MQTT Client Library Encyclopedia – Paho MQTT C Client

Guest post by Ian Craggs

Paho MQTT C Client
API-StyleBlocking and non-blocking
LicenseEPL and EDL


The Paho C client libraries started life back in 2007, when I first started writing a small MQTT server, RSMB (Really Small Message Broker). I thought I would reuse as much of the internal code of RSMB as I could, to save myself some time. As it turned out, I probably didn’t save as much time as I expected, because in some ways writing a client library for MQTT is more complex than writing a server. RSMB is single-threaded, which I mainly retained in the design of these client libraries, for better or worse.

I started writing RSMB in C++, but the template support in gcc at the time did not work well, so I switched to standard ANSI C. That’s why the client libraries are C too. It seemed to me at the time that Linux was likely to take over the world for embedded systems, with Windows CE maybe getting a look in. That’s why I wrote these libraries with Linux (and some other forms of Unix) and Windows in mind, rather than being any more portable. And that’s why the Paho embedded client libraries now exist – for all those other embedded operating systems that are now popular!

In 2008, there weren’t too many MQTT client libraries to learn from, so for the first steps I followed the IBM Java client of the time. I thought it was better to copy an existing model to make it easier for the MQTT application programmer to transfer knowledge from one to the other. As a result the synchronous client was born, just called MQTTClient. This has the following design points:

  1. threading: no background thread, or just one, regardless of how many connections are created
  2. blocking MQTT calls, to simplify application programming
  3. an inflight window for QoS 1 and 2 messages of 1 or 10 only, to limit the amount of damage that can be inflicted on a server
  4. internal tracing and memory tracking, for serviceability and elimination of memory leaks


MQTT 3.1
MQTT 3.1.1ok
Automatic Reconnectnok
Disk Persistenceok
QoS 0ok
QoS 1ok
QoS 2ok
Offline Message Bufferingnok



On Linux (or Unix) the simplest method is to clone the repo and install:

Pre-built libraries for MacOS and Windows are available on the Paho downloads page:

Unzip to a location of your choice.

First we have to include the header file.

Now we can create a client object.

Where the url is of the form host:port, and clientid is a string. The fourth parameter specifies the disk persistence required. MQTTCLIENT_PERSISTENCE_NONE says that no persistence is required. MQTTCLIENT_PERSISTENCE_DEFAULT asks for the supplied, default disk persistence to be used. This stores all inflight message data to disk, and means that the application can end, restart, and recreate the client object with the same clientid and url. Any inflight message data will be read from disk and restored so that the application can continue where its previous incarnation left off.

Next we can optionally set some callback functions. At a minimum, we must have a messageArrived callback, which will be called whenever an MQTT publish message arrives.

There is also a connectionLost callback which will be called whenever the connection to the server has been broken unexpectedly. That is, we have called connect previously, it has succeeded, we have not called disconnect, and now the connection has been broken. Often we simply want to reconnect in the connectionLost callback. The final callback function is deliveryComplete, which is called when an MQTT publish exchange finishes. The MQTTClient_publish call blocks, but only until the publish packet is written to the socket. For QoS 0, this is the end of the story. For QoS 1 and 2, the MQTT packet exchange continues. When the final acknowledgement is received from the server, deliveryComplete is called. context is a pointer which is passed to the callbacks and can contain any useful information, such as the client object for which this callback is being made.

For this synchronous client, you do have the option of not setting any callbacks. In that case, no background thread will be started.
Under these circumstances, the application, on a regular basis, must call

to receive messages, or

to allow any necessary MQTT background processing to take place. This mode was intended specifically when the application wanted no threads to be created.

Application Setup – MQTTAsync

The application setup for the asynchronous client library is very similar to the synchronous library. Header file:

client object creation:

and setting the callbacks:

There is no non-threaded mode for the async client, background threads will always be created and the arrival of MQTT publications will be notified by the messageArrived callback.


