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')

• coldbreeze16: Ty, that worked! The noob in me didn't know char arrays were Strings. I had actually tried casting the char array to a new String and used strcmp on that but for some reasons that didn't work.
• StarCat: Actually, char arrays are not Strings (note the capital S), but c-strings, which is a significant difference. Strings shouldonly not be used in devices, with limited memory (or only verys sparingly) while c-strings (char arrays) do not exhibit the same problems.
• romkey: What @StarCat said. Unfortunately C strings also let you shoot yourself in the foot very easily. It's definitely one of the less intuitive parts of C unless you're coming to C from assembly language programming :)