fastdds:传输层SHM和DATA-SHARING的区别

下图是fastdds官方的图，清晰地展示了dds支持的传输层:

根据通信双方的相对位置(跨机器、同机器跨进程、同进程)的不同选择合适的传输层，是通信中间件必须要考虑的事情。

跨机器：udp、tcp

跨机器通信，只能通过网络， fastdds支持UDP和TCP。

同机器，跨进程：SHM、DATA-SHARING

在同一个机器中，使用UDP和TCP进行通信，当然也是可以的。但是，从性能的角度来考虑，更推荐使用SHM，SHM和UDP/TCP相比有2点优势：

1、减少系统调用次数

共享内存创建完成之后，后边使用的时候直接是内存操作，而UCP/TCP每次发送或者接收的的时候，都要通过系统调用。

2、支持超长消息

UDP/TCP受协议栈的影响，可能会对消息进行分片和组装。

SHM也不完全是优点，也有自己的缺点，比如实现复杂：

1、使用UDP/TCP来收发包，功能成熟，拿来就用。使用SHM进行通信的话，很多功能需要自己实现，比如进程间的同步(通过信号量来实现)，buffer的管理(segment)等。

同进程：INTRA-PROCESS

如果writer和reader在同一个进程，那么可以使用这种传输方式，writer线程会直接调用reader的回调函数，在同一个线程中。

如下图所示，使用如下命令gdb --args ./delivery_mechanisms pubsub -m INTRA-PROCESS，并对函数PubSubApp::on_data_available设置断点，可以看到调用栈，使用INTRA-PROCESS的时候，subscriber的接收回调函数直接被发送线程调用，在同一个线程中。这是一种效率最高的方式，去掉了中间缓存的环节。

在默认情况下，在同进程的通信使用intra-process，同机器跨进程使用SHM，跨机器使用UDP。

从上图可以看出，同机器的不同进程间通信可以使用SHM，也可以使用DATA-SHARING，那么两者有什么区别呢？

本文使用fastdds中的例子delivery_mechanisms，这个例子可以测试不同的传输层。

本文先介绍SHM和DATA-SHARING传输层的数据收发流程，最后总结两者之间的异同。

1SHM收发流程

简单来说，如果要看SHM的发送流程，那么可以以函数SharedMemTransport::send为中心进行梳理，通过gdb对这个函数设置断点，可以查看这个函数的调用栈，阅读这个函数的代码，可以看这个函数的发送操作；如果要看SHM的接收过程，可以通过函数SharedMemChannelResource::perform_listen_operation来梳理，对于TCP，UDP，SHM来说，三种传输层都实现了自己的函数perform_listen_operation，这个函数监听接收数据。

上图中左边是TranportDescriptor，右边是Transport。用户在创建传输层的时候，并不需要直接创建一个Transport，而是构造一个传输层的描述符即可，fastdds内部根据描述符来创建传输层。这是典型的设计模式中的建造者模式，当构造一个对象时，如果需要传递较多的参数，并且参数之间还有一些依赖关系，那么构造函数实现起来就会比较复杂，这个时候就可以使用建造者模式，在建造者中对参数进行检查，如果没有问题则直接构造对象，让构造函数只专注于对象的构造，至于参数的检查工作，放到构造器中来完成。

上图中左图的SenderResource负责发送数据，右图的ChannelResource负责接收数据。

UDP、TCP和SHM的类有平行的关系，他们也有自己的TransportDescriptor、Tranport、SenderResource、ChannelResource类。通过一个基类或者接口派生出3个传输层，体现了c++中多态的使用。

Locator用来描述一个端点，其中kind包括LOCATOR_KIND_TCPv4、LOCATOR_KIND_UDPv4、 LOCATOR_KIND_TCPv6、LOCATOR_KIND_UDPv6、LOCATOR_KIND_SHM；port就是通信的端口，比如7400、7410这些；address是地址，在UDP、TCP中就是ip地址。只要知道要发送对象的Locator信息，那么数据就可以发送出去。

Locator {
   int32_t kind;
   uint32_t port;
   std::string address;
}

1.1发送

下图是对SharedMemTransport::send设置断点，看到的调用栈。

发送流程：

①用户调用DataWriter::write进行发送

②在DataWriterHistory中，将要发送的数据封装到一个CacheChange_t中

③在发送侧，dds和rtps之间的接口类是dds侧的DataWriterHistory，rtps侧的WriterHistory

④rtps中的BaseWriter进行发送

⑤RtpsParticipantImpl::sendSync进行发送，SenderResource属于Participant中的资源，在Participant中调用SenderResource的send函数，最终调用到SharedMemTransport的send

