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DDS (Data Distribution Service): 用于实时系统的数据分发服务标准#xff0c;是ROS 2底层通信的基础RMW (ROS Middleware): ROS中间件接口#xff0c;提供与具体DDS实现无关的抽象APIQoS (Quality of Service): 服务质量策略#xff0c;控制通信的可靠性、历史记录、…术语
DDS (Data Distribution Service): 用于实时系统的数据分发服务标准是ROS 2底层通信的基础RMW (ROS Middleware): ROS中间件接口提供与具体DDS实现无关的抽象APIQoS (Quality of Service): 服务质量策略控制通信的可靠性、历史记录、耐久性等属性符号解析: 动态库加载过程中查找和绑定函数指针的机制
1. 架构概述
ROS2采用分层设计通过多层抽象实现高度灵活性和可插拔性。其调用链如下
应用层(开发人员) → rclcpp(C客户端库) → rcl(C语言客户端库) → rmw(中间件接口) → rmw_implementation(动态加载层) → 具体DDS实现2. 发布者创建流程详解
2.1 用户代码层(rclcpp::Node::create_publisher)
通常使用以下代码创建发布者
auto node std::make_sharedrclcpp::Node(publisher_node);
auto publisher node-create_publisherstd_msgs::msg::String(topic_name, 10); // 队列深度为102.2 rclcpp层(Node类)
在node.hpp文件中对各模板函数进行声明函数的具体实现在node_impl.hpp文件中进行定义。
class Node : public std::enable_shared_from_thisNode
{templatetypename MessageT,typename AllocatorT std::allocatorvoid,typename PublisherT rclcpp::PublisherMessageT, AllocatorTstd::shared_ptrPublisherTcreate_publisher(const std::string topic_name,const rclcpp::QoS qos,const PublisherOptionsWithAllocatorAllocatorT options PublisherOptionsWithAllocatorAllocatorT());
}node_impl.hpp文件中node-create_publisher 调用一系列rclcpp函数
templatetypename MessageT, typename AllocatorT, typename PublisherT
std::shared_ptrPublisherT
Node::create_publisher(const std::string topic_name,const rclcpp::QoS qos,const PublisherOptionsWithAllocatorAllocatorT options)
{return rclcpp::create_publisherMessageT, AllocatorT, PublisherT(*this,extend_name_with_sub_namespace(topic_name, this-get_sub_namespace()),qos,options);
}该方法调用create_publisher.hpp文件的node_topics_interface-create_publisher创建类型特定的工厂
templatetypename MessageT,typename AllocatorT std::allocatorvoid,typename PublisherT rclcpp::PublisherMessageT, AllocatorT,typename NodeParametersT,typename NodeTopicsT
std::shared_ptrPublisherT
create_publisher(NodeParametersT node_parameters,NodeTopicsT node_topics,const std::string topic_name,const rclcpp::QoS qos,const rclcpp::PublisherOptionsWithAllocatorAllocatorT options (rclcpp::PublisherOptionsWithAllocatorAllocatorT())
)
{auto node_topics_interface rclcpp::node_interfaces::get_node_topics_interface(node_topics);const rclcpp::QoS actual_qos options.qos_overriding_options.get_policy_kinds().size() ?rclcpp::detail::declare_qos_parameters(options.qos_overriding_options, node_parameters,node_topics_interface-resolve_topic_name(topic_name),qos, rclcpp::detail::PublisherQosParametersTraits{}) :qos;// Create the publisher.auto pub node_topics_interface-create_publisher(topic_name,rclcpp::create_publisher_factoryMessageT, AllocatorT, PublisherT(options),actual_qos);// Add the publisher to the node topics interface.node_topics_interface-add_publisher(pub, options.callback_group);return std::dynamic_pointer_castPublisherT(pub);
}node_topics_interface-create_publisher实际调用node_topics.cpp文件的NodeTopics::create_publisher非模板方法该方法使用publisher_factory.hpp文件中工厂的create_typed_publisher函数创建实际的发布者创建的发布者进行后期初始化设置。
rclcpp::PublisherBase::SharedPtr
