IPaddress class to const char*

Projects Programming
1

Got a project going on an ESP32.

I'm using several libraries. They all need to know the IP address of a remote server. NOT the machine's local IP address, the IP address of another machine.

Most of them want IP addresses in the form of a byte array, and they seem to be happy with Arduino's weird, non-standard, practically undocumented IPAddress class.

But there's always that one guy.

EspMQTTClient requires that the MQTT Broker's IP address be input as a const char * of the format "192.168.1.1"
. And for the life of me I can't figure or find out how to convert an IPAddress object or a byte array into a const char *.

I can convert it into a String object via the following shenanigan:

String serverAddressString = String(
  String(serverAddress[0]) + '.' +
  String(serverAddress[1]) + '.' +
  String(serverAddress[2]) + '.' +
  String(serverAddress[3]));

But even from here I can't get the damn thing converted to a form that his Royal High And Mightiness EspMQTTClient will accept. From the EspMQTTClient Github, for reference:

  EspMQTTClient(
    const char* wifiSsid,
    const char* wifiPassword,
    const char* mqttServerIp,
    const char* mqttUsername,  // Omit this parameter to disable MQTT authentification
    const char* mqttPassword,  // Omit this parameter to disable MQTT authentification
    const char* mqttClientName = "ESP8266",
    const short mqttServerPort = 1883);

How do I get this done?

2

Have you tried sprintf()?

3

Once you have it in a String, you can use the String method c_str() to get a C string. But if you're going to that trouble, why not use similar shenanigans to build a C string in a char array directly?

4

wildbill:
Once you have it in a String, you can use the String method c_str() to get a C string. But if you're going to that trouble, why not use similar shenanigans to build a C string in a char array directly?

c_str() got it.

I wasn't aware you could concatenate char arrays. Doesn't GCC have a fit if you don't declare the length of an array at compile time?

5

Look at strcat for example, or sprintf as suggested. But since you have the String now, maybe just use c_str and move on. It's a bit of a waste of memory is all.

6

That's okay, I've got 520k of ram. Might as well use it for something.

7

wildbill:
It's a bit of a waste of memory is all.

Also, kind of hacky. Converting an integer to a String object just so you can then convert it to an array of ASCII char. There are much simpler ways of going from an integer to an ASCII string.

8 
String serverAddressString = String(

String(serverAddress[0]) + '.' +
 String(serverAddress[1]) + '.' +
 String(serverAddress[2]) + '.' +
 String(serverAddress[3]));

... or just

String serverAddressString = serverAddress.toString();