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

[Paul Stoffregen]

I have a perhaps related question.  It is possible to test at compile 
time if a variable is a string literal, so I could have a macro that 
automatically embeds the PSTR() and _P stuff, but also works with normal 
arrays?

For example, I often use this.

#define print(s) print_P(PSTR(s))

It's nice to just write print("something").  Of course, it only works if 
the something is a string literal.

Maybe some combination of __builtin_types_compatible_p or _built_const_p 
or typeof?  I tried playing with these but could never seem to make a 
macro that resolves one way if the arg is a string literal and another 
way if it's a pointer or anything else.


-Paul



Dave Hylands wrote:
> 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.
>
>