Reading strings from structures in PROGMEM using a loop

Question

Found a solution, see my comment below!

I am trying to read data which is placed in structures and stored in the flash memory of an Arduino Mega (ATmega 2560) using PROGMEM. The structure objects manufacturer_1 and manufacturer_2 are accessed with pointers.

Due to size of the sketch; I decided to create a (relative) small example which illustrates the problem. The following code shows how I define the structures and data.

typedef struct
{
   char info[20];
} manufacturer_def;

typedef struct
{
   unsigned int totalManufacturers;
   const manufacturer_def* manufacturer[2];
} data_def;

const manufacturer_def manufacturer_1 PROGMEM =
{
   "Manufacturer 1"
};

const manufacturer_def manufacturer_2 PROGMEM =
{
   "Manufacturer 2"
};

const data_def data PROGMEM =
{
  2,
  {
    &manufacturer_1,
    &manufacturer_2
  }
};

void setup() 
{
  // Serial monitor setup
  Serial.begin(115200);   // Begin serial monitor
}

void loop() 
{
   mainMenu();
}

The problem!

I would like to fill an array with strings using a loop. The following code is not working properly:

void mainMenu()
{
   unsigned int i = 0;

   unsigned int totalMenuItems = pgm_read_word(&data.totalManufacturers);
   String menuItems[totalMenuItems];

   char str_buf[20];

   // Create array with items for menu
   for (i = 0; i < totalMenuItems; i++)
   {
     strcpy_P(str_buf, data.manufacturer[i]->info);
     menuItems[i] = str_buf;
     Serial.println(menuItems[i]);
   }
 }

Output (section):

p�









p�

Strangely, when I place the strcpy_P command outside the loop and specify the iteration variable by hand it works:

void mainMenu()
{
  unsigned int i = 0;

  unsigned int totalMenuItems = pgm_read_word(&data.totalManufacturers);
  String menuItems[totalMenuItems];

  char str_buf[20];

  strcpy_P(str_buf, data.manufacturer[0]->info);
  menuItems[0] = str_buf;
  strcpy_P(str_buf, data.manufacturer[1]->info);
  menuItems[1] = str_buf;

  // Create array with items for menu
  for (i = 0; i < totalMenuItems; i++)
  {
    Serial.println(menuItems[i]);
  }
}

Output:

Manufacturer 1
Manufacturer 2

Why is this happening?

I am using this code on an Arduino Mega (ATmega 1280) which I believe is merely a set of C/C++. — GleNnoS
Then it's either C or C++ (or a subset or superset of one of those). That said, your code very much looks like C (typedef structs for example, use of char buffers and C-strings). — tambre

drerD drerD · Accepted Answer · 2017-05-16T22:55:26

I think it has to do with PROGMEM is storing the variable in FLASH, instead of RAM. Read this documentation on PROGMEM, so when you don't use pgm_read_word_near() and dynamically accessing the FLASH-stored variable, there will be problem. But when you were using constants(literal):

strcpy_P(str_buf, data.manufacturer[0]->info);
menuItems[0] = str_buf;

to access the variable it's fine.

And the problem can manifest itself due to the implementation of strcpy_P().

So in that documentation they did this:

const char* const string_table[] PROGMEM = {string_0, string_1, string_2, string_3, string_4, string_5};

char buffer[30];    // make sure this is large enough for the largest string it must hold

void loop()
{
  /* Using the string table in program memory requires the use of special functions to retrieve the data.
     The strcpy_P function copies a string from program space to a string in RAM ("buffer").
     Make sure your receiving string in RAM  is large enough to hold whatever
     you are retrieving from program space. */


  for (int i = 0; i < 6; i++)
  {
    strcpy_P(buffer, (char*)pgm_read_word(&(string_table[i]))); // Necessary casts and dereferencing, just copy.
    Serial.println(buffer);
    delay( 500 );
  }
}

Reading strings from structures in PROGMEM using a loop

4 Answers