Pointer between two functions in C

Question

int mqtt_connection()
{
    mqtt_broker_handle_t *broker = mqtt_connect(client_name, ip_addr, port);

    if(broker == 0) {  
    printf("Connection failed, Please check the IP and port of broker\n");  
     return 0;          
        }
    else {
        printf("Connection established successfully\n");
    }
    return 1;
}


int publish_mqtt()
{
    char msg[128] = "Test 2";

    if(mqtt_publish(broker, topic1, msg, QoS1) == -1) 
    {
            printf("publish failed\n");
    }
    else {
        printf("Sent messages\n");
    }
    return(0);
}

I am getting error while building using scons as

master.c: In function 'publish_mqtt':
master.c:39:17: error: 'broker' undeclared (first use in this function)
if(mqtt_publish(broker, topic1, msg, QoS1) == -1) 
             ^
master.c:39:17: note: each undeclared identifier is reported only once          for each function it appears in
 scons: *** [master.o] Error 1
  scons: building terminated because of errors.

How to exchange the value of broker between two functions?. Is there any other way to implement this?

Answer 1

The problem lies in the fact broker is not available in the publish_mqtt function due to variable scoping.

Ie: Once the end brace of mqtt_connection is hit, broker no longer exists.

int mqtt_connection()
{
mqtt_broker_handle_t *broker = mqtt_connect(client_name, ip_addr, port);
...
}

If you wish to use it within publish_mqtt you need to shift the scope of the variable to be one that covers both functions. Ie:

mqtt_broker_handle_t *broker = NULL;

int mqtt_connection()
{
broker = mqtt_connect(client_name, ip_addr, port);
...
}

int publish_mqtt()
{
...
}

Answer 2

How to exchange the value of broker between two functions?. Is there any other way to implement this?.

Change the signatures of both functions.

1. Change the first function to return broker.
2. Change the second function to expect broker as an argument.

---

mqtt_broker_handle_t* mqtt_connection()
{
   mqtt_broker_handle_t *broker = mqtt_connect(client_name, ip_addr, port);

   if(broker == 0)
   {
      printf("Connection failed, Please check the IP and port of broker\n");  
      return NULL;          
   }
   else
   {
      printf("Connection established successfully\n");
      return broker;
   }
}


int publish_mqtt(mqtt_broker_handle_t* broker)
{
   char msg[128] = "Test 2";

   if(mqtt_publish(broker, topic1, msg, QoS1) == -1) 
   {
      printf("publish failed\n");
   }
   else
   {
      printf("Sent messages\n");
   }
   return(0);
}

---

Change the calling function.

mqtt_broker_handle_t* broker = mqtt_connection();
publish_mqtt(broker);