NodeTopics::create_publisher(const std::string topic_name,const rclcpp::PublisherFactory publisher_factory,const rclcpp::QoS qos)
{// Create the MessageT specific Publisher using the factory, but return it as PublisherBase.return publisher_factory.create_typed_publisher(node_base_, topic_name, qos);
}而create_typed_publisher的实现如下
templatetypename MessageT, typename AllocatorT, typename PublisherT
PublisherFactory
create_publisher_factory(const rclcpp::PublisherOptionsWithAllocatorAllocatorT options)
{PublisherFactory factory {// factory function that creates a MessageT specific PublisherT[options](rclcpp::node_interfaces::NodeBaseInterface * node_base,const std::string topic_name,const rclcpp::QoS qos) - std::shared_ptrPublisherT{auto publisher std::make_sharedPublisherT(node_base, topic_name, qos, options);// This is used for setting up things like intra process comms which// require this-shared_from_this() which cannot be called from// the constructor.publisher-post_init_setup(node_base, topic_name, qos, options);return publisher;}};// return the factory now that it is populatedreturn factory;
}创建特定的Publisher时publisher.hpp文件内部会转到具体的Publisher构造函数
Publisher(rclcpp::node_interfaces::NodeBaseInterface * node_base,const std::string topic,const rclcpp::QoS qos,const rclcpp::PublisherOptionsWithAllocatorAllocatorT options)
: PublisherBase(node_base,topic,rclcpp::get_message_type_support_handleMessageT(),options.template to_rcl_publisher_optionsMessageT(qos)),options_(options),published_type_allocator_(*options.get_allocator()),ros_message_type_allocator_(*options.get_allocator())
{// 初始化内存分配器和事件回调
}PublisherBase构造函数则进行RCL层的publisher初始化
PublisherBase::PublisherBase(rclcpp::node_interfaces::NodeBaseInterface * node_base,const std::string topic,const rosidl_message_type_support_t type_support,const rcl_publisher_options_t publisher_options)
: node_base_(node_base),topic_(topic)
{// 创建rcl_publisher_t实例publisher_handle_ std::shared_ptrrcl_publisher_t(new rcl_publisher_t, [node_handle](rcl_publisher_t * publisher) {if (rcl_publisher_fini(publisher, node_handle) ! RCL_RET_OK) {RCLCPP_ERROR(/* ... */);}delete publisher;});*publisher_handle_.get() rcl_get_zero_initialized_publisher();// 关键调用初始化RCL层publisherrcl_ret_t ret rcl_publisher_init(publisher_handle_.get(),node_base-get_rcl_node_handle(),type_support,topic.c_str(),publisher_options);if (ret ! RCL_RET_OK) {// 错误处理}
}2.3 rcl层
rcl_publisher_init 函数进一步处理
rcl_ret_t
rcl_publisher_init(rcl_publisher_t * publisher,const rcl_node_t * node,const rosidl_message_type_support_t * type_support,const char * topic_name,const rcl_publisher_options_t * options
)
{// 参数验证和初始化// 向RMW层请求创建发布者// rmw_handle为rmw_publisher_t *类型publisher-impl-rmw_handle rmw_create_publisher(rcl_node_get_rmw_handle(node),type_support,remapped_topic_name,(options-qos),(options-rmw_publisher_options));// 错误处理与返回值设置
}2.4 rmw层通过rmw_implementation
此处进入rmw_implementation包的functions.cpp中其中rmw_create_publisher为rmw.h文件中定义的接口函数
RMW_INTERFACE_FN(rmw_create_publisher,rmw_publisher_t *, nullptr,5, ARG_TYPES(const rmw_node_t *, const rosidl_message_type_support_t *, const char *,const rmw_qos_profile_t *, const rmw_publisher_options_t *))这个宏展开后生成