然后我们再来看看SharedMemTransport::send函数内部的实现：

bool SharedMemTransport::send(
        const std::vector<NetworkBuffer>& buffers,
        uint32_t total_bytes,
        fastdds::rtps::LocatorsIterator* destination_locators_begin,
        fastdds::rtps::LocatorsIterator* destination_locators_end,
        const std::chrono::steady_clock::time_point& max_blocking_time_point)
{
    ...
    fastdds::rtps::LocatorsIterator& it = *destination_locators_begin;

    bool ret = true;

    std::shared_ptr<SharedMemManager::Buffer> shared_buffer;

    try
    {
        while (it != *destination_locators_end)
        {
            if (IsLocatorSupported(*it))
            {
                if (shared_buffer == nullptr)
                {
                    shared_buffer = copy_to_shared_buffer(buffers, total_bytes, max_blocking_time_point);
                }

                ret &= send(shared_buffer, *it);
                ...
            }

            ++it;
        }
    }
    ...
    return ret;

}

①对于第一次循环，需要通过函数copy_to_shared_buffer将数据保存到共享内存中

在函数copy_to_shared_buffer中打印shared_mem_segment_的信息，可以看到segment name是fastdds_aec0cadd732cebe2，这个就是发送数据要拷贝的共享内存。所以说，使用共享内存发送数据时，会进行一次数据拷贝。

将数据拷贝到共享内存之后，就是通知接收者，通知接收者需要两个步骤来实现：

①构造一个BufferDescrptor(简称BD)，并将BD发送到对应的端口

②环形监听者

构造一个BufferDescriptor:

唤醒监听者：

1.2接收

对Subscriber的接收回调设置断点，调用栈如下：

2DATA-SHARING收发流程

DATA-SHARING不是一个标准的传输层，没有TransportDescriptor、Transport、SenderResource、ChannelResource这些资源。

2.1发送

delivery mechanisms这个例子，publish的时候，并不是构造了一个临时的变量发送出去的，而是通过load_sample首先从底层申请了一块内存，而这块内存就是从data sharing的writer pool中申请的，这样数据直接就是保存在这里的，payload pool就是从这里申请的，申请的是data sharing的writer pool，而writer pool就是基于共享内存创建的。所以说，在发送时，没有拷贝。

std::shared_ptr<IPayloadPool> DataWriterImpl::get_payload_pool()

{

if (!payload_pool_)

{

// Avoid calling the serialization size functors on PREALLOCATED mode

fixed_payload_size_ =

pool_config_.memory_policy == PREALLOCATED_MEMORY_MODE ? pool_config_.payload_initial_size : 0u;

// Get payload pool reference and allocate space for our history

if (is_data_sharing_compatible_)

{

payload_pool_ = DataSharingPayloadPool::get_writer_pool(pool_config_);

}

else

{

payload_pool_ = TopicPayloadPoolRegistry::get(topic_->get_name(), pool_config_);

if (!std::static_pointer_cast<ITopicPayloadPool>(payload_pool_)->reserve_history(pool_config_, false))

{

payload_pool_.reset();

}

}

// Prepare loans collection for plain types only

if (type_->is_plain(data_representation_))

{

loans_.reset(new LoanCollection(pool_config_));

}

}

return payload_pool_;

}

在这个函数里把数据发送到对方

DeliveryRetCode StatefulWriter::deliver_sample_nts(

CacheChange_t* cache_change,

RTPSMessageGroup& group,

LocatorSelectorSender& locator_selector, // Object locked by FlowControllerImpl

const std::chrono::time_point<std::chrono::steady_clock>& max_blocking_time)

{

DeliveryRetCode ret_code = DeliveryRetCode::DELIVERED;

if (there_are_local_readers_)

{

deliver_sample_to_intraprocesses(cache_change);

}

// Process datasharing then

if (there_are_datasharing_readers_)

{

deliver_sample_to_datasharing(cache_change);

}

if (there_are_remote_readers_)

{

ret_code = deliver_sample_to_network(cache_change, group, locator_selector, max_blocking_time);

}

check_acked_status();

return ret_code;

