mqtt-example-c

CMakeLists.txt

cmake_minimum_required(VERSION 3.10)

project(run C)

add_executable(run mqtt_example.c)
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -pthread")

target_link_libraries(run 
    mosquitto
)

mqtt_example.c

#include <signal.h>
#include <stdio.h>
#include <stdint.h>
#include <string.h>
#include <pthread.h>
#include <string.h>
#include <unistd.h>

#include <mosquitto.h>

#define mqtt_host "localhost"
#define mqtt_port 1883

static int run = 1;
pthread_t g_publish;

void handle_signal(int s)
{
    run = 0;
}

void connect_callback(struct mosquitto *mosq, void *obj, int result)
{
    printf("connect callback, rc=%d\n", result);
}

void message_callback(struct mosquitto *mosq, void *obj, const struct mosquitto_message *message)
{
    bool match = false;
    printf("'%s'(%d): '%.*s'\n", message->topic, message->payloadlen, message->payloadlen, (char *)message->payload);

    mosquitto_topic_matches_sub("test-match", message->topic, &match);
    if (match)
    {
        printf("got message for test-match topic\n");
    }
}

void *run_publish_topic(void *arg)
{
    char message[255];
    int counter = 0;
    while (true)
    {
        sprintf(message, "こんにちは: %d", counter);
        int message_len = strlen(message);
        mosquitto_publish(arg, NULL, "test1", message_len, message, 0, false);
        counter++;
        sleep(1);
    }
}

int main(int argc, char *argv[])
{
    bool reconnect = true;
    struct mosquitto *mosq;
    int rc = 0;

    signal(SIGINT, handle_signal);
    signal(SIGTERM, handle_signal);

    mosquitto_lib_init();

    mosq = mosquitto_new(NULL, true, 0);

    if (mosq)
    {
        mosquitto_connect_callback_set(mosq, connect_callback);
        mosquitto_message_callback_set(mosq, message_callback);

        rc = mosquitto_connect(mosq, mqtt_host, mqtt_port, 60);

    label_run:
        mosquitto_subscribe(mosq, NULL, "test1", 0);
        mosquitto_subscribe(mosq, NULL, "test2", 0);
        mosquitto_subscribe(mosq, NULL, "test-match", 0);

        pthread_create(&g_publish, NULL, run_publish_topic, mosq);

        while (run)
        {
            rc = mosquitto_loop(mosq, -1, 1);
            if (run && rc)
            {
                printf("connection error!\n");
                pthread_cancel(g_publish);
                sleep(4);
                if (reconnect)
                {
                    mosquitto_reconnect(mosq);
                    goto label_run;
                }
                else
                {
                    break;
                }
            }
        }
        mosquitto_destroy(mosq);
    }

    mosquitto_lib_cleanup();

    return rc;
}

// reference: 
// https://gist.github.com/evgeny-boger/8cefa502779f98efaf24
// https://github.com/Johannes4Linux/libmosquitto_examples

Build

gcc mqtt_example.c  -o run -lmosquitto -lpthread

Build with CMAKE

mkdir build
cd build
cmake ..
make

Publish from bash

 q=0; while true; do mosquitto_pub -h 127.0.0.1 -t "test2" -m "Hello, world $q" -p 1883; echo ${q}; ((q++)); sleep 0.5; done