#define RMW_INTERFACE_FN(name, ReturnType, error_value, _NR, ...) \void * symbol_ ## name nullptr; \ReturnType name(EXPAND(ARGS_ ## _NR(__VA_ARGS__))) \{ \CALL_SYMBOL( \name, ReturnType, error_value, ARG_TYPES(__VA_ARGS__), \EXPAND(ARG_VALUES_ ## _NR(__VA_ARGS__))); \}CALL_SYMBOL宏进一步展开
#define CALL_SYMBOL(symbol_name, ReturnType, error_value, ArgTypes, arg_values) \if (!symbol_ ## symbol_name) { \/* only necessary for functions called before rmw_init */ \//获取库中的函数符号symbol_ ## symbol_name get_symbol(#symbol_name); \} \if (!symbol_ ## symbol_name) { \/* error message set by get_symbol() */ \return error_value; \} \typedef ReturnType (* FunctionSignature)(ArgTypes); \// 根据函数地址调用加载的函数FunctionSignature func reinterpret_castFunctionSignature(symbol_ ## symbol_name); \return func(arg_values);get_symbol 函数获取函数符号
void *
get_symbol(const char * symbol_name)
{try {return lookup_symbol(get_library(), symbol_name);} catch (const std::exception e) {RMW_SET_ERROR_MSG_WITH_FORMAT_STRING(failed to get symbol %s due to %s,symbol_name, e.what());return nullptr;}
}2.5 加载和符号查找
关键的lookup_symbol函数负责从已加载的库中查找符号
void *
lookup_symbol(std::shared_ptrrcpputils::SharedLibrary lib, const std::string symbol_name)
{if (!lib) {if (!rmw_error_is_set()) {RMW_SET_ERROR_MSG(no shared library to lookup);} // else assume library loading failedreturn nullptr;}if (!lib-has_symbol(symbol_name)) {try {std::string library_path lib-get_library_path();RMW_SET_ERROR_MSG_WITH_FORMAT_STRING(failed to resolve symbol %s in shared library %s,symbol_name.c_str(), library_path.c_str());} catch (const std::exception e) {RMW_SET_ERROR_MSG_WITH_FORMAT_STRING(failed to resolve symbol %s in shared library due to %s,symbol_name.c_str(), e.what());}return nullptr;}// 返回找到的函数指针return lib-get_symbol(symbol_name);
}get_library()函数负责加载RMW实现库
std::shared_ptrrcpputils::SharedLibrary get_library()
{if (!g_rmw_lib) {// 懒加载策略 - 首次使用时加载g_rmw_lib load_library();}return g_rmw_lib;
}2.6 具体DDS实现
在rmw_publisher.cpp文件中调用rmw_fastrtps_cpp::create_publisher函数进行publisher创建。最终通过符号解析得到的函数指针指向特定DDS实现如FastDDS、CycloneDDS提供的具体rmw_create_publisher实现
// 在FastDDS中的实现示例
rmw_publisher_t *
rmw_fastrtps_cpp::create_publisher(const CustomParticipantInfo * participant_info,const rosidl_message_type_support_t * type_supports,const char * topic_name,const rmw_qos_profile_t * qos_policies,const rmw_publisher_options_t * publisher_options,bool keyed,bool create_publisher_listener)
{rmw_publisher_t * publisher rmw_fastrtps_cpp::create_publisher(participant_info,type_supports,topic_name,qos_policies,publisher_options,false,true);// 创建并返回rmw_publisher_t结构
}rmw_publisher_t *
rmw_fastrtps_cpp::create_publisher(const CustomParticipantInfo * participant_info,const rosidl_message_type_support_t * type_supports,const char * topic_name,const rmw_qos_profile_t * qos_policies,const rmw_publisher_options_t * publisher_options,bool keyed,bool create_publisher_listener)
{eprosima::fastdds::dds::Publisher * publisher participant_info-publisher_;...
