c++ - Boost进程间共享内存删除、权限和输出文件

Question

我正在使用 Boost Interprocess 库在两个进程之间共享内存。

我正在使用以下内容来分配共享内存块、附加 vector 、命名互斥量和命名条件变量:

using ShmemAllocator = allocator<T, managed_shared_memory::segment_manager>;
using MyVector = vector<T, ShmemAllocator>;

segment.reset(new managed_shared_memory(create_only, blockName, numBytes));

const ShmemAllocator alloc_inst(segment->get_segment_manager());
vec = segment->construct<MyVector>(sharedVectorName)(alloc_inst);

named_mutex.reset(new named_mutex(create_only, mutexName));
cond_empty.reset(new named_condition(create_only, conditionVName));

以及要删除的以下内容:

named_mutex::remove(mutexName);
named_condition::remove(conditionVName);
shared_memory_object::remove(blockName);

为了检查内存是否被正确删除，我进行了压力测试:

while(counter < 1000000)
{
    MySharedMemoryObj s;
    ++counter;
}

(依赖RAII为析构函数删除共享内存)

我有三个问题:

1. 我是否需要删除 vector ，因为它无论如何都是线段的一部分？

2. 上面的方法有效，但在一个特定的情况下它没有，并抛出一个 Boost Interprocess 异常，说它没有访问内存的权限。是什么原因导致的/有没有办法避免它？

3. 我注意到上面的代码似乎在/tmp 中生成了名为 outputXXXXXXXXXXX 的二进制文件。这些是什么？它们不会被删除，因此会累积。

Answer 1

1. Do I need to delete the vector, because it's part of the segment anyway?

从技术上讲，这里并非如此(假设您也使用共享内存分配器)。但是，跳过销毁是一种不好的做法，尤其是当您的析构函数具有逻辑(不是微不足道)时。

2. The above works, but on one particular occasion it didn't and threw a Boost Interprocess exception, saying it did not have permission to access the memory. What causes this/is there a way to avoid it?

确保程序在创建共享段时与预期用户一起运行。这就是赋予它文件级访问权限的原因。

例如如果您以 root 身份创建段，则您应该无法以其他用户身份打开它。

3. I have noticed that the above code seems to generate binary files in /tmp named outputXXXXXXXXXXX. What are these? They aren't being deleted and therefore accumulate.

这没有多大意义。这些路径表明您使用的是 POSIX 系统。在 POSIX 上，shmem 通常存在于 /dev/shm 中，我认为不需要临时文件。

我建议临时文件可能是其他程序(比如您的 IDE？)的产物

建议的简化:

#include <boost/interprocess/managed_shared_memory.hpp>
#include <boost/interprocess/sync/named_mutex.hpp>
#include <boost/interprocess/sync/named_condition.hpp>
#include <vector>
namespace bip = boost::interprocess;

static auto blockName        = "4f72909d-8265-4260-9bb1-6bd58f63812c";
static auto sharedVectorName = "54714711";
static auto mutexName        = "b4eb63e0";
static auto conditionVName   = "f7a95857";

template <typename T> using ShmemAllocator = bip::allocator<T, bip::managed_shared_memory::segment_manager>;
template <typename T> using MyVector = std::vector<T, ShmemAllocator<T> >;

int main() {
    bip::managed_shared_memory segment(bip::create_only, "blockName", 10<<20u);

    auto vec = segment.construct<MyVector<int> >(sharedVectorName)(segment.get_segment_manager());

    bip::named_mutex named_mutex(bip::create_only, mutexName);
    bip::named_condition named_condition(bip::create_only, conditionVName);
}

或者，根据您要同步的内容，使同步原语成为共享数据的成员:

struct SharedData {
    using allocator_type = ShmemAllocator<int>;

    template <typename A>
    SharedData(A alloc) : _vec(alloc) {}

    MyVector<int> _vec;
    bip::interprocess_mutex _mx;
    bip::interprocess_condition _cond;
};

int main(int argc, char**) {
    bip::managed_shared_memory segment(bip::open_or_create, "2fc51845-3d9b-442b-88ee-f6fd1725e8b0", 10<<20u);

    auto& data = *segment.find_or_construct<SharedData>("sharedData")(segment.get_segment_manager());
}

简单的多生产者/多消费者队列:

模拟最大容量为 10 个元素的队列。

#include <boost/interprocess/managed_shared_memory.hpp>
#include <boost/interprocess/sync/interprocess_mutex.hpp>
#include <boost/interprocess/sync/interprocess_condition.hpp>
#include <boost/date_time/posix_time/posix_time.hpp>
#include <vector>
#include <mutex> // unique_lock
namespace bip = boost::interprocess;

template <typename T> using ShmemAllocator = bip::allocator<T, bip::managed_shared_memory::segment_manager>;
template <typename T> using MyVector = std::vector<T, ShmemAllocator<T> >;

struct SharedData {
    using allocator_type = ShmemAllocator<int>;

    template <typename A>
    SharedData(A alloc) : _vec(alloc) {}

    MyVector<int> _vec;
    bip::interprocess_mutex _mx;
    bip::interprocess_condition _cond;

    using lock_type = std::unique_lock<bip::interprocess_mutex>;
    lock_type wait_for_empty() {
        lock_type lk(_mx);
        _cond.wait(lk, [=] { return _vec.empty(); });
        return lk;
    }

    void push(int v) {
        lock_type lk(_mx);
        _cond.wait(lk, [=] { return _vec.size() < 10; }); // wait for free space
        _vec.push_back(v);
        _cond.notify_all();
    }

    bool pop(boost::posix_time::time_duration timeout, int& out) {
        lock_type lk(_mx);

        // wait for message
        auto deadline = boost::posix_time::microsec_clock::universal_time() + timeout;
        if (_cond.timed_wait(lk, deadline, [=] { return !_vec.empty(); })) {
            out = _vec.back();
            _vec.pop_back();
            _cond.notify_all();
            return true;
        } 
        return false;
    }
};

int main(int argc, char**) {
    bip::managed_shared_memory segment(bip::open_or_create, "2fc51845-3d9b-442b-88ee-f6fd1725e8b0", 10<<20u);

    auto& data = *segment.find_or_construct<SharedData>("sharedData")(segment.get_segment_manager());

    if (argc>1) {
        // "server"
        std::cout << "Waiting for queue to be depleted\n";
        data.wait_for_empty();

        for (int i = 0; i<20; ++i) {
            std::cout << "Pushing " << i << "\n";
            data.push(i);
        }
    } else {
        // "client"
        int what;
        while (data.pop(boost::posix_time::seconds(1), what))
            std::cout << "Popped " << what << "\n";

        std::cout << "Timeout reached, bye\n";
    }

}