}

发送侧通知接收侧

(gdb) bt
#0 eprosima::fastdds::rtps::DataSharingNotifier::notify (this=0x7fffe00029b0) at /root/Fast-DDS/Fast-DDS/src/cpp/rtps/DataSharing/DataSharingNotifier.hpp:78
#1 0x00007ffff7788956 in eprosima::fastdds::rtps::ReaderLocator::datasharing_notify (this=0x7fffe0002568) at /root/Fast-DDS/Fast-DDS/src/cpp/rtps/writer/ReaderLocator.cpp:237
#2 0x00007ffff77a3cf8 in eprosima::fastdds::rtps::ReaderProxy::datasharing_notify (this=0x7fffe0002560) at /root/Fast-DDS/Fast-DDS/src/cpp/rtps/writer/ReaderProxy.hpp:388
#3 0x00007ffff779761a in eprosima::fastdds::rtps::StatefulWriter::deliver_sample_to_datasharing (this=0x555555789150, change=0x555555786f70)
at /root/Fast-DDS/Fast-DDS/src/cpp/rtps/writer/StatefulWriter.cpp:612
#4 0x00007ffff779eab1 in eprosima::fastdds::rtps::StatefulWriter::deliver_sample_nts (this=0x555555789150, cache_change=0x555555786f70, group=..., locator_selector=..., max_blocking_time=...)
at /root/Fast-DDS/Fast-DDS/src/cpp/rtps/writer/StatefulWriter.cpp:2130
#5 0x00007ffff762d4af in eprosima::fastdds::rtps::FlowControllerImpl<eprosima::fastdds::rtps::FlowControllerSyncPublishMode, eprosima::fastdds::rtps::FlowControllerFifoSchedule>::add_new_sample_impl<eprosima::fastdds::rtps::FlowControllerSyncPublishMode> (this=0x55555567f9d0, writer=0x555555789150, change=0x555555786f70, max_blocking_time=...)
at /root/Fast-DDS/Fast-DDS/src/cpp/rtps/flowcontrol/FlowControllerImpl.hpp:1191
#6 0x00007ffff762a44c in eprosima::fastdds::rtps::FlowControllerImpl<eprosima::fastdds::rtps::FlowControllerSyncPublishMode, eprosima::fastdds::rtps::FlowControllerFifoSchedule>::add_new_sample (
this=0x55555567f9d0, writer=0x555555789150, change=0x555555786f70, max_blocking_time=...) at /root/Fast-DDS/Fast-DDS/src/cpp/rtps/flowcontrol/FlowControllerImpl.hpp:1012
#7 0x00007ffff7796415 in eprosima::fastdds::rtps::StatefulWriter::unsent_change_added_to_history (this=0x555555789150, change=0x555555786f70, max_blocking_time=...)
at /root/Fast-DDS/Fast-DDS/src/cpp/rtps/writer/StatefulWriter.cpp:386
#8 0x00007ffff7644784 in eprosima::fastdds::rtps::WriterHistory::notify_writer (this=0x555555788ca0, a_change=0x555555786f70, max_blocking_time=...)
at /root/Fast-DDS/Fast-DDS/src/cpp/rtps/history/WriterHistory.cpp:201
#9 0x00007ffff71efd05 in eprosima::fastdds::rtps::WriterHistory::add_change_with_commit_hook<eprosima::fastdds::dds::DataWriterImpl::perform_create_new_change(eprosima::fastdds::rtps::ChangeKind_t, void const*, eprosima::fastdds::rtps::WriteParams&, const InstanceHandle_t&)::<lambda(eprosima::fastdds::dds::DataWriterImpl::CacheChange_t&)> >(eprosima::fastdds::rtps::CacheChange_t *, eprosima::fastdds::rtps::WriteParams &, struct {...}, std::chrono::time_point<std::chrono::_V2::steady_clock, std::chrono::duration<long, std::ratio<1, 1000000000> > >) (this=0x555555788ca0, a_change=0x555555786f70,
wparams=..., pre_commit=..., max_blocking_time=...) at /root/Fast-DDS/Fast-DDS/include/fastdds/rtps/history/WriterHistory.hpp:299
#10 0x00007ffff71ef6a0 in eprosima::fastdds::dds::DataWriterHistory::add_pub_change_with_commit_hook<eprosima::fastdds::dds::DataWriterImpl::perform_create_new_change(eprosima::fastdds::rtps::ChangeKind_t, void const*, eprosima::fastdds::rtps::WriteParams&, const InstanceHandle_t&)::<lambda(eprosima::fastdds::dds::DataWriterImpl::CacheChange_t&)> >(eprosima::fastdds::rtps::CacheChange_t *, eprosima::fastdds::rtps::WriteParams &, struct {...}, std::unique_lock<std::recursive_timed_mutex> &, const std::chrono::time_point<std::chrono::_V2::steady_clock, std::chrono::duration<long, std::ratio<1, 1000000000> > > &) (this=0x555555788ca0, change=0x555555786f70, wparams=..., pre_commit=..., lock=..., max_blocking_time=...) at /root/Fast-DDS/Fast-DDS/src/cpp/fastdds/publisher/DataWriterHistory.hpp:159
