Re: [avr-gcc-list] C++ with PSTR

[Dave Hylands]

Hi,

On Sun, Dec 6, 2009 at 4:20 AM, darkschine <address@hidden> wrote:
>
> This topic is covered all over avr forums in bits and pieces so please keep
> responses to a minimum. The problem is that when using PSTR("String") in a
> cpp file, the compiler generates the warning:
>
> warning: only initialized variables can be placed into program memory area
...snip...

> Does anyone have a proven solution or workaround for this problem? Is there
> anything that can be done to at least suppress the warning without
> suppressing other warnings?

I think I found a way.

I tested this with WinAVR-20080610 (gcc 4.3.0) and with the latest
(WinAVR-20090313 gcc 4.3.2).

My source file, called test.cpp was as follows:

----- Start of test.cpp -----
#include <avr/pgmspace.h>

#if 0
#undef PROGMEM
#define PROGMEM __attribute__(( section(".progmem.data") ))

#undef PSTR
#define PSTR(s) (__extension__({static prog_char __c[] PROGMEM = (s);
&__c[0];}))
#endif

void Log_P( const char *fmt, ... );

void foo( void )
{
    Log_P( PSTR( "foo was called\n" ));
}
----- End of test.cpp -----

As shown above, I compiled it with the following command line:

603 >avr-g++ -mmcu=atmega168 -c test.cpp -o test1.o
test.cpp: In function 'void foo()':
test.cpp:15: warning: only initialized variables can be placed into
program memory area

and then ran

604 >avr-objdump --full-contents test1.o

test1.o:     file format elf32-avr

Contents of section .text:
 0000 df93cf93 cdb7deb7 80e090e0 00d0edb7  ................
 0010 feb73196 91838083 0e940000 0f900f90  ..1.............
 0020 cf91df91 0895                        ......
Contents of section .progmem.data:
 0000 666f6f20 77617320 63616c6c 65640a00  foo was called..

I then changed the #if 0 to #if 1 and repeated, replacing test1.o with test2.o

605 >avr-g++ -mmcu=atmega168 -c test.cpp -o test2.o
606 >avr-objdump --full-contents test2.o

test2.o:     file format elf32-avr

Contents of section .text:
 0000 df93cf93 cdb7deb7 80e090e0 00d0edb7  ................
 0010 feb73196 91838083 0e940000 0f900f90  ..1.............
 0020 cf91df91 0895                        ......
Contents of section .progmem.data:
 0000 666f6f20 77617320 63616c6c 65640a00  foo was called..

607 >cmp test1.o test2.o

shows that the two objects are in fact 100% identical, but the 2nd
version has no warning.

-- 
Dave Hylands
Shuswap, BC, Canada
http://www.DaveHylands.com/