Why two const qualifiers required in PROGMEM definition?

Projects Programming
1

I have recently upgraded my Arduino to avr-gcc 4.7.0 (and supporting libraries etc) and need to modify my sketches to suit.

Th Progmem syntax has changed and to get it right I wrote a small test sketch as shown below:

//#include <avr/io.h>
#include <avr/pgmspace.h>

PROGMEM const char MenuItem1[] = "Menu Item 1";
PROGMEM const char MenuItem2[] = "Menu Item 2";
PROGMEM const char MenuItem3[] = "Menu Item 3";

PROGMEM const char* const MenuItemPointers[] = {MenuItem1, MenuItem2, MenuItem3};

void setup(void)
{
  char buffer[20];
  strcpy_P(buffer, (char*)pgm_read_word(&MenuItemPointers[1]));
}

void loop(void)
{  
}

This compiles without errors however I don't understand why it requires the two constant qualifiers rather than just the first one in the line:

PROGMEM const char* const MenuItemPointers[] = {MenuItem1, MenuItem2, MenuItem3};

Can someone enlighten me?

2

I believe that says that MenuItemPointers won't change, and neither will the things the table entries point to.

3

Thanks afremont.

That clarifies the the code

Its would seem that the current 'standard' Arduino release (that only requires the first const) is more lax and would allow "things the table entries point to" to be changed in flash.

4

Its would seem that the current 'standard' Arduino release (that only requires the first const) is more lax and would allow "things the table entries point to" to be changed in flash.

True. But, since flash is read only, that hardly seems possible.