#11 0x00007ffff71e830d in eprosima::fastdds::dds::DataWriterImpl::perform_create_new_change (this=0x55555577d430, change_kind=eprosima::fastdds::rtps::ALIVE, data=0x7ffff6233ef4, wparams=..., handle=...)
at /root/Fast-DDS/Fast-DDS/src/cpp/fastdds/publisher/DataWriterImpl.cpp:1057
#12 0x00007ffff71e88e0 in eprosima::fastdds::dds::DataWriterImpl::create_new_change_with_params (this=0x55555577d430, changeKind=eprosima::fastdds::rtps::ALIVE, data=0x7ffff6233ef4, wparams=...)
at /root/Fast-DDS/Fast-DDS/src/cpp/fastdds/publisher/DataWriterImpl.cpp:1131
#13 0x00007ffff71e7ba9 in eprosima::fastdds::dds::DataWriterImpl::create_new_change (this=0x55555577d430, changeKind=eprosima::fastdds::rtps::ALIVE, data=0x7ffff6233ef4)
at /root/Fast-DDS/Fast-DDS/src/cpp/fastdds/publisher/DataWriterImpl.cpp:976
#14 0x00007ffff71e6312 in eprosima::fastdds::dds::DataWriterImpl::write (this=0x55555577d430, data=0x7ffff6233ef4) at /root/Fast-DDS/Fast-DDS/src/cpp/fastdds/publisher/DataWriterImpl.cpp:655
#15 0x00007ffff71d9d8f in eprosima::fastdds::dds::DataWriter::write (this=0x55555577db90, data=0x7ffff6233ef4) at /root/Fast-DDS/Fast-DDS/src/cpp/fastdds/publisher/DataWriter.cpp:84
#16 0x0000555555601386 in eprosima::fastdds::examples::delivery_mechanisms::PublisherApp::publish (this=0x555555657c60) at /root/Fast-DDS/Fast-DDS/examples/cpp/delivery_mechanisms/PublisherApp.cpp:295
#17 0x00005555556010b3 in eprosima::fastdds::examples::delivery_mechanisms::PublisherApp::run (this=0x555555657c60) at /root/Fast-DDS/Fast-DDS/examples/cpp/delivery_mechanisms/PublisherApp.cpp:266
#18 0x0000555555609105 in std::__invoke_impl<void, void (eprosima::fastdds::examples::delivery_mechanisms::Application::*)(), std::shared_ptr<eprosima::fastdds::examples::delivery_mechanisms::Application>>(std::__invoke_memfun_deref, void (eprosima::fastdds::examples::delivery_mechanisms::Application::*&&)(), std::shared_ptr<eprosima::fastdds::examples::delivery_mechanisms::Application>&&) (
__f=@0x55555565e578: &virtual table offset 16, __t=...) at /usr/include/c++/11/bits/invoke.h:74
#19 0x000055555560903d in std::__invoke<void (eprosima::fastdds::examples::delivery_mechanisms::Application::*)(), std::shared_ptr<eprosima::fastdds::examples::delivery_mechanisms::Application> > (
__fn=@0x55555565e578: &virtual table offset 16) at /usr/include/c++/11/bits/invoke.h:96
#20 0x0000555555608f9d in std::thread::_Invoker<std::tuple<void (eprosima::fastdds::examples::delivery_mechanisms::Application::*)(), std::shared_ptr<eprosima::fastdds::examples::delivery_mechanisms::Application> > >::_M_invoke<0ul, 1ul> (this=0x55555565e568) at /usr/include/c++/11/bits/std_thread.h:259
#21 0x0000555555608f52 in std::thread::_Invoker<std::tuple<void (eprosima::fastdds::examples::delivery_mechanisms::Application::*)(), std::shared_ptr<eprosima::fastdds::examples::delivery_mechanisms::Application> > >::operator() (this=0x55555565e568) at /usr/include/c++/11/bits/std_thread.h:266
#22 0x0000555555608f32 in std::thread::_State_impl<std::thread::_Invoker<std::tuple<void (eprosima::fastdds::examples::delivery_mechanisms::Application::*)(), std::shared_ptr<eprosima::fastdds::examples::delivery_mechanisms::Application> > > >::_M_run (this=0x55555565e560) at /usr/include/c++/11/bits/std_thread.h:211
#23 0x00007ffff60dc253 in ?? () from /lib/x86_64-linux-gnu/libstdc++.so.6
#24 0x00007ffff5c94ac3 in start_thread (arg=<optimized out>) at ./nptl/pthread_create.c:442
#25 0x00007ffff5d26850 in clone3 () at ../sysdeps/unix/sysv/linux/x86_64/clone3.S:81
(gdb)

