icediscovery_in_c

icediscovery in C

Introduction

This example demonstrates how to search for specific services using iceoryx's service discovery. It provides two applications - one offering different services and one searching for those with different search queries. The behavior and structure is quite similar to the icediscovery C++ example.

Code walkthrough

Offer services

We create several publishers which offer their services on construction by default. For more dynamism the cameraPublishers offer/stop their services periodically. If you want more information on how to create publishers, have a look at the icedelivery C example.

Find services

To be able to search for services, we need to include:

#include "iceoryx_binding_c/service_discovery.h"

We create some storage for the service discovery and initialize it.

iox_service_discovery_storage_t storage;
iox_service_discovery_t serviceDiscovery = iox_service_discovery_init(&storage);

We can now call three different find service functions. Let's start with

iox_service_discovery_find_service_apply_callable(
    serviceDiscovery, "Radar", "FrontLeft", "Objects", printSearchResult, MessagingPattern_PUB_SUB);

which searches for all {Radar, FrontLeft, Objects} services offered by publishers and applies the provided function printSearchResult on each of them. The function must have the signature void(const iox_service_description_t). Here we pass a function that prints the found services on the console:

void printSearchResult(const iox_service_description_t service)
{
    printf(
        "- Service: %s, Instance: %s, Event: %s\n", service.serviceString, service.instanceString, service.eventString);
}

We cannot only search for exact matching services. In combination with wildcards (NULL) we can also search for all instances and events with the service Radar:

iox_service_discovery_find_service_apply_callable(
    serviceDiscovery, "Radar", NULL, NULL, printSearchResult, MessagingPattern_PUB_SUB);

The wildcard can be used for all strings which describe a service, i.e. service, instance and event.

Let's now search for all Camera services and store the results in an array:

numberFoundServices = iox_service_discovery_find_service(serviceDiscovery,
                                                         "Camera",
                                                         NULL,
                                                         NULL,
                                                         searchResult,
                                                         SEARCH_RESULT_CAPACITY,
                                                         &missedServices,
                                                         MessagingPattern_PUB_SUB);

searchResult is a iox_service_description_t array of size SEARCH_RESULT_CAPACITY. The matching services are written into this array, up to a maximum of SEARCH_RESULT_CAPACITY. If the number of found services exceeds the array's capacity, the number of services that could not be stored is written to missedServices. The number of stored services is returned. Since the cameraPublishers periodically offer/stop their services, you should see sometimes 5 Camera services and sometimes none.

Finally, let's try out the third find service function. We search again for all Camera services but additionally count the front camera services:

iox_service_discovery_find_service_apply_callable_with_context_data(
    serviceDiscovery, "Camera", NULL, NULL, searchFrontDevices, &numberFrontCameras, MessagingPattern_PUB_SUB);

This function is quite similar to the first find service function, but we pass an additional argument numberFrontCameras:

uint32_t numberFrontCameras = 0U;

We use this variable to store the number of front cameras and provide it as second argument count to the function searchFrontDevices which is applied to all found services:

void searchFrontDevices(const iox_service_description_t service, void* count)
{
    if (strncmp(service.instanceString, "FrontLeft", IOX_CONFIG_SERVICE_STRING_SIZE) == 0
        || strncmp(service.instanceString, "FrontRight", IOX_CONFIG_SERVICE_STRING_SIZE) == 0)
    {
        ++*(uint32_t*)count;
    }
}

[Check out icediscovery on GitHub :fontawesome-brands-github:](https://github.com/eclipse-iceoryx/iceoryx/tree/master/iceoryx_examples/icediscovery_in_c){ .md-button }