Re: [avr-gcc-list] prog_mem limitation?

[Mark Litwack]

On Tuesday 13 May 2008 05:36:25 am Marc Wetzel wrote:
> Hi,
>
> I just found out the hard way, that the allocation of prog_mem variables
> is not location independend.
>
> So, if you alloc a variable outside of a function, e.g. in the
> "standard" top area of your c-file, everything is working as expected.
> But if you declare it inside a function, it does not work (as expected).
>
> Working Example:
> --
> const prog_mem char teststring[]= "Teststring";
>
> void * test(void) {
>     fprintf_P(uart, teststring);
> }
> --
>
> Non-Working Example (but compiling fine):
> --
> void * test(void) {
>     const prog_mem char teststring[]= "Teststring";
>     fprintf_P(uart, teststring);
> }
> --
>
> Is this the intended behaviour?
> What cause is here the trigger?


I'm not eaxctly sure why they behave differently, but the
latter causes teststring[] to be allocated in the .data
section instead of .progmem.  So, a regular fprintf() would
work.

You might want to use PSTR() and the other pgmspace.h macros
instead.  For your example it would be:

  void * test(void) {
    PGM_P teststring = PSTR("Teststring");
    fprintf_P(uart, teststring);
  }

Or even:

  void * test(void) {
    fprintf_P(uart, PSTR("Teststring"));
  }

These work as expected.

-mark



>
> TIA
> Marc
>
>
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