# Connect to Deployment via Paho C

This article mainly introduces how to use `Eclipse Paho C` in the C project, and implement the connection, subscription, messaging, unsubscribing and other functions between the client and MQTT broker.

Eclipse Paho Embedded C can be used on the desktop operating system, but mainly for embedded environments such as mbed, Arduino and FreeRTOS .

## Prerequisites

Before connecting your application to the EMQX Cloud using the Paho C client, ensure you have deployed an MQTT broker.

### Deploy MQTT Broker

<!-- @include: ./deploy_mqtt_broker.md -->

## Installation Dependency

```bash
sudo apt-get update
sudo apt-get -y install build-essential git cmake
```

### Building from source

The continuous integration builds can be found on Travis-CI for Linux and Mac, and AppVeyor for Windows.

#### Linux/Mac

```bash
git clone https://github.com/eclipse/paho.mqtt.c.git
cd org.eclipse.paho.mqtt.c.git
make
sudo make install
```

#### Windows

```bash
mkdir build.paho

cd build.paho

call "C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat" x64

cmake -G "NMake Makefiles" -DPAHO_WITH_SSL=TRUE -DPAHO_BUILD_DOCUMENTATION=FALSE -DPAHO_BUILD_SAMPLES=TRUE -DCMAKE_BUILD_TYPE=Release -DCMAKE_VERBOSE_MAKEFILE=TRUE ..

nmake
```

## Connect over TCP Protocol

This section introduces how to connect to the MQTT Broker over TCP port.

1. Include dependency library

   ```c
   #include "stdlib.h"
   #include "string.h"
   #include "unistd.h"
   #include "MQTTClient.h"
   ```

2. Define connection addresses, authentication information, and message publishing and receiving topic

   ```c
   // Enable or disable SSL/TLS connection (1 for SSL/TLS, 0 for TCP)
   #define USE_SSL 0
   
   #if USE_SSL
   #define ADDRESS "ssl://broker.emqx.io:8883"
   #else
   #define ADDRESS "tcp://broker.emqx.io:1883"
   #endif
   
   #define USERNAME    "emqx"
   #define PASSWORD    "public"
   #define CLIENTID    "c-client"
   #define QOS         0
   #define TOPIC       "emqx/c-test"
   #define TIMEOUT     10000L
   ```

3. Connect to broker

   ```c
   int rc;
   MQTTClient client;
   
   MQTTClient_create(&client, ADDRESS, CLIENTID, 0, NULL);
   MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;
   conn_opts.username = USERNAME;
   conn_opts.password = PASSWORD;
   
   if ((rc = MQTTClient_connect(client, &conn_opts)) != MQTTCLIENT_SUCCESS) {
       printf("Failed to connect, return code %d\n", rc);
       exit(-1);
   } else {
       printf("Connected to MQTT Broker!\n");
   }
   ```

## Connect over SSL/TLS Protocol
This section introduces how to connect to the MQTT Broker over SSL/TLS port.

1. Define connection addresses. Set `USE_SSL` to 1.

   ``` c
   // Enable or disable SSL/TLS connection (1 for SSL/TLS, 0 for TCP)
   #define USE_SSL 1
   ```

2. Connect to broker by SSL/TLS

   Set SSL Option by function `configureSSLOptions()`. If you need to configure the server certificate, you can load the certificate by `ssl_opts.trustStore`. If not, set `ssl_opts.enableServerCertAuth` to 0.

   ``` c
   MQTTClient_SSLOptions configureSSLOptions() {
       MQTTClient_SSLOptions ssl_opts = MQTTClient_SSLOptions_initializer;
       ssl_opts.enableServerCertAuth = 1;
       // CA_CERTIFICATE_FILE_Path specify server CA
       // ssl_opts.trustStore = CA_CERTIFICATE_FILE_Path;
       return ssl_opts;
   }
   
   ···
   
   #if USE_SSL
    MQTTClient_SSLOptions ssl_opts = configureSSLOptions();
    conn_opts.ssl = &ssl_opts;
   #endif
   ```

## Publish and Subscribe

This section introduces how to subscribe to topics and publish messages after you successfully connect to the MQTT broker.

### Subscribe to Topics

Set the topic for subscription and the [QoS Level](https://www.emqx.com/en/blog/introduction-to-mqtt-qos) of the topic.

``` c 
// subscribe topic
MQTTClient_subscribe(client, TOPIC, QOS);
```

### Unsubscribe to Topics

Use the following codes to unsubscribe to topics. You need to define the topic for unsubscription.

``` c 
MQTTClient_unsubscribe(client, TOPIC);
```

### Publish Messages

Inform MQTT Broker about the topic and payload when publishing messages.

```c
void publish(MQTTClient client, char *topic, char *payload) {
    MQTTClient_message message = MQTTClient_message_initializer;
    message.payload = payload;
    message.payloadlen = strlen(payload);
    message.qos = QOS;
    message.retained = 0;
    MQTTClient_deliveryToken token;
    MQTTClient_publishMessage(client, topic, &message, &token);
    MQTTClient_waitForCompletion(client, token, TIMEOUT);
    printf("Send `%s` to topic `%s` \n", payload, TOPIC);
}

// Call the `publish` function in a loop to finish publishing the message
char payload[16];
for (int i = 0; i < 100; i += 1) {
    // publish message to broker
    snprintf(payload, 16, "message-%d", i);
    publish(client, TOPIC, payload);
    sleep(1);
}
```

