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Listener in C (or how to use callbacks with iceoryx)🔗

Thread Safety🔗

The Listener is thread-safe and can be used without restrictions. But be aware that all provided callbacks are executed concurrently in the background thread of the Listener. If you access structures inside this callback you have to either ensure that you are the only one accessing it or that it is accessed with a guard like a mutex.


For a general introduction into the Listener concept please take a look at the first part of the Listener C++ example and at the Glossary of the WaitSet C++ example.

Expected Output🔗


Code Walkthrough🔗


Please be aware about the thread-safety restrictions of the Listener and read the Thread Safety chapter carefully.

The C version of the callbacks example performs the identical tasks as the C++ version. We have again an application which offers two services called Radar.FrontLeft.Counter and Radar.FrontRight.Counter. Every time we have received a sample from each service we calculate the sum of it.


The publisher contains only already known iceoryx features. If some of them are not known to you please take a look at the icedelivery in C example.


int main()🔗

The subscriber starts as usual by registering the process at the runtime. In the next step, we set up some listenerStorage and initialize the listener which will start a background thread for the upcoming event-triggered callbacks.

iox_listener_storage_t listenerStorage;
iox_listener_t listener = iox_listener_init(&listenerStorage);

Besides the subscribers we also would like to have an event that will be triggered by our self - the heartbeat.

iox_user_trigger_storage_t heartbeatStorage;
heartbeat = iox_user_trigger_init(&heartbeatStorage);

Both subscribers use the same options which we set up with:

iox_sub_options_t options;
options.historyRequest = 10U;
options.queueCapacity = 50U;
options.nodeName = "iox-c-callback-subscriber-node";

and then we can construct the two subscribers subscriberLeft and subscriberRight.

iox_sub_t subscriberLeft = iox_sub_init(&subscriberLeftStorage, "Radar", "FrontLeft", "Counter", &options);
iox_sub_t subscriberRight = iox_sub_init(&subscriberRightStorage, "Radar", "FrontRight", "Counter", &options);

Now that everything is initialized, we start our heartbeatTriggerThread which triggers our heartbeat every 4 seconds.

pthread_t heartbeatTriggerThread;
if (pthread_create(&heartbeatTriggerThread, NULL, cyclicHeartbeatTrigger, NULL))
    printf("failed to create thread\n");
    return -1;

Attaching the subscribers and the heartbeat allows the Listener to call the callbacks whenever the event is signaled by the EventOrigin.

// from here on the callbacks are called when an event occurs
iox_listener_attach_user_trigger_event(listener, heartbeat, &heartbeatCallback);
    listener, subscriberLeft, SubscriberEvent_DATA_RECEIVED, &onSampleReceivedCallback);
    listener, subscriberRight, SubscriberEvent_DATA_RECEIVED, &onSampleReceivedCallback);

A user trigger can emit only one event therefore we do not provide the event type as an argument in the user trigger attach call.

Since we are following a push-based approach, i.e. without an event loop that is pulling the events and processing them, we require a blocking call that waits until the process is signaled to terminate.

while (keepRunning)

When keepRunning is set to false, we clean up all the resources. First, we detach the events from the Listener. This is an optional step since the Listener detaches all events by itself when it is deinitialized. This applies also for all the EventOrigins, if you for instance deinitialize an attached subscriber it will automatically detach itself from the Listener.

iox_listener_detach_user_trigger_event(listener, heartbeat);
iox_listener_detach_subscriber_event(listener, subscriberLeft, SubscriberEvent_DATA_RECEIVED);
iox_listener_detach_subscriber_event(listener, subscriberRight, SubscriberEvent_DATA_RECEIVED);

In a last step we have to release all acquired resources


The callbacks🔗

Every callback must have a signature like void (iox_event_origin_t). Our heartbeatCallback just prints the message heartbeat received onto the console.

void heartbeatCallback(iox_user_trigger_t userTrigger)
    printf("heartbeat received\n");

The onSampleReceivedCallback is a little bit more complex. First we acquire the chunk and then we have to find out which subscriber received the chunk. For that we acquire the service description of the subscriber and if its instance equals FrontLeft we store the chunk value in the leftCache otherwise in the rightCache.

void onSampleReceivedCallback(iox_sub_t subscriber)
    const struct CounterTopic* userPayload;
    if (iox_sub_take_chunk(subscriber, (const void**)&userPayload) == ChunkReceiveResult_SUCCESS)
        iox_service_description_t serviceDescription = iox_sub_get_service_description(subscriber);
        if (strcmp(serviceDescription.instanceString, "FrontLeft") == 0)
            leftCache.value = *userPayload;
            leftCache.isSet = true;
        else if (strcmp(serviceDescription.instanceString, "FrontRight") == 0)
            rightCache.value = *userPayload;
            rightCache.isSet = true;
        printf("received: %d\n", userPayload->counter);
    // ...

If both caches are set, we can calculate the sum of both chunks and print them to the console. To start fresh in the next cycle, we reset the leftCache and the rightCache afterward.

void onSampleReceivedCallback(iox_sub_t subscriber)
    // ...
    if (leftCache.isSet && rightCache.isSet)
        printf("Received samples from FrontLeft and FrontRight. Sum of %d + %d = %d\n",
               leftCache.value.counter + rightCache.value.counter);
        leftCache.isSet = false;
        rightCache.isSet = false;

Additional context data for callbacks (ice_c_callbacks_with_context_data.c)🔗

Sometimes we would like to modify data structures which are not globally available within the callback. To facilitate this we provide the functions called iox_listener_attach_***_event_with_context_data which allow to provide an additional void pointer for the callback as second argument.

The following example is a simplified version of the ice_c_callbacks_subscriber.c example where we removed the cyclic heartbeat trigger. The key difference is that we have a local variable called counterService in which we store the leftCache and rightCache and we let the callback update that variable directly.

CounterService counterService;
counterService.leftCache.isSet = false;
counterService.rightCache.isSet = false;

The callback takes an additional void pointer argument which we cast then to our CounterService to perform the same tasks as in the previous example but now on CounterService * self.

void onSampleReceivedCallback(iox_sub_t subscriber, void* contextData)
    if (contextData == NULL)
        fprintf(stderr, "aborting onSampleReceivedCallback since contextData is a null pointer\n");

    CounterService* self = (CounterService*)contextData;
    // ...

Finally, we have to attach both subscribers and provide the pointer to counterService as additional argument so that we can access it in the callback.


The user has to ensure that the contextData (&counterService) in iox_listener_attach_subscriber_event_with_context_data lives as long as the attachment with its callback is attached, otherwise the callback context data pointer is dangling.

    listener, subscriberLeft, SubscriberEvent_DATA_RECEIVED, &onSampleReceivedCallback, &counterService);
    listener, subscriberRight, SubscriberEvent_DATA_RECEIVED, &onSampleReceivedCallback, &counterService);

Check out callbacks_in_c on GitHub