Extracting the value of a program variable and sending it via UDP (LUA language)

Hi. I am controlling an ESP8266 + NodeMCU v2 WiFi module (communication via UART) from an XMEGA processor. I send commands in LUA language to the module in this way. An example can be seen below:

.

When I connect, I want to send the value of a software variable indicating the currently measured voltage on one of the processor peripherals (let's call it working name bat_lvl). The problem is that I don't know how to "extract" and send such a variable. So far, all I send is "static" text:

.

.

Trying to send the value of the variable doesn't work (I can't see anything reaching the terminal I'm tapping on):

.

Does anyone perhaps know the solution to my problem? I have looked on the internet for a solution but cannot find any example that answers my question.

Is there really no one who can help me?

Judging by the name of the function and the way it is called, it is used to send data from the microcontroller's flash memory. You need to send a variable from ram.

As a colleague above rightly pointed out, you need a function that operates on RAM. I'm guessing that you are using the example codes from the book "AVR. Practical projects"? There is no buffered UART function reading the string placed in RAM, so you need to add such a function (you can also modify the existing one using the so-called named address spaces). I don't know which example you are using, for the interrupt version the function could look like this (yes, the buf parameter is not used, but that's how it is in the original codes :-/):
.
Case 2.
socket:send takes a string in the parameter, so the value of the variable has to be converted to a string and only then the whole command is sent via the UART to the NodeMCU.
.

I understand thank you for your hints. I will follow your advice, as soon as I achieve the desired effect I will present the solution for posterity .

EDIT: Yes ex-or I am using the code from the book you mentioned.

After applying the code suggested by you ex-or I obtained the intended effect. Professional help , thank you .