DATA-SHARING初始化writer pool

(gdb) bt
#0 eprosima::fastdds::rtps::WriterPool::init_shared_segment<eprosima::fastdds::rtps::SharedSegment<boost::interprocess::basic_managed_shared_memory<char, boost::interprocess::rbtree_best_fit<boost::interprocess::mutex_family, boost::interprocess::offset_ptr<void, unsigned int, unsigned long, 0ul>, 0ul>, boost::interprocess::iset_index>, boost::interprocess::shared_memory_object> > (this=0x5555556633c0,
writer=0x555555789150, shared_dir="") at /root/Fast-DDS/Fast-DDS/src/cpp/rtps/DataSharing/./WriterPool.hpp:132
#1 0x00007ffff760f455 in eprosima::fastdds::rtps::WriterPool::init_shared_memory (this=0x5555556633c0, writer=0x555555789150, shared_dir="")
at /root/Fast-DDS/Fast-DDS/src/cpp/rtps/DataSharing/./WriterPool.hpp:248
#2 0x00007ffff777f315 in eprosima::fastdds::rtps::BaseWriter::init (this=0x555555789150, att=...) at /root/Fast-DDS/Fast-DDS/src/cpp/rtps/writer/BaseWriter.cpp:388
#3 0x00007ffff777e14f in eprosima::fastdds::rtps::BaseWriter::BaseWriter (this=0x555555789150, impl=0x555555663600, guid=..., att=..., flow_controller=0x55555567f9d0, hist=0x555555788ca0,
listen=0x55555577d930) at /root/Fast-DDS/Fast-DDS/src/cpp/rtps/writer/BaseWriter.cpp:82
#4 0x00007ffff7794db9 in eprosima::fastdds::rtps::StatefulWriter::StatefulWriter (this=0x555555789150, pimpl=0x555555663600, guid=..., att=..., flow_controller=0x55555567f9d0, history=0x555555788ca0,
listener=0x55555577d930) at /root/Fast-DDS/Fast-DDS/src/cpp/rtps/writer/StatefulWriter.cpp:203
#5 0x00007ffff7685df0 in operator() (__closure=0x7fffffffbc50, guid=..., watt=..., flow_controller=0x55555567f9d0, persistence=0x0, is_reliable=true)
at /root/Fast-DDS/Fast-DDS/src/cpp/rtps/participant/RTPSParticipantImpl.cpp:1246
#6 0x00007ffff768f132 in eprosima::fastdds::rtps::RTPSParticipantImpl::create_writer<eprosima::fastdds::rtps::RTPSParticipantImpl::create_writer(eprosima::fastdds::rtps::RTPSWriter**, eprosima::fastdds::rtps::WriterAttributes&, eprosima::fastdds::rtps::WriterHistory*, eprosima::fastdds::rtps::WriterListener*, const eprosima::fastdds::rtps::EntityId_t&, bool)::<lambda(const GUID_t&, eprosima::fastdds::rtps::WriterAttributes&, eprosima::fastdds::rtps::FlowController*, eprosima::fastdds::rtps::IPersistenceService*, bool)> >(eprosima::fastdds::rtps::RTPSWriter **, eprosima::fastdds::rtps::WriterAttributes &, const eprosima::fastdds::rtps::EntityId_t &, bool, const struct {...} &) (this=0x555555663600, writer_out=0x7fffffffbe88, param=..., entity_id=..., is_builtin=false, callback=...)
at /root/Fast-DDS/Fast-DDS/src/cpp/rtps/participant/RTPSParticipantImpl.cpp:955
#7 0x00007ffff7686266 in eprosima::fastdds::rtps::RTPSParticipantImpl::create_writer (this=0x555555663600, WriterOut=0x7fffffffbe88, watt=..., hist=0x555555788ca0, listen=0x55555577d930, entityId=...,
isBuiltin=false) at /root/Fast-DDS/Fast-DDS/src/cpp/rtps/participant/RTPSParticipantImpl.cpp:1264
#8 0x00007ffff76d5f74 in eprosima::fastdds::rtps::RTPSDomainImpl::create_rtps_writer (p=0x5555556631f0, entity_id=..., watt=..., hist=0x555555788ca0, listen=0x55555577d930)
