Message parsing from MQTT server

Question

I'm making a NodeMCU based home automation. I adapted code I found on internet for controlling a device (say LED) via MQTT broker hosted on a Raspberry Pi. It works fine. But when I try to scale it up for more than one device, I'm hitting a wall.

void callback(char* topic, byte* payload, unsigned int length)
{
  
  const char* t_bedroom_light1 = "/home/bedroom/light1";
  
  Serial.print("Topic from MQTT broker is : ");
  Serial.print(topic);
  char p = payload[0];

  // if MQTT comes a 0 turn OFF pin D4
  if (topic == "/home/bedroom/light1" && p == '0')
  {
    digitalWrite(LIGHT_PIN, LOW);
    Serial.println(" Turn Light Off! ");
  }

  // if MQTT comes a 1, turn ON pin D4
  if (topic == "/home/bedroom/light1" && p == '1')
  {
    digitalWrite(LIGHT_PIN, HIGH);
    Serial.println(" Turn Light On! " );
  }
}

Serial monitor receives and prints the topic correctly from MQTT server. But topic == "/home/bedroom/light1" never returns true. If I delete that part then p =='0' or p=='1' works fine by itself. I've tried comparing the topic with the locally declared char array, tried converting it into String and doing strcmp. Nothing. What am I doing wrong? Here is the Serial monitor output

12:43:20.346 -> .......
12:43:24.187 -> WiFi connected
12:43:24.187 -> IP address: 
12:43:24.187 -> 192.168.1.5
12:43:24.187 -> Attempting MQTT connection...connected
12:43:30.752 -> Topic from MQTT broker is : /home/bedroom/light1

Answer 1

That's not how you compare C strings.

When you write

if (topic == "/home/bedroom/light1")

you're comparing whether the character pointer topic is the same value as the address of the string "/home/bedroom/light1", which of course it is not in this case.

To compare C strings, use the strcmp() function, which returns 0 if they're identical.

if (strcmp(topic, "/home/bedroom/light1") == 0 && p == '0')