Example of sdbusplus in Action

Explore practical examples and code to master sdbusplus usage

linux desktop application d bus interface

Highlights

Modern C++ Bindings: sdbusplus offers a strong, type-safe interface for interacting with D-Bus.
Comprehensive Examples: Learn by exploring examples ranging from basic service setup to asynchronous operations.
Integration with async frameworks: Utilize asynchronous programming, C++ coroutines, and Boost.Asio for non-blocking operations.

Understanding sdbusplus

sdbusplus is designed to bridge the gap between traditional D-Bus IPC mechanisms and modern C++ programming paradigms. Built on top of the sd-bus library from systemd, it provides a clean and intuitive C++ interface for interacting with the Linux D-Bus system. With a focus on type safety, memory management, and asynchronous programming models, sdbusplus caters to developers building robust and high-performance applications.

The library not only wraps the lower-level sd-bus API but also introduces additional features such as asynchronous method calls, easy property management, and signal handling. These capabilities are instrumental when developing applications that need to handle real-time inter-process communication, especially in environments like OpenBMC.

Core Examples and Use Cases

1. Basic D-Bus Service Setup

One common use of sdbusplus is to set up a basic D-Bus service. In this scenario, the service registers a name on the bus and starts processing incoming D-Bus messages. The example below demonstrates how to create a D-Bus service that listens for events.

Service Setup Example Code


// Include the necessary header for the D-Bus server
#include "server.hpp"

int main()
{
    // Create a new bus connection (typically the system bus)
    auto bus = sdbusplus::bus::new_default();
    
    // Request a well-known service name on the bus
    bus.request_name("org.example.Service");
    
    // Instantiate the D-Bus service object at the provided object path
    auto obj = std::make_shared<org::example::Interface::server::Service>(bus, "/org/example/Service");
    
    // Main loop to process incoming signals and method calls
    while (true)
    {
        bus.process();
        bus.flush();
    }
    
    return 0;
}

In this example, a default bus connection is created and a service name "org.example.Service" is requested. The service object is then bound to a specific object path ("/org/example/Service") and enters a loop that continuously processes D-Bus message events.

2. Signal Handling

Handling signals is a critical part of D-Bus communication. sdbusplus provides an elegant way to listen for and respond to signals emitted by other services. Signal handling helps applications dynamically react to changes in the system.

Signal Listening Example Code


#include <sdbusplus/bus/match.hpp>

// Callback function that processes the received signal
void signalHandler(sdbusplus::message::message& msg)
{
    // Process the signal message
    // Typically, msg can be parsed to retrieve necessary information
}

int main()
{
    // Establish a bus connection
    auto bus = sdbusplus::bus::new_default();
    
    // Construct a rule to match signals based on type, interface, and the member (signal name)
    std::string matchRule = "type='signal',interface='org.example.Interface',member='SignalName'";
    
    // Create a match object and register the signal handler
    auto match = std::make_unique<sdbusplus::bus::match::match>(bus, matchRule, signalHandler);
    
    // Process incoming messages in a continuous loop
    while (true)
    {
        bus.process();
        bus.flush();
    }
    
    return 0;
}

This example sets up a match rule to catch signals with a specific signature from the interface 'org.example.Interface'. The provided handler function, signalHandler, is invoked whenever such a signal is received, affording the developer the ability to perform run-time operations based on system events.

3. Asynchronous Operations and Coroutines

Modern C++ encourages non-blocking, asynchronous operations for efficient resource utilization and improved performance. sdbusplus supports asynchronous D-Bus method calls using C++ coroutines, which provide a means to write asynchronous code in a synchronous style.

Coroutine-Based Async Example


#include <coroutine>
#include <future>
#include <sdbusplus/bus.hpp>

// An asynchronous function using C++ coroutines to perform method calls
std::future<void> asyncMethod(sdbusplus::bus::bus& bus)
{
    // Await an asynchronous D-Bus method call
    co_await bus.async_call_method("org.example.Service", "/org/example/Service", "org.example.Interface", "MethodName");
    
    // After the call completes, further processing can occur here
}

int main()
{
    // Create a bus connection to the default context (usually system bus)
    sdbusplus::bus::bus bus = sdbusplus::bus::new_default();
    
    // Start the asynchronous method call
    asyncMethod(bus);
    
    // Continue processing D-Bus events
    while (true)
    {
        bus.process();
        bus.flush();
    }
    
    return 0;
}

In this snippet, the coroutine asyncMethod asynchronously calls a method on the D-Bus service identified by "org.example.Service". The usage of co_await signifies that the function will suspend until the call completes, enabling non-blocking asynchronous operations.

