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);
}

我在使用scons构建时遇到错误

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.

如何在两个函数之间交换代理的价值？还有其他方法可以实现吗？

Answer 1

如何在两个函数之间交换代理的价值？还有其他方法可以实现吗？。

更改两个功能的签名。

更改第一个函数以返回broker。
将第二个函数更改为期望broker作为参数。

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);
}

更改通话功能。

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

Answer 2

问题在于broker函数由于变量作用域而无法在publish_mqtt函数中使用。

即：一旦mqtt_connection的结束符号被击中，经纪人就不再存在。

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

如果您希望在publish_mqtt内使用它，则需要将变量的范围转换为涵盖这两个函数的范围。即：

mqtt_broker_handle_t *broker = NULL;

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

int publish_mqtt()
{
...
}

C中两个函数之间的指针

2 个答案: