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icedelivery🔗

Introduction🔗

This example showcases a data transmission setup with zero-copy inter-process communication (IPC) on iceoryx. It provides publisher and subscriber applications. They come in two C++ API flavors (untyped and typed).

Expected Output🔗

asciicast

Code walkthrough🔗

This example makes use of two kinds of API flavors. With the untyped API, you have the most flexibility. It enables you to define higher level APIs with a different look and feel on top of iceoryx, e.g. the ara::com API of AUTOSAR Adaptive or the ROS 2 API. It is not meant to be used by developers in daily life, the assumption is that there will always be a higher abstraction. A simple example how such an abstraction could look like is given in the second step with the typed example. The typed API provides type safety combined with RAII.

Publisher application (untyped)🔗

First off, let's include the publisher, the runtime and the signal handler:

#include "iceoryx_hoofs/posix_wrapper/signal_watcher.hpp"
#include "iceoryx_posh/popo/untyped_publisher.hpp"
#include "iceoryx_posh/runtime/posh_runtime.hpp"

You might be wondering what the publisher application is sending? It's this struct.

struct RadarObject
{
    RadarObject() noexcept
    {
    }
    RadarObject(double x, double y, double z) noexcept
        : x(x)
        , y(y)
        , z(z)
    {
    }
    double x = 0.0;
    double y = 0.0;
    double z = 0.0;
};

It is included by:

#include "topic_data.hpp"

For the communication with RouDi a runtime object is created. The parameter of the method initRuntime() contains a unique string identifier for this publisher.

constexpr char APP_NAME[] = "iox-cpp-publisher-untyped";
iox::runtime::PoshRuntime::initRuntime(APP_NAME);

Now that RouDi knows our publisher application is existing, let's create a publisher instance for sending our charming struct to everyone:

iox::popo::UntypedPublisher publisher({"Radar", "FrontLeft", "Object"});

Now comes the work mode. Data needs to be created. But hang on.. we need memory first! Let's reserve a memory chunk which fits our RadarObject struct.

publisher.loan(sizeof(RadarObject))
    .and_then([&](auto& userPayload) {
        // ...
    })
    .or_else([&](auto& error) {
        // ...
    });

The call to loan() returns a cxx::expected. By concatenating and_then and or_else branches are implicitly taken and your code becomes less error-prone compared to using if() { .. } else { .. }. Well, it's a bit of a lambda jungle. Read it like a story in a book: "Loan memory and then if it succeeds, fill it with some data or else if it fails, handle the error".

Remember, the untyped API will always be bare-metal!

Hence, the RadarObject needs to be constructed with a placement new:

RadarObject* data = new (userPayload) RadarObject(ct, ct, ct);

Then, we can write some values:

data->x = ct;
data->y = ct;
data->z = ct;

Finally, the value is made available to any subscriber with

publisher.publish(userPayload);

Incrementing the counter and sending the data happens in a loop every second until the user presses Ctrl-C. It is captured with the signal handler and stops the loop.

Subscriber application (untyped)🔗

How can the subscriber application receive the data the publisher application just transmitted?

Similar to the publisher, we include the topic data, the subscriber, the runtime as well as the signal handler header:

#include "topic_data.hpp"

#include "iceoryx_hoofs/posix_wrapper/signal_watcher.hpp"
#include "iceoryx_posh/popo/untyped_subscriber.hpp"
#include "iceoryx_posh/runtime/posh_runtime.hpp"

To make RouDi aware of the subscriber a runtime object is created, once again with a unique identifier string:

constexpr char APP_NAME[] = "iox-cpp-subscriber-untyped";
iox::runtime::PoshRuntime::initRuntime(APP_NAME);

In the next step a subscriber object is created, matching exactly the capro::ServiceDescription that the publisher offered:

iox::popo::UntypedSubscriber subscriber({"Radar", "FrontLeft", "Object"});

Again in a while-loop we do the following:

while (!iox::posix::hasTerminationRequested())
{
    subscriber
        .take()
        .and_then([&](const void* userPayload) {
            // ...
        })
        .or_else([](auto& result) {
            if (result != iox::popo::ChunkReceiveResult::NO_CHUNK_AVAILABLE)
            {
                std::cout << "Error receiving chunk." << std::endl;
            }
        });

    std::this_thread::sleep_for(std::chrono::milliseconds(100));
}

The program execution is stopped when the user presses Ctrl-C.

Let's translate it into a story again: "Take the data and then if this succeeds, work with the sample, or else if an error other than NO_CHUNK_AVAILABLE occurred, print the string 'Error receiving chunk.'". Of course, you don't need to take care of all cases, but we advise doing so.

In the and_then case the content of the sample is printed to the command line:

auto object = static_cast<const RadarObject*>(userPayload);
std::cout << APP_NAME << " got value: " << object->x << std::endl;

Please note the static_cast before reading out the data. It is necessary because the untyped subscriber is unaware of the type of the transmitted data.

After accessing the value, the chunk of memory needs to be explicitly released by calling:

// note that we explicitly have to release the sample
// and afterwards the pointer access is undefined behavior
subscriber.release(userPayload);

The subscriber runs 10x times faster than the publisher, to make sure that all data samples are received.

Publisher application (typed)🔗

The typed publisher application is an example for a high-level user API and does the same thing as the untyped publisher described before. In this summary, just the differences to the prior publisher application are described.

Starting again with the includes, there is now a different one:

#include "iceoryx_posh/popo/publisher.hpp"

When it comes to the runtime, things are the same as in the untyped publisher. However, a typed publisher object is created and the transmitted data type is provided as template parameter:

iox::popo::Publisher<RadarObject> publisher({"Radar", "FrontLeft", "Object"});

A similar while-loop is used to send the data to the subscriber. In contrast to the untyped publisher the typed one offers three additional possibilities.

#1 Loan and publish🔗

Usage #1 default constructs the data type in-place:

//  * Retrieve a typed sample from shared memory.
//  * Sample can be held until ready to publish.
//  * Data is default constructed during loan
publisher.loan()
    .and_then([&](auto& sample) {
        sample->x = sampleValue1;
        sample->y = sampleValue1;
        sample->z = sampleValue1;
        sample.publish();
    })
    .or_else([](auto& error) {
        // Do something with error
        std::cerr << "Unable to loan sample, error: " << error << std::endl;
    });

#2 Loan, construct in-place and publish🔗

Usage #2 constructs the data type with the values provided in loan:

//  * Retrieve a typed sample from shared memory and construct data in-place
//  * Sample can be held until ready to publish.
//  * Data is constructed with the arguments provided.
publisher.loan(sampleValue2, sampleValue2, sampleValue2)
    .and_then([](auto& sample) { sample.publish(); })
    .or_else([](auto& error) {
        // Do something with error
        std::cerr << "Unable to loan sample, error: " << error << std::endl;
    });

One might wonder what the type of the variable sample is? It is iox::popo::Sample<RadarObject>. This class behaves similar to a std::unique_ptr and makes sure that the ownership handling is done automatically and memory is freed when going out of scope on subscriber side.

#3 Publish by copy🔗

Usage #3 does a copy-and-publish in one call. This should only be used for small data types, as otherwise copies can lead to a larger runtime.

//  * Basic copy-and-publish. Useful for smaller data types.
auto object = RadarObject(sampleValue3, sampleValue3, sampleValue3);
publisher.publishCopyOf(object).or_else([](auto& error) {
    // Do something with error.
    std::cerr << "Unable to publishCopyOf, error: " << error << std::endl;
});

#4 Publish the result of a computation🔗

Usage #4 can be useful if you have a callable, e.g. a function or functor that should always be called. The callable needs to have the signature void(SampleType*, ...). The semantics are as follows: The publisher loans a sample from shared memory and if loaning was successful the callable is called with a pointer to the SampleType as first argument. If loaning was unsuccessful, the callable is not called, but instead the or_else branch is taken.

//  * Provide a callable that will be used to populate the loaned sample.
//  * The first argument of the callable must be T* and is the location that the callable should
//      write its result to.
publisher.publishResultOf(getRadarObject, ct).or_else([](auto& error) {
    // Do something with error.
    std::cerr << "Unable to publishResultOf, error: " << error << std::endl;
});
publisher
    .publishResultOf([&sampleValue4](RadarObject* object) {
        *object = RadarObject(sampleValue4, sampleValue4, sampleValue4);
    })
    .or_else([](auto& error) {
        // Do something with error.
        std::cerr << "Unable to publishResultOf, error: " << error << std::endl;
    });

Subscriber application (typed)🔗

As with the typed publisher application there is a different include compared to the untyped subscriber:

#include "iceoryx_posh/popo/subscriber.hpp"

An instance of Subscriber is created:

iox::popo::Subscriber<RadarObject> subscriber({"Radar", "FrontLeft", "Object"});

Everything else is nearly the same. However, there is one crucial difference which makes the Subscriber typed.

Compare this line from the UntypedSubscriber

.and_then([&](const void* userPayload) {
    // ...
})

with

.and_then([](auto& sample) {
    // ...
})

In case of the typed Subscriber, auto is deduced to iox::popo::Sample<const RadarObject>. With the UntypedSubscriber the parameter is const void* as no type information is available.

Check out icedelivery on GitHub