at /root/Fast-DDS/Fast-DDS/src/cpp/rtps/RTPSDomain.cpp:384
#9 0x00007ffff71e4834 in eprosima::fastdds::dds::DataWriterImpl::enable (this=0x55555577d430) at /root/Fast-DDS/Fast-DDS/src/cpp/fastdds/publisher/DataWriterImpl.cpp:375
#10 0x00007ffff783570a in eprosima::fastdds::statistics::dds::DataWriterImpl::enable (this=0x55555577d430) at /root/Fast-DDS/Fast-DDS/src/cpp/statistics/fastdds/publisher/DataWriterImpl.hpp:77
#11 0x00007ffff71d9ceb in eprosima::fastdds::dds::DataWriter::enable (this=0x55555577db90) at /root/Fast-DDS/Fast-DDS/src/cpp/fastdds/publisher/DataWriter.cpp:63
#12 0x00007ffff7204a76 in eprosima::fastdds::dds::PublisherImpl::create_datawriter (this=0x55555577c8a0, topic=0x555555668dd0, impl=0x55555577d430, mask=...)
at /root/Fast-DDS/Fast-DDS/src/cpp/fastdds/publisher/PublisherImpl.cpp:277
#13 0x00007ffff720480c in eprosima::fastdds::dds::PublisherImpl::create_datawriter (this=0x55555577c8a0, topic=0x555555668dd0, qos=..., listener=0x555555657c68, mask=...,
payload_pool=std::shared_ptr<eprosima::fastdds::rtps::IPayloadPool> (empty) = {...}) at /root/Fast-DDS/Fast-DDS/src/cpp/fastdds/publisher/PublisherImpl.cpp:257
#14 0x00007ffff7202b0c in eprosima::fastdds::dds::Publisher::create_datawriter (this=0x55555577ced0, topic=0x555555668dd0, qos=..., listener=0x555555657c68, mask=...,
payload_pool=std::shared_ptr<eprosima::fastdds::rtps::IPayloadPool> (empty) = {...}) at /root/Fast-DDS/Fast-DDS/src/cpp/fastdds/publisher/Publisher.cpp:119
#15 0x0000555555600ae1 in eprosima::fastdds::examples::delivery_mechanisms::PublisherApp::PublisherApp (this=0x555555657c60, config=..., topic_name="delivery_mechanisms_topic")
at /root/Fast-DDS/Fast-DDS/examples/cpp/delivery_mechanisms/PublisherApp.cpp:221
#16 0x00005555555daf5f in __gnu_cxx::new_allocator<eprosima::fastdds::examples::delivery_mechanisms::PublisherApp>::construct<eprosima::fastdds::examples::delivery_mechanisms::PublisherApp, eprosima::fastdds::examples::delivery_mechanisms::CLIParser::delivery_mechanisms_config const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&> (this=0x7fffffffdddf,
__p=0x555555657c60) at /usr/include/c++/11/ext/new_allocator.h:162
#17 0x00005555555daafc in std::allocator_traits<std::allocator<eprosima::fastdds::examples::delivery_mechanisms::PublisherApp> >::construct<eprosima::fastdds::examples::delivery_mechanisms::PublisherApp, eprosima::fastdds::examples::delivery_mechanisms::CLIParser::delivery_mechanisms_config const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&> (__a=...,
__p=0x555555657c60) at /usr/include/c++/11/bits/alloc_traits.h:516
#18 0x00005555555da3d7 in std::_Sp_counted_ptr_inplace<eprosima::fastdds::examples::delivery_mechanisms::PublisherApp, std::allocator<eprosima::fastdds::examples::delivery_mechanisms::PublisherApp>, (__gnu_cxx::_Lock_policy)2>::_Sp_counted_ptr_inplace<eprosima::fastdds::examples::delivery_mechanisms::CLIParser::delivery_mechanisms_config const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&> (this=0x555555657c50, __a=...) at /usr/include/c++/11/bits/shared_ptr_base.h:519
