Fix for "only initialized variables can be placed into program memory area" warning

[Lauszus]

As discussed on the developer mailing list: https://groups.google.com/a/arduino.cc/forum/#!msg/developers/5iaId6nPzqA/jpgd38UdBqQJ.

Here is how I got rid of the incorrect "only initialized variables can be placed into program memory area" warning: https://github.com/felis/USB_Host_Shield_2.0/blob/f676812f84b3212779365b9950927a01209e4045/settings.h#L82-L96. This was done for the USB Host library, but could easily be used in the Arduino core as well.

See: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34734.
The fixed is based on the following comment: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34734#c4.

I have tested it myself on my local copy of the Arduino IDE and it works fine. I can send a pull request for that if you like?

[matthijskooijman]

I wonder if this workaround would have other side effects. A quick test using below test program shows the generated code (I compared avr-objdump -D output) is identical in both cases, so that sounds good.

  #include <avr/pgmspace.h> 

  //PROGMEM const int foo = 0xdead;
  const int foo __attribute__((section(".progmem.data"))) = 0xdead;

  int main() {
      return pgm_read_word(&foo);
  }

[cmaglie]

@Lauszus after applying your patch the following code:

prog_char a0[] PROGMEM = "Hello!";
prog_char a1[] PROGMEM = "Hi!";

PROGMEM const char *b[] = { a0, a1 };

void setup() {}
void loop() {}

gives an error:

sketch_feb05b.ino:5: error: b causes a section type conflict

any suggestion?

[cmaglie]

For the record, the Robot Control library uses this in:

utility/scripts_Hello_User.h

[Lauszus]

@cmaglie it looks like the fix is not that good after all then. Anyway this will be fixed when the toolchain hopefully gets updated ;)

[cmaglie]

This was fixed from 1.5.6 with the new toolchain. Closing...