This tutorial will introduce you to the fundamentals of the Solace API by connecting a client, adding a topic subscription and sending a message matching this topic subscription. This forms the basis for any publish / subscribe message exchange.


This tutorial assumes the following:

  • You are familiar with Solace core concepts.
  • You have access to Solace messaging with the following configuration details:
    • Connectivity information for a Solace message-VPN
    • Enabled client username and password

One simple way to get access to Solace messaging quickly is to create a messaging service in Solace Cloud as outlined here. You can find other ways to get access to Solace messaging below.


The goal of this tutorial is to demonstrate the most basic messaging interaction using Solace. This tutorial will show you:

  1. How to build and send a message on a topic
  2. How to subscribe to a topic and receive a message

Get Solace Messaging

This tutorial requires access Solace messaging and requires that you know several connectivity properties about your Solace messaging. Specifically you need to know the following:

Resource Value Description
Host String This is the address clients use when connecting to the Solace messaging to send and receive messages. (Format: DNS_NAME:Port or IP:Port)
Message VPN String The Solace message router Message VPN that this client should connect to.
Client Username String The client username. (See Notes below)
Client Password String The client password. (See Notes below)

There are several ways you can get access to Solace Messaging and find these required properties.

Option 1: Use Solace Cloud

  • Follow these instructions to quickly spin up a cloud-based Solace messaging service for your applications.
  • The messaging connectivity information is found in the service details in the connectivity tab (shown below). You will need:
    • Host:Port (use the SMF URI)
    • Message VPN
    • Client Username
    • Client Password

Option 2: Start a Solace VMR

  • Follow these instructions to start the Solace VMR in leading Clouds, Container Platforms or Hypervisors. The tutorials outline where to download and how to install the Solace VMR.
  • The messaging connectivity information are the following:
    • Host: <public_ip> (IP address assigned to the VMR in tutorial instructions)
    • Message VPN: default
    • Client Username: sampleUser (can be any value)
    • Client Password: samplePassword (can be any value)

    Note: By default, the Solace VMR “default” message VPN has authentication disabled.

Option 3: Get access to a Solace appliance

  • Contact your Solace appliance administrators and obtain the following:
    • A Solace Message-VPN where you can produce and consume direct and persistent messages
    • The host name or IP address of the Solace appliance hosting your Message-VPN
    • A username and password to access the Solace appliance

Obtaining the Solace API

This tutorial depends on you having the C API downloaded and available. The C API library can be downloaded here. The C API is distributed as a gzipped tar file for the platform you’re working with. The API developer documentation is downloaded a separate tar file. The instructions in this tutorial assume you have downloaded the C API library, unpacked it to a known location and are executing the provided build instructions from a sub-directory within the C API library root. If your environment differs then adjust the build instructions appropriately.

Connecting to the Solace message router

In order to send or receive messages, an application must connect a Solace session. The Solace session is the basis for all client communication with the Solace message router.

In the Solace messaging API for C (SolClient), a few distinct steps are required to create and connect a Solace session.

  • The API must be initialized
  • Appropriate asynchronous callbacks must be declared
  • A SolClient Context is needed to control application threading
  • The SolClient session must be created

Initializing the CCSMP API

To initialize the SolClient API, you call the initialize method with arguments that control logging.

/* solClient needs to be initialized before any other API calls. */
solClient_initialize ( SOLCLIENT_LOG_DEFAULT_FILTER, NULL );

This call must be made prior to making any other calls to the SolClient API. It allows the API to initialize internal state and buffer pools.

