1

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

  • 2
    bool myJSON::getINTValue(const char *key, int &retval){ – Juraj Jun 5 at 18:56
  • @Juraj can you explain why? and why is it different from char variable ? – Guy . D Jun 5 at 18:58
  • it is not an Arduino question. & is 'reference'. the parameter with & is not a value, but a reference to a variable – Juraj Jun 5 at 18:59
  • @Juraj - I know the mean of &, but why it doest not pass its value a to retval? – Guy . D Jun 5 at 19:10
3

the parameters of a function call are copied into parameters in the function. if the pointer is copied it still points to a memory where you write the data. if int is copied then changing the value changes the copy, not the variable used as parameter. reference is kind of pointer

how to make an in-out parameter by passing a parameter by reference

void setup() {

  Serial.begin(115200);
  int x = 3;
  fnc(x);
  Serial.println(x);
}

void loop() {

}

void fnc(int &x) {
  x = 5;
}

prints 5

example with a complex type

#include <IPAddress.h>

void setup() {

  Serial.begin(115200);
  IPAddress ip(192, 168, 1, 5);
  fnc(ip);
  Serial.println(ip);
}

void loop() {

}

void fnc(IPAddress& ip) {
  ip = IPAddress(192, 168, 1, 3);
}

with &ip it prints 192.168.1.3. without & it prints 192.168.1.5

  • Tnx ! and the reason char behaves differently is because it points to a pointer ? – Guy . D Jun 5 at 19:39
  • 1
    char* is a pointer. and char[] is a pointer too. it is copied so assigning a different pointer to it would not work. but if you change the memory where it points you will read outside of the function the same memory location with the changed content – Juraj Jun 5 at 19:41
2

Your problem is not directly how to return a variable, but how to give a variable from outsite the function as parameter to be filled by the function. There are basically 2 types how you can give a variable to a function: by value or by reference

The definition of getINTValue() says

getINTValue (const char *key, int retval)

so retval will be an integer and is called by value. This means, that a local variable with that name is created and filled with the value from the function call parameters. You are writing to this variable, but it get's thrown away when the function returns. And the writing to the local variable didn't change the outsite variable.

You need to use a call by reference. Your function must take a pointer to an integer variable. Then you dereference it and write to it:

getINTValue (const char *key, int *retval){
    ...
    *retval = tempJDOC[key];
    ...
}

When you call the getINTValue() function, it must be done like this:

int value;
bool result = getINTValue("int_key", &value);

The & operator gets you the pointer to the variable value, which the function then uses to write a value at that memory location.


char* is different, because you give an array to the function in this case. The compiler sees, that the used variable is an array and uses the pointer to the first element of the array as parameter. That is a common way, because you can iterate over an array by using pointers (without index numbers). You can do the same with any array of any type, but in this case you only have a single variable, not an array.

  • Oh, sorry, have read it wrong. Can you elaborate your comment shortly or give me a hint for what to google here? I learned it the way I've shown in my answer and while googling I couldn't find your way of doing it (though I didn't google for long). Would be great to learn more. – chrisl Jun 5 at 19:16
  • I wrote it as answer – Juraj Jun 5 at 19:36

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.