4. Integration with Boost.Asio for Advanced Asynchronous Handling

For developers who require a more robust asynchronous framework, sdbusplus can be integrated with Boost.Asio. This enables you to use sophisticated event handling and asynchronous I/O operations in tandem with D-Bus communications.

Asio Integration Example Code


#include <boost/asio.hpp>
#include <sdbusplus/asio/connection.hpp>

// Function demonstrating asynchronous method call invocation using Boost.Asio
void do_start_async_method_call_one(
    std::shared_ptr<sdbusplus::asio::connection> conn,
    boost::asio::yield_context yield)
{
    boost::system::error_code ec;
    // Example of a variant data type call (assuming a proper type conversion)
    auto testValue = conn->yield_method_call<int>(
        yield[ec], 
        "xyz.openbmc_project.asio-test", 
        "/xyz/openbmc_project/test",
        "org.freedesktop.DBus.Properties", 
        "Set", 
        "xyz.openbmc_project.test", 
        "int", 
        24
    );

    // Retrieve the property as a test to ensure the set was successful.
    auto propertyValue = conn->yield_method_call<int>(
        yield[ec],
        "xyz.openbmc_project.asio-test",
        "/xyz/openbmc_project/test",
        "org.freedesktop.DBus.Properties",
        "Get",
        "xyz.openbmc_project.test",
        "int"
    );

    if (!ec && propertyValue == 24)
    {
        std::cout << "Asynchronous call via Boost.Asio successful!" << std::endl;
    }
}

int main()
{
    boost::asio::io_context io;
    
    // Create a sdbusplus connection that works with Boost.Asio
    auto conn = std::make_shared<sdbusplus::asio::connection>(io);
    
    // Start an asynchronous operation using a coroutine
    boost::asio::spawn(io, [&](boost::asio::yield_context yield)
    {
        do_start_async_method_call_one(conn, yield);
    });
    
    io.run();
    
    return 0;
}

This code demonstrates how to blend sdbusplus with Boost.Asio to perform asynchronous D-Bus property operations. The Boost.Asio coroutine mechanism (using yield_context) ensures that the asynchronous logic is handled efficiently without blocking other I/O operations.

Practical Table of sdbusplus Examples

Below is a comprehensive table summarizing various sdbusplus examples along with key details, usage scenarios, and corresponding code references:

Example Type	Description	Key Operation	Reference Code/File
Basic Service Setup	Registers a service and listens on a D-Bus object path	bus.request_name(), bus.process()	server.hpp example
Signal Handling	Listens to D-Bus signals and processes events	bus::match, signalHandler()	Signal match example
Coroutine-based Async Operations	Performs asynchronous method calls using C++ coroutines	co_await, async_call_method()	coroutine-example.cpp
Boost.Asio Integration	Matches asynchronous D-Bus calls with Boost.Asio event loops	boost::asio::spawn, yield_method_call()	asio-example.cpp
Interface Generation	Automatically generates D-Bus interface bindings from YAML definitions	sdbus++ tool usage	Documentation and YAML examples

Additional Examples and Advanced Techniques

Beyond basic examples, sdbusplus provides advanced functionalities aimed at complex D-Bus interactions. Developers often combine property handling, method invocations, and signal handling in a single application. Here are additional ideas to elevate your sdbusplus applications:

Property Management

sdbusplus simplifies the registration and management of properties on D-Bus objects. This is particularly useful for exposing application state. Examples found in the library documentation, such as "register-property.cpp", demonstrate how to programmatically define properties along with their access and change notifications.

Property Registration Example


#include <sdbusplus/bus.hpp>
#include <sdbusplus/server.hpp>

int main()
{
    auto bus = sdbusplus::bus::new_default();
    bus.request_name("org.example.PropertyService");

    // Example: Create a property server object and register properties
    auto service_obj = std::make_shared<org::example::Interface::server::Service>(
        bus, "/org/example/PropertyService"
    );

    // The service's API would define how properties are read or updated
    while (true)
    {
        bus.process();
        bus.flush();
    }
    return 0;
}

This snippet illustrates the process of setting up a property server. The approach aids in clearly isolating logic related to properties, making maintenance and debugging simpler.

Using sdbus++ for Interface Automation

The companion tool, sdbus++, allows developers to generate C++ bindings directly from YAML interface definitions. This procedural workflow minimizes boilerplate code and maintains consistency across different D-Bus services.

Developers define interfaces in YAML format which are then converted into C++ classes. These auto-generated bindings facilitate method calls, property access, and signal processing, ensuring that your code adheres to the defined D-Bus specification with minimal manual errors.