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 <zhpctw3d...@hotmail.com> 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.
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