all locks held by a process are dropped any time the process closes any file descriptor that corresponds to the locked file, even if those locks were made using a still-open file descriptor. It is this detail that catches most programmers by surprise as it requires that a program take extra care not to close a file descriptor until it is certain that locks held on that file are able to be dropped.

看来我应该使用 flock 或其他使用 flock.

的平台库

boost::interprocess::file_lock 与 std::ostream 一起使用时的错误行为

boost::interprocess::file_lock erroneous behavior when used with std::ostream

c++

linux

boost

boost-interprocess

filelock