Re: [avr-gcc-list] printf and printf_P suggestions

[Joerg Wunsch]

(Please start a new thread for a new topic, rather than replying to an
old mail.)

"Trampas" <address@hidden> wrote:

> ..., however if you have:
> 
> const char data[]="hello world";
> 
> printf(data);

> This code is a problem, actually the compiler should catch the
> error.

There is no error to catch here.

> However what I did was make it such that in the printf() routine I
> checked the address of the format string. Thus if the address was in
> flash I knew to call the printf_P() version if it was in RAM then
> standard printf was used.

This is not possible on the AVR, as it is a true Harvard: the routine
gets passed a pointer, and from looking at the address, you cannot say
whether it's ROM or RAM -- it's up to the callee to interpret it his
way.  That's why there are _P() pendants.  If the function gets an
address 0 passed, it could point to the beginning of ROM or of RAM
(the latter would actually constitute an alias for register r0, but
that's another issue).

AVR-GCC marks the RAM addresses by an offset of 0x800000 internally
(and EEPROM addresses with 0x810000), but that's just a convention
followed inside the object files only: when it gets translated to the
target system (e. g. by producing an ihex file with avr-objcopy), this
offset gets filtered away.  It's only a virtual flat address space, as
that is what GCC currently wants to work with.

> I would personally like to have a command line option to tell the
> compiler not to copy the strings to data memory in the above case.

This would make any and all strings violate the C standard (they
became incompatible with standard <string.h> functions).

> The reason I personally would like such features is that I am
> porting a large amount of code which has:

> sprintf(str, "results %d\n",data); 

> Thus having the option would save me a lot of data memory and/or
> time correcting the sprintf calls to sprintf_P.

Using an editor that understands regular expressions, it's a matter of
5.2 minutes to convert these: five minutes to think over the RE, and
0.2 minutes to run the actual conversion. ;-)

For example, for the classical Unix editor vi, you might try this:

:g/sprintf/s/sprintf(\([^"]*\)\(".*"\)/sprintf_P(\1 PSTR(\2)/

-- 

J"org Wunsch                                           Unix support engineer
address@hidden        http://www.interface-systems.de/~j/