Basic subscribe example.

examples/publish/basic-1.c

@@ -17,6 +17,12 @@ void on_connect(struct mosquitto *mosq, void *obj, int reason_code)
  * clients is mosquitto_reason_string().
  */
  printf("on_connect: %s\n", mosquitto_connack_string(reason_code));
+ if(reason_code != 0){
+ /* If the connection fails for any reason, we don't want to keep on
+ * retrying in this example, so disconnect. Without this, the client
+ * will attempt to reconnect. */
+ mosquitto_disconnect(mosq);
+ }
 
  /* You may wish to set a flag here to indicate to your application that the
  * client is now connected. */

examples/subscribe/basic-1.c

@@ -0,0 +1,119 @@
+/*
+ * This example shows how to write a client that subscribes to a topic and does
+ * not do anything other than handle the messages that are received.
+ */
+
+#include <mosquitto.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+
+/* Callback called when the client receives a CONNACK message from the broker. */
+void on_connect(struct mosquitto *mosq, void *obj, int reason_code)
+{
+ int rc;
+ /* Print out the connection result. mosquitto_connack_string() produces an
+ * appropriate string for MQTT v3.x clients, the equivalent for MQTT v5.0
+ * clients is mosquitto_reason_string().
+ */
+ printf("on_connect: %s\n", mosquitto_connack_string(reason_code));
+ if(reason_code != 0){
+ /* If the connection fails for any reason, we don't want to keep on
+ * retrying in this example, so disconnect. Without this, the client
+ * will attempt to reconnect. */
+ mosquitto_disconnect(mosq);
+ }
+
+ /* Making subscriptions in the on_connect() callback means that if the
+ * connection drops and is automatically resumed by the client, then the
+ * subscriptions will be recreated when the client reconnects. */
+ rc = mosquitto_subscribe(mosq, NULL, "example/temperature", 1);
+ if(rc != MOSQ_ERR_SUCCESS){
+ fprintf(stderr, "Error subscribing: %s\n", mosquitto_strerror(rc));
+ /* We might as well disconnect if we were unable to subscribe */
+ mosquitto_disconnect(mosq);
+ }
+}
+
+
+/* Callback called when the broker sends a SUBACK in response to a SUBSCRIBE. */
+void on_subscribe(struct mosquitto *mosq, void *obj, int mid, int qos_count, const int *granted_qos)
+{
+ int i;
+ bool have_subscription = false;
+
+ /* In this example we only subscribe to a single topic at once, but a
+ * SUBSCRIBE can contain many topics at once, so this is one way to check
+ * them all. */
+ for(i=0; i<qos_count; i++){
+ printf("on_subscribe: %d:granted qos = %d\n", i, granted_qos[i]);
+ if(granted_qos[i] <= 2){
+ have_subscription = true;
+ }
+ }
+ if(have_subscription == false){
+ /* The broker rejected all of our subscriptions, we know we only sent
+ * the one SUBSCRIBE, so there is no point remaining connected. */
+ fprintf(stderr, "Error: All subscriptions rejected.\n");
+ mosquitto_disconnect(mosq);
+ }
+}
+
+
+/* Callback called when the client receives a message. */
+void on_message(struct mosquitto *mosq, void *obj, const struct mosquitto_message *msg)
+{
+ /* This blindly prints the payload, but the payload can be anything so take care. */
+ printf("%s %d %s\n", msg->topic, msg->qos, (char *)msg->payload);
+}
+
+
+int main(int argc, char *argv[])
+{
+ struct mosquitto *mosq;
+ int rc;
+
+ /* Required before calling other mosquitto functions */
+ mosquitto_lib_init();
+
+ /* Create a new client instance.
+ * id = NULL -> ask the broker to generate a client id for us
+ * clean session = true -> the broker should remove old sessions when we connect
+ * obj = NULL -> we aren't passing any of our private data for callbacks
+ */
+ mosq = mosquitto_new(NULL, true, NULL);
+ if(mosq == NULL){
+ fprintf(stderr, "Error: Out of memory.\n");
+ return 1;
+ }
+
+ /* Configure callbacks. This should be done before connecting ideally. */
+ mosquitto_connect_callback_set(mosq, on_connect);
+ mosquitto_subscribe_callback_set(mosq, on_subscribe);
+ mosquitto_message_callback_set(mosq, on_message);
+
+ /* Connect to test.mosquitto.org on port 1883, with a keepalive of 60 seconds.
+ * This call makes the socket connection only, it does not complete the MQTT
+ * CONNECT/CONNACK flow, you should use mosquitto_loop_start() or
+ * mosquitto_loop_forever() for processing net traffic. */
+ rc = mosquitto_connect(mosq, "test.mosquitto.org", 1883, 60);
+ if(rc != MOSQ_ERR_SUCCESS){
+ mosquitto_destroy(mosq);
+ fprintf(stderr, "Error: %s\n", mosquitto_strerror(rc));
+ return 1;
+ }
+
+ /* Run the network loop in a blocking call. The only thing we do in this
+ * example is to print incoming messages, so a blocking call here is fine.
+ *
+ * This call will continue forever, carrying automatic reconnections if
+ * necessary, until the user calls mosquitto_disconnect().
+ */
+ mosquitto_loop_forever(mosq, -1, 1);
+
+ mosquitto_lib_cleanup();
+ return 0;
+}
+