I'm having a real head scratch here, hopefully it's just because its late but here goes ...
I need to compare some data received via an MQTT message (which comes in as a byte*) with a piece of known data (a char* in this case). Obviously I have to convert the byte* to a char* in order for this to work but I'm obviously missing a step as the data sent over is different to what is compared at the other end. The code I'm struggling with is ...
void callback(char* topic, byte* payload, unsigned int length) {
// In order to republish this payload, a copy must be made
// as the orignal payload buffer will be overwritten whilst
// constructing the PUBLISH packet.
// Allocate the correct amount of memory for the payload copy
byte* p = (byte*)malloc(length);
// Copy the payload to the new buffer
memcpy(p,payload,length);
//client.publish("SHHNoT/frontDoor/secSensor/doorLock", p, length);
char* rcvdMQTT = (char*)p; //need to convert byte* p variable into a char* for comparision
client.publish("SHHNoT/frontDoor/secSensor/doorLock", rcvdMQTT); // debugging - republish after conversion to see what you've actually got
if(rcvdMQTT=="Steve") {
client.publish("SHHNoT/frontDoor/secSensor/doorLock", "Hello Steve. Come on in.");
}
else {
client.publish("SHHNoT/frontDoor/secSensor/doorLock", "Who are you? I'm not opening the door.");
}
// Free the memory
free(p);
}
When the code receives a message it converts the byte* to a char* and then republishes that out for debugging purposes. What I can see has been republished is the original data with some extra characters on the on end (two colour inverted question marks and a question mark). Due, presumably, to these additional characters even if I send the correct data over it fails the check.
I'm sure it's just a conversion issue but I just can't see it.
You can not receive a byte*, that is a pointer to a byte, it does not make any sense. If you are receiving a pointer from an other device that byte is still being stored in that device.
thermalhound:
I am trying to check whether the text message (via MQTT) received (byte* p) is equal to a known text - in this case "Steve"
Exactly. What your code will do is check if the memory location rcvdMQTT (which you got from malloc) is the same as the memory location where the compiler has placed the static text in the double quotes.
It won't be.
You need to use strcmp() to compare the content of the memory at those two locations. Docs are here
Note that this function compares strings. Strings are null-terminated.
// enough space for the payload and a nul terminator
byte* p = (byte*)malloc(length+1);
// Copy the payload to the new buffer
memcpy(p,payload,length);
// force null termination
p[length] = '\0';
Don't use the C++ String class - it chews up memory and is not a suitable solution for microcontrollers with 2K of the stuff.
-- EDIT --
mistergreen suggests using strncmp. With that, you don't need to explicitly add a nul terminator. It's a better solution - that function was built for exactly this purpose.
