c++ - 使用 Boost 的进程间读/写锁

Question

This thread在解释如何使用 Boost 实现读/写锁时是黄金。它看起来相对简单，我真的很喜欢它，但它似乎也使用了一个非命名锁，我需要一个进程间解决方案(不需要可移植，可以是仅限 Windows)。

有没有办法让进程间shared_mutex？我看到有一个 named_mutex但我无法让它与 shared_lock 一起使用其他锁。

感谢任何指点。

[编辑]

与此同时，我遇到了this thread这几乎击中了头部的钉子。我有两个问题:

1. 它没有显示完整的代码(我猜我需要使用 named_upgradable_mutex 但我不太确定)和
2. 我不喜欢修改后的“编写器”的答案，它不使用在析构函数中解锁的现成类，而是对互斥锁进行 3 次原始调用。

仍然欢迎提出意见或好的解决方案。

Answer 1

Boost.Interprocess 文档描述了所谓的 upgradable mutexes它支持和 upgradable mutex operations对于两种支持的可升级互斥体类型:

• boost::interprocess::interprocess_upgradable_mutex ，一种非递归、匿名的可升级互斥量，可以放置在共享内存或内存映射文件中。
• boost::interprocess::named_upgradable_mutex ，一个非递归的、命名的可升级互斥体。

编辑:我相信这行得通:

#include <iostream>
#include <string>
#include <unistd.h>

#include <boost/scope_exit.hpp>
#include <boost/interprocess/mapped_region.hpp>
#include <boost/interprocess/shared_memory_object.hpp>
#include <boost/interprocess/sync/interprocess_upgradable_mutex.hpp>
#include <boost/interprocess/sync/scoped_lock.hpp>
#include <boost/interprocess/sync/sharable_lock.hpp>
#include <boost/interprocess/sync/upgradable_lock.hpp>

// http://stackoverflow.com/questions/12439099/interprocess-reader-writer-lock-with-boost/

#define SHARED_MEMORY_NAME "SO12439099-MySharedMemory"

struct shared_data {
private:
    typedef boost::interprocess::interprocess_upgradable_mutex upgradable_mutex_type;

    mutable upgradable_mutex_type mutex;
    volatile int counter;

public:
    shared_data()
        : counter(0)
    {
    }

    int count() const {
        boost::interprocess::sharable_lock<upgradable_mutex_type> lock(mutex);
        return counter;
    }

    void set_counter(int counter) {
        boost::interprocess::scoped_lock<upgradable_mutex_type> lock(mutex);
        this->counter = counter;
    }
};

int main(int argc, char *argv[])
{
    using namespace boost::interprocess;

    if (argc != 2) {
        std::cerr << "Usage: " << argv[0] << " WHICH" << std::endl;
        return 1;
    }

    const std::string which = argv[1];
    if (which == "parent") {
        shared_memory_object::remove(SHARED_MEMORY_NAME);
        shared_memory_object shm(create_only, SHARED_MEMORY_NAME, read_write);

        BOOST_SCOPE_EXIT(argc) {
            shared_memory_object::remove(SHARED_MEMORY_NAME);
        } BOOST_SCOPE_EXIT_END;

        shm.truncate(sizeof (shared_data));

        // Map the whole shared memory into this process.
        mapped_region region(shm, read_write);

        // Construct the shared_data.
        new (region.get_address()) shared_data;

        // Go to sleep for a minute.
        sleep(60);

        return 0;
    } else if (which == "reader_child") {
        shared_memory_object shm(open_only, SHARED_MEMORY_NAME, read_write);

        mapped_region region(shm, read_write);
        shared_data& d = *static_cast<shared_data *>(region.get_address());

        for (int i = 0; i < 100000; ++i) {
            std::cout << "reader_child: " << d.count() << std::endl;
        }
    } else if (which == "writer_child") {
        shared_memory_object shm(open_only, SHARED_MEMORY_NAME, read_write);

        mapped_region region(shm, read_write);
        shared_data& d = *static_cast<shared_data *>(region.get_address());

        for (int i = 0; i < 100000; ++i) {
            d.set_counter(i);
            std::cout << "writer_child: " << i << std::endl;
        }
    }
}

我在 Mac 上用以下脚本试过这个:

#!/usr/bin/env sh
./a.out reader_child &
./a.out reader_child &
./a.out writer_child &
./a.out reader_child &
./a.out reader_child &

(你必须先启动父级:./a.out parent)

输出显示“reader_child”和“writer_child”行的交错(所有“reader_child”行在第一个“writer_child”行之后显示非零值)，因此它似乎有效。