}
3. 层级交互总结 #mermaid-svg-UIx8O6cfNvf5oZ7e {font-family:"trebuchet ms",verdana,arial,sans-serif;font-size:16px;fill:#333;}#mermaid-svg-UIx8O6cfNvf5oZ7e .error-icon{fill:#552222;}#mermaid-svg-UIx8O6cfNvf5oZ7e .error-text{fill:#552222;stroke:#552222;}#mermaid-svg-UIx8O6cfNvf5oZ7e .edge-thickness-normal{stroke-width:2px;}#mermaid-svg-UIx8O6cfNvf5oZ7e .edge-thickness-thick{stroke-width:3.5px;}#mermaid-svg-UIx8O6cfNvf5oZ7e .edge-pattern-solid{stroke-dasharray:0;}#mermaid-svg-UIx8O6cfNvf5oZ7e .edge-pattern-dashed{stroke-dasharray:3;}#mermaid-svg-UIx8O6cfNvf5oZ7e .edge-pattern-dotted{stroke-dasharray:2;}#mermaid-svg-UIx8O6cfNvf5oZ7e .marker{fill:#333333;stroke:#333333;}#mermaid-svg-UIx8O6cfNvf5oZ7e .marker.cross{stroke:#333333;}#mermaid-svg-UIx8O6cfNvf5oZ7e svg{font-family:"trebuchet ms",verdana,arial,sans-serif;font-size:16px;}#mermaid-svg-UIx8O6cfNvf5oZ7e .label{font-family:"trebuchet ms",verdana,arial,sans-serif;color:#333;}#mermaid-svg-UIx8O6cfNvf5oZ7e .cluster-label text{fill:#333;}#mermaid-svg-UIx8O6cfNvf5oZ7e .cluster-label span{color:#333;}#mermaid-svg-UIx8O6cfNvf5oZ7e .label text,#mermaid-svg-UIx8O6cfNvf5oZ7e span{fill:#333;color:#333;}#mermaid-svg-UIx8O6cfNvf5oZ7e .node rect,#mermaid-svg-UIx8O6cfNvf5oZ7e .node circle,#mermaid-svg-UIx8O6cfNvf5oZ7e .node ellipse,#mermaid-svg-UIx8O6cfNvf5oZ7e .node polygon,#mermaid-svg-UIx8O6cfNvf5oZ7e .node path{fill:#ECECFF;stroke:#9370DB;stroke-width:1px;}#mermaid-svg-UIx8O6cfNvf5oZ7e .node .label{text-align:center;}#mermaid-svg-UIx8O6cfNvf5oZ7e .node.clickable{cursor:pointer;}#mermaid-svg-UIx8O6cfNvf5oZ7e .arrowheadPath{fill:#333333;}#mermaid-svg-UIx8O6cfNvf5oZ7e .edgePath .path{stroke:#333333;stroke-width:2.0px;}#mermaid-svg-UIx8O6cfNvf5oZ7e .flowchart-link{stroke:#333333;fill:none;}#mermaid-svg-UIx8O6cfNvf5oZ7e .edgeLabel{background-color:#e8e8e8;text-align:center;}#mermaid-svg-UIx8O6cfNvf5oZ7e .edgeLabel rect{opacity:0.5;background-color:#e8e8e8;fill:#e8e8e8;}#mermaid-svg-UIx8O6cfNvf5oZ7e .cluster rect{fill:#ffffde;stroke:#aaaa33;stroke-width:1px;}#mermaid-svg-UIx8O6cfNvf5oZ7e .cluster text{fill:#333;}#mermaid-svg-UIx8O6cfNvf5oZ7e .cluster span{color:#333;}#mermaid-svg-UIx8O6cfNvf5oZ7e div.mermaidTooltip{position:absolute;text-align:center;max-width:200px;padding:2px;font-family:"trebuchet ms",verdana,arial,sans-serif;font-size:12px;background:hsl(80, 100%, 96.2745098039%);border:1px solid #aaaa33;border-radius:2px;pointer-events:none;z-index:100;}#mermaid-svg-UIx8O6cfNvf5oZ7e :root{--mermaid-font-family:"trebuchet ms",verdana,arial,sans-serif;}#mermaid-svg-UIx8O6cfNvf5oZ7e .userCode*{fill:#e1f5fe!important;stroke:#01579b!important;stroke-width:2px!important;}#mermaid-svg-UIx8O6cfNvf5oZ7e .userCode span{fill:#e1f5fe!important;stroke:#01579b!important;stroke-width:2px!important;}#mermaid-svg-UIx8O6cfNvf5oZ7e .clientLib*{fill:#e8f5e9!important;stroke:#2e7d32!important;stroke-width:2px!important;}#mermaid-svg-UIx8O6cfNvf5oZ7e .clientLib span{fill:#e8f5e9!important;stroke:#2e7d32!important;stroke-width:2px!important;}#mermaid-svg-UIx8O6cfNvf5oZ7e .middleware*{fill:#fff3e0!important;stroke:#e65100!important;stroke-width:2px!important;}#mermaid-svg-UIx8O6cfNvf5oZ7e .middleware span{fill:#fff3e0!important;stroke:#e65100!important;stroke-width:2px!important;}#mermaid-svg-UIx8O6cfNvf5oZ7e .implementation*{fill:#f3e5f5!important;stroke:#6a1b9a!important;stroke-width:2px!important;}#mermaid-svg-UIx8O6cfNvf5oZ7e .implementation span{fill:#f3e5f5!important;stroke:#6a1b9a!important;stroke-width:2px!important;}#mermaid-svg-UIx8O6cfNvf5oZ7e .ddsImpl*{fill:#ffebee!important;stroke:#b71c1c!important;stroke-width:2px!important;}#mermaid-svg-UIx8O6cfNvf5oZ7e .ddsImpl span{fill:#ffebee!important;stroke:#b71c1c!important;stroke-width:2px!important;}#mermaid-svg-UIx8O6cfNvf5oZ7e .shared*{fill:#ede7f6!important;stroke:#4527a0!important;stroke-width:2px!important;}#mermaid-svg-UIx8O6cfNvf5oZ7e .shared span{fill:#ede7f6!important;stroke:#4527a0!important;stroke-width:2px!important;} DDS层 实现层 动态加载层 中间件接口层 客户端库层 用户应用层 环境变量决定 环境变量决定 环境变量决定 环境变量决定 eProsima FastDDS Eclipse CycloneDDS RTI Connext DDS rmw_fastrtps_shared_cpp FastDDS共享实现 rmw_fastrtps_cpp FastDDS C实现 rmw_fastrtps_dynamic_cpp FastDDS动态类型支持 rmw_cyclonedds_cpp CycloneDDS实现 rmw_connextdds Connext DDS实现 rmw_implementation 动态加载机制 根据RMW_IMPLEMENTATION 环境变量加载具体实现 rmw 定义中间件接口API 所有RMW实现必须遵循的 接口规范 rmw_dds_common 通用DDS功能 为所有基于DDS的 RMW实现提供共享功能 rclcpp层 C客户端库 提供面向对象API 管理C对象生命周期 rcl层 C语言客户端库 处理资源分配 错误处理和参数验证 用户代码 node-create_publisher(...) rclcpp层提供面向对象的友好API管理C对象生命周期rcl层提供C语言API处理资源分配、错误处理和参数验证rmw层定义中间件抽象接口与具体DDS无关rmw_implementation层 使用环境变量决定加载哪个RMW实现通过动态符号查找机制(lookup_symbol)获取函数指针通过函数指针转发调用到实际DDS实现 具体DDS实现执行真正的DDS操作与网络通信
4. 实际应用示例
在ROS 2系统中可以通过环境变量轻松切换底层DDS实现
# 使用FastDDS (默认)
export RMW_IMPLEMENTATIONrmw_fastrtps_cpp# 使用CycloneDDS
export RMW_IMPLEMENTATIONrmw_cyclonedds_cpp# 使用Connext DDS
export RMW_IMPLEMENTATIONrmw_connextdds# 查看当前使用的RMW实现
ros2 doctor --report | grep middleware