### Receive Messages

The following code specifies that the client listens for message events and executes a callback function after receiving a message, printing the received message and its topic to the console.

``` c 
MQTTClient_setCallbacks(client, NULL, NULL, on_message, NULL);
···
// Define the on_message callback function to print the content of the messages received by the subscribed topic
int on_message(void *context, char *topicName, int topicLen, MQTTClient_message *message) {
    char *payload = message->payload;
    printf("Received `%s` from `%s` topic \n", payload, topicName);
    MQTTClient_freeMessage(&message);
    MQTTClient_free(topicName);
    return 1;
}
```

### Disconnect from MQTT Broker
If the client wants to disconnect actively, use the following code:

``` c 
MQTTClient_disconnect(client, TIMEOUT);
MQTTClient_destroy(&client);
```

The above section only lists some key code, the complete code of the project can be found at: [mqtt-client-C-paho]https://github.com/emqx/MQTT-Client-Examples/tree/master/mqtt-client-C-paho), you can download and experience it.

### The full code

```c
#include "stdlib.h"
#include "string.h"
#include "unistd.h"
#include "MQTTClient.h"

// Enable or disable SSL/TLS connection (1 for SSL/TLS, 0 for TCP)
#define USE_SSL 1

#if USE_SSL
#define ADDRESS "ssl://broker.emqx.io:8883"
#else
#define ADDRESS "tcp://broker.emqx.io:1883"
#endif

#define USERNAME    "emqx"
#define PASSWORD    "public"
#define CLIENTID    "c-client"
#define QOS         0
#define TOPIC       "emqx/c-test"
#define TIMEOUT     10000L

MQTTClient_SSLOptions configureSSLOptions() {
    MQTTClient_SSLOptions ssl_opts = MQTTClient_SSLOptions_initializer;
    ssl_opts.enableServerCertAuth = 1;
    ssl_opts.trustStore = CA_CERTIFICATE_FILE;
    return ssl_opts;
}


void publish(MQTTClient client, char *topic, char *payload) {
    MQTTClient_message message = MQTTClient_message_initializer;
    message.payload = payload;
    message.payloadlen = strlen(payload);
    message.qos = QOS;
    message.retained = 0;
    MQTTClient_deliveryToken token;
    MQTTClient_publishMessage(client, topic, &message, &token);
    MQTTClient_waitForCompletion(client, token, TIMEOUT);
    printf("Send `%s` to topic `%s` \n", payload, TOPIC);
}

int on_message(void *context, char *topicName, int topicLen, MQTTClient_message *message) {
    char *payload = message->payload;
    printf("Received `%s` from `%s` topic \n", payload, topicName);
    MQTTClient_freeMessage(&message);
    MQTTClient_free(topicName);
    return 1;
}

int main(int argc, char *argv[]) {
    int rc;
    MQTTClient client;

    MQTTClient_create(&client, ADDRESS, CLIENTID, 0, NULL);
    MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;
    conn_opts.username = USERNAME;
    conn_opts.password = PASSWORD;

#if USE_SSL
    MQTTClient_SSLOptions ssl_opts = configureSSLOptions();
    conn_opts.ssl = &ssl_opts;
#endif

    MQTTClient_setCallbacks(client, NULL, NULL, on_message, NULL);
    if ((rc = MQTTClient_connect(client, &conn_opts)) != MQTTCLIENT_SUCCESS) {
        printf("Failed to connect, return code %d\n", rc);
        exit(-1);
    } else {
        printf("Connected to MQTT Broker!\n");
    }
    // subscribe topic
    MQTTClient_subscribe(client, TOPIC, QOS);
    
    char payload[16];
    for (int i = 0; i < 10; i += 1) {
        // publish message to broker
        snprintf(payload, 16, "message-%d", i);
        publish(client, TOPIC, payload);
        sleep(1);
    }
    MQTTClient_unsubscribe(client, TOPIC);
    MQTTClient_disconnect(client, TIMEOUT);
    MQTTClient_destroy(&client);
    return rc;
}
```

## Test

1. Write the CMakeLists.txt file
   ```
   cmake_minimum_required(VERSION 3.17)
   find_package(eclipse-paho-mqtt-c 1.3.9 REQUIRED)
   project(mqtt_c C)
   include_directories(/usr/local/include)
   link_directories(/usr/local/lib)
   set(CMAKE_C_STANDARD 99)
   add_executable(mqtt_c main.c)
   target_link_libraries(mqtt_c paho-mqtt3c)
   ```
2. Compile and run code
   ![c_mqtt](./_assets/c_mqtt.png)
   
## More

In summary, we have implemented the creation of an MQTT connection in a C project, simulated subscribing, sending and receiving messages, unsubscribing, and disconnecting between the client and MQTT broker. You can download the source code of the example [here](https://github.com/emqx/MQTT-Client-Examples/tree/master/mqtt-client-c), and you can also find more demo examples in other languages on [GitHub](https://github.com/emqx/MQTT-Client-Examples).