I have created myJSON library to simplify read and write values from/ to FLASH of my ESP8266 NodeMCU/ Wemos Mini.

I do not want to "mess up" the question, posting my entire code/ library - but to try to explain what happens -

I get a 0 value when reading a specific key (and not its saved value ). My assumption is that is has nothing to do with myJSON library, but my understanding in returning a value from a function.

Code is as follows:

(some explanations first):

1. json is an instance of myJSON library.

2. json.setValue(key, value) - gets a key and a value ( char/ int) and save it to FLASH.

3)json.getINTValue(key, return_var)- gets a key and return its int value to return_var. function return 1 - for successful read, and 0 when not.

inside setup()

int int_val;
// json.setValue("int_key",343); // commented out after first run - to be sure is is saved and read from flash.
Serial.println(json.getINTValue("int_key",int_val)); // NOTE 2
Serial.print("Value is: "); // NOTE 3
Serial.println(int_val);   //  NOTE 3

myJSON ( only relevant code )

bool myJSON::getINTValue (const char *key, int retval){
        StaticJsonDocument<DOC_SIZE> tempJDOC;
        myJSON::readJSON_file(tempJDOC);
        bool hasKey = tempJDOC.containsKey(key);
        if (hasKey){
          retval = tempJDOC[key];
          Serial.print("retval :"); // NOTE 1
          Serial.println(retval);  // NOTE 1
          return 1;
        }
        else {
          return 0; // when key is not present
        }
}

void myJSON::setValue(const char *key, char *value){
        StaticJsonDocument<DOC_SIZE> tempJDOC;
        myJSON::readJSON_file(tempJDOC);
        tempJDOC[key]=value;
        myJSON::saveJSON2file(tempJDOC);
}
void myJSON::setValue(const char *key, int value){
        StaticJsonDocument<DOC_SIZE> tempJDOC;
        myJSON::readJSON_file(tempJDOC);
        tempJDOC[key]=value;
        myJSON::saveJSON2file(tempJDOC);
}

result is (SEE NOTATIONS):

21:01:03.198 -> retval :343  // OUTPUT NOTE 1
21:01:03.198 -> 1            // OUTPUT NOTE 2 
21:01:03.198 -> Value is: 0  // OUTPUT NOTE 3

As far a I could understand :

1. setValue works as needed ( inc. saving to flash ).
2. unexplained besides getting value = 0, is when setting a char value ( there are 2 myJSON::setValue functions to deal with int and char), code works as needed.

Appreciate any help ( and sorry for my English ).

Guy