#19 0x00005555555d9d73 in std::__shared_count<(__gnu_cxx::_Lock_policy)2>::__shared_count<eprosima::fastdds::examples::delivery_mechanisms::PublisherApp, std::allocator<eprosima::fastdds::examples::delivery_mechanisms::PublisherApp>, eprosima::fastdds::examples::delivery_mechanisms::CLIParser::delivery_mechanisms_config const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&> (this=0x7fffffffdf88, __p=@0x7fffffffdf80: 0x0, __a=...) at /usr/include/c++/11/bits/shared_ptr_base.h:650
#20 0x00005555555d9ade in std::__shared_ptr<eprosima::fastdds::examples::delivery_mechanisms::PublisherApp, (__gnu_cxx::_Lock_policy)2>::__shared_ptr<std::allocator<eprosima::fastdds::examples::delivery_mechanisms::PublisherApp>, eprosima::fastdds::examples::delivery_mechanisms::CLIParser::delivery_mechanisms_config const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&> (this=0x7fffffffdf80, __tag=...) at /usr/include/c++/11/bits/shared_ptr_base.h:1342
#21 0x00005555555d97e1 in std::shared_ptr<eprosima::fastdds::examples::delivery_mechanisms::PublisherApp>::shared_ptr<std::allocator<eprosima::fastdds::examples::delivery_mechanisms::PublisherApp>, eprosima::fastdds::examples::delivery_mechanisms::CLIParser::delivery_mechanisms_config const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&> (this=0x7fffffffdf80,
__tag=...) at /usr/include/c++/11/bits/shared_ptr.h:409
#22 0x00005555555d948f in std::allocate_shared<eprosima::fastdds::examples::delivery_mechanisms::PublisherApp, std::allocator<eprosima::fastdds::examples::delivery_mechanisms::PublisherApp>, eprosima::fastdds::examples::delivery_mechanisms::CLIParser::delivery_mechanisms_config const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&> (__a=...)
at /usr/include/c++/11/bits/shared_ptr.h:863
#23 0x00005555555d916d in std::make_shared<eprosima::fastdds::examples::delivery_mechanisms::PublisherApp, eprosima::fastdds::examples::delivery_mechanisms::CLIParser::delivery_mechanisms_config const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&> () at /usr/include/c++/11/bits/shared_ptr.h:879
#24 0x00005555555d885a in eprosima::fastdds::examples::delivery_mechanisms::Application::make_app (config=..., topic_name="delivery_mechanisms_topic")
at /root/Fast-DDS/Fast-DDS/examples/cpp/delivery_mechanisms/Application.cpp:41
--Type <RET> for more, q to quit, c to continue without paging--
#25 0x0000555555603cb9 in main (argc=4, argv=0x7fffffffe398) at /root/Fast-DDS/Fast-DDS/examples/cpp/delivery_mechanisms/main.cpp:54
(gdb) p shared_dir
$1 = ""
(gdb) s
136 segment_id_ = writer->getGuid();
(gdb) n
137 segment_name_ = generate_segment_name(shared_dir, segment_id_);
(gdb) n
138 std::unique_ptr<T> local_segment;
(gdb) p segment_id_
$2 = {guidPrefix = {static size = 12, value = "\001\017\325\227\065\253k[\000\000\000"}, entityId = {static size = 4, value = "\000\000\001\003"}}
(gdb) p segment_name_
$3 = "fast_datasharing_01.0f.d5.97.35.ab.6b.5b.00.00.00.00_0.0.1.3"
(gdb)

