Retrieve data structure from flash memory

Projects Programming
1

Dear all,

I've faced some trouble trying to retrieve some fields from a data structure that I tried to store on flash memory.
Here is the code:

#include <avr/pgmspace.h>

PROGMEM struct frankie {
  PROGMEM const char *key;
  PROGMEM const char *value;  
};

char buffer[30];

void setup() {
    Serial.begin(9600);
    
    PROGMEM frankie test[2];
    test[0].key = "KEY1";
    test[0].value = "VALUE1";
    test[1].key = "KEY2";
    test[1].value = "VALUE2";
    
    for (int i = 0; i < 2; i++) {
      strcpy_P(buffer, (char*)pgm_read_word(&(test[i].key)));
      Serial.println( buffer );
      strcpy_P(buffer, (char*)pgm_read_word(&(test[i].value)));
      Serial.println( buffer );
    }
}

void loop() { }

Would someone please offer me some guidance on this topic?
Thanks in advanced

2 
test[0].key = "KEY1";

That's never going to work, I'm afraid - you can't write to PROGMEM at runtime, you can only initialise PROGMEM structures at compile-time.

3

Look into EEPROM WriteAnything library.

4

Thank you both.

@tuxduino: I'll take a better look at EEPROM lib option, it seems to solve my issue...

5

Even if you could write to PROGMEM at run time, writing bytes to PROGMEM, and then reading words hardly makes sense.

6

The variable you're trying to store in progmem needs to be global, it doesn't make sense to try to store a local variable in progmem.

The variable needs to be initialised as part of its definition; you can't define it and then assign values to it separately.

If you want persistent storage which can be modified at runtime, the EEPROM might be a better bet. If using EEPROM, make sure your code doesn't update the EEPROM stored values repeatedly because the storage only supports a limited number of write cycles.

7

Dear all,

I would like to thank you all, specially to AWOL who showed me the peculiarity of Harvard architecture.
So, I've solved my problem with this piece of code:

#include <avr/pgmspace.h>

prog_char string_0[] PROGMEM = "String 0";   // "String 0" etc are strings to store - change to suit.
prog_char string_1[] PROGMEM = "String 1";
prog_char string_2[] PROGMEM = "String 2";   // "String 0" etc are strings to store - change to suit.
prog_char string_3[] PROGMEM = "String 3";

struct map_t
{
  prog_char *a;
  prog_char *b;
};

const map_t some[2] PROGMEM = {
  {string_0, string_1},
  {string_2, string_3}
} ;

void setup () {
    Serial.begin(9600);
    Serial.println(availableMemory());
    for (int i = 0; i < 2; i++){
      int N = strlen_P((char*)pgm_read_word(&(some[i].b)));
      char buffer[N];
      strcpy_P(buffer, (char*)pgm_read_word(&(some[i].a))); // Necessary casts and dereferencing, just copy. 
      Serial.println( buffer );
      strcpy_P(buffer, (char*)pgm_read_word(&(some[i].b))); // Necessary casts and dereferencing, just copy. 
      Serial.println( buffer );
    }
    delay( 500 );
}
void loop () {}

My best regards