To connect to a server, we create a connect options structure and call connect:

which will block until the MQTT connect is complete, or has failed. Similarly, for the async client:

except that we add two pointers to callback functions. The connect call will not block, success or failure will be notified by the invocation of one or other of the callback functions. For the other connect examples, I’ll just show the synchronous client, but the pattern is the same for the async client. A typical pattern for the async client is to make all MQTT calls in callback functions: a subscribe or publish call can be made in the success callback for connect, for instance.

Connect with LWT

Connect with Username and Password


Publish looks like the following, for the synchronous client:

There is also another version of the call which takes a message structure:

The delivery token can be used in a waitForCompletion call, to synchronize on the completion of the MQTT packet exchange. The async calls for publish look very similar, except I renamed publish to send, and topic to destination. This was to be compatible with the JavaScript Paho client, because the asynchronous programming models are very similar. It seemed
like a good idea at the time!

Again, the biggest change is the addition of the success and failure callbacks.


The subscribe call is straightforward:

and for the async client:


The unsubscribe call is also straightforward:

and for the async client:


There is a timeout parameter in milliseconds on the disconnect call, which allows outstanding MQTT packet exchanges to complete.


When you’ve finished with a client object, call destroy to free up the memory:


Receiving Messages

A typical message arrived callback function might look like this:

Note the calls to free memory, and the return value: 1 means that the message was received properly, 0 means it wasn’t. The messageArrived callback will be invoked again for the same message if you return 0.

Using TLS / SSL

To use TLS, you need to add the SSL options data to the connect options:

The TLS implementation uses OpenSSL, so the configuration parameters are the same:

Example application

Sample application for the Paho C clients are in the samples directory of the github repository:

and in the download packages.

Author Information

Ian Craggs | IBM
Ian Craggs works for IBM, and has been involved with MQTT for more than 10 years. He wrote the IBM MQTT server Really Small Message Broker which became the inspiration for the Eclipse Mosquitto project. He contributed C client libraries to the Eclipse Paho project at its onset and is now the project leader.


  1. Wouter says:

    thanks for the info

    the best explination i have seen so far between sync and async

    I have managed to compile the sync code to publish on the RPI using netbeans.

    I have tested the publish to from the RPI and use MQTT.FX to subscribe to the topic and all was working

    No i have changed to the async mode

    I get a compile error
    undefined reference to `MQTTAsync_create’
    my linker flag was -lpaho-mqtt3c for the sync and it worked
    must i change it for the async mode??


    1. Hi Wouter,

      since the guest authors do not monitor this site regularly, best would be to ask at the Paho Mailing list, the developers are very helpful and can answer your questions:

      All the best,
      Dominik from the HiveMQ Team

  2. Himanshu says:

    Do we have a MQTT Client Library in c programming for Windows CE 6.0


    1. I believe the Paho embedded C library ( can be integrated with Windows CE. The best way to check this would to reach out to the Paho folks via the mailing list at, they’re always very helpful.

      Hope this helps,
      Dominik from the HiveMQ Team

  3. Shakti Gupta says:

    I am beginner in MQTT. i am using eclipse paho C. i am facing some problem in a sample program. Please have a look into code. i have pasted code and output of publisher and subscriber below. In subscriber, it show some corrupted string and then it lost connection. Please help me to find mistake.


    #include “stdio.h”
    #include “stdlib.h”
    #include “string.h”
    #include “MQTTClient.h”

    #define ADDRESS “tcp://”
    #define CLIENTID “ExampleClientPub”
    #define TOPIC “MQTT Examples”
    #define PAYLOAD “Hellooooooo”
    #define QOS 1
    #define TIMEOUT 10000L

    int main(int argc, char* argv[])
    MQTTClient client;
    MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;
    MQTTClient_message pubmsg = MQTTClient_message_initializer;
    MQTTClient_deliveryToken token;
    int rc;
    int status;
    char* str=”HELLO”;