2.2接收

DataSharingListener::run

3相同点：均通过共享内存进行通信

3.1SHM

在linux下，共享内存默认使用/dev/shm临时文件系统。如下是使用SHM的时候，启动了一个publisher和subscriber，在/dev/shm中创建的文件。

（1）SHM使用了端口的概念

fastdds中对SHM的使用，借鉴了UDP的通信方式，也使用了端口这个概念。在通信中，端口的表示这个通信通道被哪个进程使用。

上图中7400、7410、7411、7412、7413端口与下文中UDP的端口，作用是相同的。

fastdds:传输层端口号计算规则_fastdds mechanism-CSDN博客

下图中显示了SHM传输层的通信时序图。可以看到port是用来传输descriptor的，真正的数据是通过fastdds_56f311faa23f4389这样的文件进行传输的。descriptor在网络通信中是经常见到的一个概念，作为数据的描述符描述了数据的基本信息，比如数据的起始地址，数据的长度等。在网卡驱动中，经常说的BD，全称是buffer descriptor，也是网络收发包的描述符，与这里的descriptor是类似的。

（2）文件的作用

①锁，进程间同步

上图中以sem开头，以mutex结尾的文件用于进程间同步。以sem.xxx.port7413_mutex为例，如果一个进程监听7413这个端口的数据，那么就会wait这个锁；如果有进程要向7413发数据，那么写好数据之后，便会notiy这个锁，从而接收方就会被唤醒，进而接收并处理数据。

②fastdds_port7413

这个是端口文件，用于传输descriptor，如果有进程要向7413发送数据，那么便会将descriptor写到这个文件中。

③fastdds_56f311faa23f4389

数据文件，进程要发送的数据都会写到这个文件中。

也就是说，通过共享内存发送数据的时候，首先要将数据写到这个文件中，然后再填充一个desctiptor，将descriptor文件放到port文件中，最后调用notify，环形对应port的进程。

当我们想要了解这些文件是什么用的时候，除了从文件的名字、代码、文档中去查找确认之外，还可以通过在gdb中对shm_open设置断点，从而查看调用栈的方式去学习。/dev/shm中的文件通过shm_open打开。

3.2DATA-SHARING

如下是使用DATA-SHARING，启动了一个publisher和subscriber的情况下，在/dev/shm下创建的文件。从下图中可以看出，使用DATA-SHARING 的时候，/dev/shm下的文件，除了使用SHM时的哪些文件之外，还多了一个文件，比如fast_datasharing_01.0f.d5.97.90.46.db.3a.00.00.00.00_0.0.1.3。

那么文件fast_datasharing_01.0f.d5.97.90.46.db.3a.00.00.00.00_0.0.1.3是做什么用的呢？

首先我们可以在gdb中对shm_open设断点，查看这个函数的调用栈。调用栈如下图所示：

从中我们可以看到这个函数，可以大胆猜测在使用DATA-SHARING的时候，这个文件用于数据的传输。而原来与SHM传输层相同的文件，用于服务发现阶段。

#8 0x00007ffff7610de5 in eprosima::fastdds::rtps::WriterPool::init_shared_segment<eprosima::fastdds::rtps::SharedSegment<boost::interprocess::basic_managed_shared_memory<char, boost::interprocess::rbtree_best_fit<boost::interprocess::mutex_family, boost::interprocess::offset_ptr<void, unsigned int, unsigned long, 0ul>, 0ul>, boost::interprocess::iset_index>, boost::interprocess::shared_memory_object> > (
this=0x5555556633c0, writer=0x555555789150, shared_dir="") at /root/Fast-DDS/Fast-DDS/src/cpp/rtps/DataSharing/./WriterPool.hpp:183

4不同点

（1）SHM是标准的传输层port based的传输层，DATA-SHARING不是

（2）相对于SHM，DATA-SHARING在发送侧少一次拷贝

（3）SHM数据和Descriptor保存在了不同的文件中，数据保存在fastdds_683379ac5b6c3ba5中，descriptor发送到fastdds_port7413中；而DATA-SHARING的数据和描述符保存在了同一块内存中，下边的函数，node就相当于SHM中的descriptor，可以看到node和数据所在的内存是爱着的。

void add_to_shared_history(

const CacheChange_t* cache_change)

{

assert(cache_change);

assert(cache_change->serializedPayload.data);

assert(cache_change->serializedPayload.payload_owner == this);

assert(free_history_size_ > 0);

// Fill the payload metadata with the change info

PayloadNode* node = PayloadNode::get_from_data(cache_change->serializedPayload.data);

node->status(ALIVE);

node->data_length(cache_change->serializedPayload.length);

node->source_timestamp(cache_change->sourceTimestamp);

node->writer_GUID(cache_change->writerGUID);

node->instance_handle(cache_change->instanceHandle);

if (cache_change->write_params.related_sample_identity() != SampleIdentity::unknown())

{

node->related_sample_identity(cache_change->write_params.related_sample_identity());

}

// Set the sequence number last, it signals the data is ready

node->sequence_number(cache_change->sequenceNumber);

// Add it to the history

history_[static_cast<uint32_t>(descriptor_->notified_end)] = segment_->get_offset_from_address(node);

EPROSIMA_LOG_INFO(DATASHARING_PAYLOADPOOL, "Change added to shared history"

<< " with SN " << cache_change->sequenceNumber);

advance(descriptor_->notified_end);

--