SolClient Asynchronous Callbacks

The SolClient API is predominantly an asynchronous API designed for the highest speed and lowest latency. As such most events and notifications occur through callbacks. In order to get up and running, the following basic callbacks are required at a minimum.

static int msgCount = 0;

messageReceiveCallback ( solClient_opaqueSession_pt opaqueSession_p, solClient_opaqueMsg_pt msg_p, void *user_p )
    printf ( "Received message:\n" );
    solClient_msg_dump ( msg_p, NULL, 0 );
    printf ( "\n" );


eventCallback ( solClient_opaqueSession_pt opaqueSession_p,
                solClient_session_eventCallbackInfo_pt eventInfo_p, void *user_p )
    printf("Session EventCallback() called:  %s\n", solClient_session_eventToString ( eventInfo_p->sessionEvent));

The messageReceiveCallback is invoked for each Direct message received by the Session. In this sample, the message is printed to the screen.

The eventCallback is invoked for various significant session events like connection, disconnection, and other SolClient session events. In this sample, simply prints the events. See the SolClient API documentation and samples for details on the session events.

Context Creation

As outlined in the core concepts, the context object is used to control threading that drives network I/O and message delivery and acts as containers for sessions. The easiest way to create a context is to use the context initializer with default thread creation.

/* Context */
solClient_opaqueContext_pt context_p;
solClient_context_createFuncInfo_t contextFuncInfo = SOLCLIENT_CONTEXT_CREATEFUNC_INITIALIZER;

                           &context_p, &contextFuncInfo, sizeof ( contextFuncInfo ) );

Session Creation

Finally a session is needed to actually connect to the Solace message router. This is accomplished by creating a properties array and connecting the session.

/* Session */
solClient_opaqueSession_pt session_p;
solClient_session_createFuncInfo_t sessionFuncInfo = SOLCLIENT_SESSION_CREATEFUNC_INITIALIZER;

/* Session Properties */
const char     *sessionProps[50];
int             propIndex = 0;
char *username,*password,*vpnname,*host;

/* Configure the Session function information. */
sessionFuncInfo.rxMsgInfo.callback_p = messageReceiveCallback;
sessionFuncInfo.rxMsgInfo.user_p = NULL;
sessionFuncInfo.eventInfo.callback_p = eventCallback;
sessionFuncInfo.eventInfo.user_p = NULL;

/* Configure the Session properties. */
propIndex = 0;
host = argv[1];
vpnname = argv[2];
username = strsep(&vpnname,"@");
password = argv[3];

sessionProps[propIndex++] = SOLCLIENT_SESSION_PROP_HOST;
sessionProps[propIndex++] = host;

sessionProps[propIndex++] = SOLCLIENT_SESSION_PROP_VPN_NAME;
sessionProps[propIndex++] = vpnname;

sessionProps[propIndex++] = SOLCLIENT_SESSION_PROP_USERNAME;
sessionProps[propIndex++] = username;

sessionProps[propIndex++] = SOLCLIENT_SESSION_PROP_PASSWORD;
sessionProps[propIndex] = password;

/* Create the Session. */
solClient_session_create ( ( char ** ) sessionProps,
                           &session_p, &sessionFuncInfo, sizeof ( sessionFuncInfo ) );

/* Connect the Session. */
solClient_session_connect ( session_p );
printf ( "Connected.\n" );

When creating the session, the factory method takes the session properties, the session pointer and information about the session callback functions. The API then creates the session within the supplied context and returns a reference in the session pointer. The final call to solClient_session_connect establishes the connection to the Solace message router which makes the session ready for use.

At this point your client is connected to the Solace message router. You can use SolAdmin to view the client connection and related details.

Receiving a message

This tutorial is uses “Direct” messages which are at most once delivery messages. So first, let’s express interest in the messages by subscribing to a Solace topic. Then you can look at publishing a matching message and see it received.

With a session connected in the previous step, then you must subscribe to a topic in order to express interest in receiving messages. This tutorial uses the topics “tutorial/topic”.

solClient_session_topicSubscribeExt ( session_p,

Then after the subscription is added, the consumer is started. At this point the consumer is ready to receive messages. So wait in a loop for the expected message to arrive.

printf ( "Waiting for message......\n" );
fflush ( stdout );
while ( msgCount < 1 ) {
    sleepInSec ( 1 );

Sending a message

Now it is time to send a message to the waiting consumer.