    MQTTClient_create(&client, ADDRESS, CLIENTID,
    conn_opts.keepAliveInterval = 20;
    conn_opts.cleansession = 1;

    if ((rc = MQTTClient_connect(client, &conn_opts)) != MQTTCLIENT_SUCCESS)
    printf(“Failed to connect, return code %d\n”, rc);
    pubmsg.payload = (void*)str;
    pubmsg.payloadlen = strlen(str);
    pubmsg.qos = QOS;
    pubmsg.retained = 0;
    printf(“lenght is %d”, pubmsg.payloadlen);
    status=MQTTClient_publishMessage(client, TOPIC, &pubmsg, &token);
    printf(“status is %d \n”,status);
    printf(“Waiting for up to %d seconds for publication of %s\n”
    “on topic %s for client with ClientID: %s\n”,
    rc = MQTTClient_waitForCompletion(client, token, TIMEOUT);
    printf(“Message with delivery token %d delivered\n”, token);
    MQTTClient_disconnect(client, 10000);
    return rc;

    Output of Sample_publish

    lenght is 5status is 0
    Waiting for up to 10 seconds for publication of Hellooooooo
    on topic MQTT Examples for client with ClientID: ExampleClientPub
    Message with delivery token 1 delivered


    * sample_subscribe.c
    * Created on: 22-Sep-2016
    * Author: shakti

    #include “stdio.h”
    #include “stdlib.h”
    #include “string.h”
    #include “MQTTClient.h”
    #include “sample_subscribe.h”

    #define ADDRESS “tcp://”
    #define CLIENTID “ExampleClientPub”
    #define TOPIC “MQTT Examples”
    #define PAYLOAD “Hellooooooo”
    #define QOS 1
    #define TIMEOUT 10000L

    int main(int argc, char* argv[])
    MQTTClient client;
    MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;
    int rc;
    int ch;
    //a. Create an instance of MQTT client
    //b. Prepare connection options
    conn_opts.keepAliveInterval = 20;
    conn_opts.cleansession = 1;
    MQTTClient_setCallbacks(client, NULL, connlost, msgarrvd, delivered);
    //c. Connect to broker with the connection options
    if ((rc = MQTTClient_connect(client, &conn_opts)) != MQTTCLIENT_SUCCESS)
    printf(“Failed to connect, return code %d\n”, rc);
    //d. Subscribe interested topics.
    printf(“Subscribing to topic %s\nfor client %s using QoS%d\n\n”
    “Press Q to quit\n\n”, TOPIC, CLIENTID, QOS);
    MQTTClient_subscribe(client, TOPIC, QOS);
    ch = getchar();
    } while(ch!=’Q’ && ch != ‘q’);
    //MQTTClient_setCallbacks(client, NULL, connlost, msgarrvd, delivered);
    //e. Disconnect to broker
    MQTTClient_disconnect(client, 10000);
    //f. Release resources
    return rc;



    volatile MQTTClient_deliveryToken deliveredtoken;
    void delivered(void *context, MQTTClient_deliveryToken dt)
    printf(“Message with token value %d delivery confirmed\n”, dt);
    deliveredtoken = dt;

    int msgarrvd(void *context, char *topicName, int topicLen, MQTTClient_message* message)
    int i;
    char* payloadptr;
    printf(“Message arrived\n”);
    printf(” topic: %s\n”, topicName);
    printf(” message: “);
    printf(“address of message is %u \n”,message);
    payloadptr = (char*)message->payload;
    printf(“11111 \n”);
    printf(“message is…. %s”,payloadptr);
    for(i=0; ipayloadlen; i++)
    printf(“2222 \n”);
    printf(“33333 \n”);
    printf(“555 \n”);
    printf(“666 \n”);
    printf(“4444 \n”);
    return 1;

    void connlost(void *context, char *cause)
    printf(“\nConnection lost\n”);
    printf(” cause: %s\n”, cause);


    Output of sample_subscribe

    Subscribing to topic MQTT Examples
    for client ExampleClientPub using QoS1

    Press Q to quit

    Message arrived
    topic: MQTT Examples
    message: address of message is 2281704372
    message is…. crashedket.c2222


    Connection lost

    cause (null)

    Thank You.
    Shakti Gupta

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