共享内存中带有 boost::interprocess 分配器的 std::unordered_map - 缺点? [英] std::unordered_map with boost::interprocess allocator in shared memory - drawbacks?

问题描述

我现在正在使用 boost::interprocess 进入共享内存.

I'm right now getting into shared memory using boost::interprocess.

我以如下方式定义了一些 std::unordered_map 和 std::unordered_set 类型:

I've defined a few std::unordered_map and std::unordered_set types like in the following manner:

#include <boost/interprocess/allocators/allocator.hpp>

#include <unordered_map> // NOT the boost implementation ...

...

namespace ipc = boost::interprocess;

 /**
 * allocator type needed to construct maps in shared memory
 */
typedef ipc::allocator<std::pair<const size_t, std::string>,
           ipc::managed_shared_memory::segment_manager> OBJ_MAP_ALLOCATOR;

/**
 * map type to construct maps in shared memory
 */
typedef std::unordered_map<size_t,
               std::string,
               std::hash<size_t>,
               std::equal_to<size_t>,
               OBJ_MAP_ALLOCATOR> OBJ_MAP_TYPE;

我这样初始化它们:

ipc::managed_shared_memory segment;

// allocate segment etc ... 

OBJ_MAP_ALLOCATOR alloc_inst(segment.get_segment_manager());
objMap = segment.find_or_construct<OBJ_MAP_TYPE> (ipc::unique_instance)(alloc_inst);

这似乎工作正常，我在编译或运行期间没有发现任何问题(在 macOS 上工作，Apple LLVM version 9.1.0 (clang-902.0.39.1)，使用 C++14 标准).

This seems to work fine, i haven't found any problems during compile- or runtime (working on macOS, Apple LLVM version 9.1.0 (clang-902.0.39.1), with C++14 standard).

在 Boost 文档中，只提到了 Boost 容器或特定于进程间的实现.不幸的是，它们似乎不包含无序版本.

In the Boost documentation, only the Boost containers, or the interprocess-specific implementations are mentioned. Unfortunately, they do not seem to contain the unordered versions.

那么，我想知道将默认 STL 容器与 Boost 分配器一起使用是否有任何问题?也许在不同的平台上?

So, i wonder if there's anything problematic about using the default STL containers with the Boost allocators ? Maybe on a different platform ?

任何提示表示赞赏！

更新:

我想知道它是否在不同的环境中工作，所以我写了一个关于 Coliru 的最小示例(它令人惊讶地与 std::string 一起使用):

I was wondering if it was working in a different environment, so i wrote a minimal example on Coliru (which surprisingly works with std::string):

http://coliru.stacked-crooked.com/a/91d1a143778cf3e9

推荐答案

unordered_map 将处理 Boost Interprocess 分配器 如果您的库实现支持有状态分配器¹和使用非原始指针类型的分配器.

unordered_map will cope with Boost Interprocess allocators IFF your library implementation has support for stateful allocators¹ and allocators using non-raw pointer types.

即便如此，就像@rustyx 提到的那样，如果您实际上与另一个进程共享内存，您将会遇到很大的麻烦.另一个进程可能会将段映射到不同的基地址，从而使存储在内存区域内的所有指针都无效.

Even so, like @rustyx mentions, you're going to be in deep trouble if you actually share the memory with another process. The other process is likely to map the segment at a different base address, making all pointers stored inside the memory region invalid.

☞你需要也使用带有字符串的进程间分配器！

☞ You need to use a Interprocess allocator with the string too!

以下是我通常更喜欢写的内容:

Here's what I usually prefer to write:

#include <boost/interprocess/allocators/allocator.hpp>
#include <boost/interprocess/containers/string.hpp>
#include <boost/interprocess/managed_shared_memory.hpp>

#include <unordered_map>

namespace ipc = boost::interprocess;

namespace Shared {
    using Segment = ipc::managed_shared_memory;
    using Manager = Segment::segment_manager;
    template <typename T> using Alloc = ipc::allocator<T, Manager>;
    template <typename K, typename V, typename KH = std::hash<K>, typename KEq = std::equal_to<K> >
        using HashMap = std::unordered_map<K, V, KH, KEq, Alloc<std::pair<const K, V>> >;

    using String = ipc::basic_string<char, std::char_traits<char>, Alloc<char> >;
}

using OBJ_MAP_TYPE = Shared::HashMap<size_t, Shared::String>;

int main() {
    Shared::Segment msm(ipc::open_or_create, "test", 10ul<<20);

    Shared::Manager* mgr = msm.get_segment_manager();
    OBJ_MAP_TYPE& m = *msm.find_or_construct<OBJ_MAP_TYPE>("aname")(msm.get_segment_manager());

    m.emplace(42, Shared::String("LtUaE", msm.get_segment_manager()));
}

值得注意的细节:

1. 这一点:

1. This bit:

Shared::Manager* mgr = msm.get_segment_manager();
OBJ_MAP_TYPE& m = *msm.find_or_construct<OBJ_MAP_TYPE>("aname")(mgr);

是一个方便的捷径:

Shared::Alloc<OBJ_MAP_TYPE::value_type> alloc_inst(msm.get_segment_manager());
OBJ_MAP_TYPE& m = *msm.find_or_construct<OBJ_MAP_TYPE>("aname")(alloc_inst);

这是可行的，因为允许从段管理器指针到分配器实例的隐式转换.

This works because the implicit conversion from segment-manager pointer to allocator instance is allowed.

进入魔法

您会注意到嵌套分配器使用起来很笨拙:

Enter MAGIC

You'll note that the nested allocator is clumsy to work with:

m.emplace(42, Shared::String("LtUaE", msm.get_segment_manager()));

这就是 scoped_allocator_adaptor 的设计者试图解决的问题.如果将分配器更改为:

That's what the designers of scoped_allocator_adaptor tried to solve. If you change the allocator into:

template <typename T> using Alloc = std::scoped_allocator_adaptor<ipc::allocator<T, Manager> >;

你可以突然写:

m.emplace(42, "LtUaE");

这是因为就地构造是根据用途定义的-分配器构造(参见[allocator.uses.construction])

This is because in-place construction is defined in terms of uses- allocator construction (see [allocator.uses.construction])

看到它住在 Coliru

¹ 准备好惊讶吧，@SergeyA.我上次检查时 Libstdc++ 不支持这个，但它的 unordered_map 自 GCC 4.9.0 开始支持它，而且 OP 似乎有传闻证据表明 libc++ 支持(尽管我们甚至不知道是否有曾经是 typedef 的一个实例 :))

¹ prepare to be surprised, @SergeyA. Libstdc++ didn't support this last time I checked, but its unordered_map supports it since GCC 4.9.0, and OP seems to have anecdotal evidence that libc++ does (although we don't even know whether there was ever an instance of the typedef :))

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