To send a message, you must create a message and a topic destination. This tutorial will send a Solace binary message with contents “Hello world!”. Then you must send the message to the Solace message router.

/* Message */
solClient_opaqueMsg_pt msg_p = NULL;ss
solClient_destination_t destination;
const char *text_p = "Hello world!";

/* Allocate memory for the message that is to be sent. */
solClient_msg_alloc ( &msg_p );

/* Set the message delivery mode. */
solClient_msg_setDeliveryMode ( msg_p, SOLCLIENT_DELIVERY_MODE_DIRECT );

/* Set the destination. */
destination.dest = "tutorial/topic";
solClient_msg_setDestination ( msg_p, &destination, sizeof ( destination ) );

/* Add some content to the message. */
solClient_msg_setBinaryAttachment ( msg_p, text_p, ( solClient_uint32_t ) strlen ( (char *)text_p ) );

/* Send the message. */
printf ( "About to send message '%s' to topic '%s'...\n", (char *)text_p, argv[4] );
solClient_session_sendMsg ( session_p, msg_p );

printf ( "Message sent. Exiting.\n" );
solClient_msg_free ( &msg_p );

In the SolClient API, messages are allocated and freed from an internal API message pool for greatest performance and efficiency. Therefore as shown, messages must be acquired by calls to solClient_msg_alloc and then later freed back to the pool by calls to solClient_msg_free.

The minimum properties required to create a SolClient message that can be sent is to set the delivery mode, queue or topic destination, and message contents as shown in the above code. Once the message is created it is sent to the Solace message router with a call to solClient_session_sendMsg.

At this point the producer has sent a message to the Solace message router and your waiting consumer will have received the message and printed its contents to the screen.


The full source code for this example is available in GitHub. If you combine the example source code shown above results in the following source:

The OS source code simply provides platform abstraction. The subscriber sample makes use of this for the sleep in the main loop.


Building these examples is simple. The following provides an example using Linux. For ideas on how to build on other platforms you can consult the README of the C API library.

gcc -g -Wall -I ../include -L ../lib -lsolclient -lpthread HelloWorldPub.c -o HelloWorldPub
gcc -g -Wall -I ../include -L ../lib -lsolclient -lpthread os.c HelloWorldSub.c -o HelloWorldSub

Referencing the downloaded SolClient library include and lib file is required. For more advanced build control, consider adapting the makefile found in the “Intro” directory of the SolClient package. The above samples very closely mirror the samples found there.

If you start the HelloWoldSub with the required arguments of your Solace messaging, it will connect and wait for a message.

$ LD_LIBRARY_PATH=../lib:$LD_LIBRARY_PATH ./HelloWorldSub <host:port> <client-username@message-vpn> <client-password>
HelloWorldSub initializing...
Session EventCallback() called:  Session up
Connected. Awaiting message...

Then you can send a message using the HelloWorldPub with the same arguments. If successful, the output for the producer will look like the following:

$ LD_LIBRARY_PATH=../lib:$LD_LIBRARY_PATH ./HelloWorldPub <host:port> <client-username@message-vpn> <client-password>
HelloWorldPub initializing...
Session EventCallback() called:  Session up
Connected. About to send message 'Hello world!' to topic 'tutorial/topic'...
Message sent. Exiting.

With the message delivered the subscriber output will look like the following:

Received message:
Destination:         Topic 'tutorial/topic'
Class Of Service:    COS_1
DeliveryMode:        DIRECT
Binary Attachment:   len=12
48 65 6c 6c 6f 20 77 6f 72 6c 64 21           Hello world!

The received message is printed to the screen. The message contents was “Hello world!” as expected and shown in the contents of the message dump along with additional information about the Solace message that was received.

If you have any issues sending and receiving a message, check the Solace community for answers to common issues seen.

You have now successfully connected a client, subscribed to a topic and exchanged messages using this topic.