Re: AW: Constant at fixed address

[David Brown <david.brown@hesbynett.no>]

On 25/02/2022 19:18, stefan@franke.ms wrote:
> 
> 
>> -----Ursprüngliche Nachricht-----
>> Von: Gcc-help <gcc-help-bounces+bebbo=bejy.net@gcc.gnu.org> Im
>> Auftrag von David Brown
>> Gesendet: Freitag, 25. Februar 2022 18:57
>> An: Martin Sebor <msebor@gmail.com>; Henrique Coser
>> <henrique.coser@tex.com.br>; gcc-help@gcc.gnu.org
>> Betreff: Re: Constant at fixed address
>>
>> On 25/02/2022 17:45, Martin Sebor via Gcc-help wrote:
>>> On 2/25/22 09:01, Henrique Coser wrote:
>>>> Hello,
>>>>
>>>> I need a help. I'm trying to solve a problem for weeks.
>>>> I have a embeded software that is a boot loader. It puts the boot
>>>> load version at a specific address.
>>>> My memmory starts at 0x400000 with 0x1400 size. My constant version
>>>> string value must be placed @0x401000 with 8bytes length.
>>>> If I place this const value into a section like this:
>>>>
>>>>   const unsigned char Version[8] __attribute__ ((section
>>>> (".bootversion")))  = "V1.0.1a";
>>>>
>>>> I got this error:
>>>>
>>>> section .bootversion LMA [00401000,00401007] overlaps section .text
>>>> LMA [00400000,00401013]collect2.exe(0,0): error: ld returned 1 exit
>>>> status
>>>>
>>>> I have already tried to split flash memmory using linker script but
>>>> it does not worked.
>>>> I wish to find something like "automatic" split.
>>>>
>>>> For example, this code was compiled using ARM Keil. With ARM Keil I
>>>> have the attribute that makes all the magic :
>>>>   const unsigned char Version[8] __attribute__((at(0x0401000))) =
>>>> "V1.0.1a";
>>>>
>>>> I dont know if is possible to have something as pratical as ARM Keil
>>>> attribute in GCC.
>>>
>>> GCC for the AVR target supports a couple of attributes that can be
>>> used to pin a variable declaration to a fixed address: address and io.
>>> It doesn't look to me like they're put in their own sections like in
>>> the ARM Keil compiler (but the section attribute can be used for
>>> that).
>>>
>>> Beside your use case, exposing at least the address attribute in all
>>> targets would make would also solve a long-standing problem with GCC
>>> issuing warnings for accesses to hardwired addresses).
>>>
>>> I suggest opening an enhancement request in Bugzilla.
>>>
>>> Martin
>>>
>>>
>>
>> I second that request - it would definitely be convenient to be able to put a
>> variable or section at a specific address without having to modify a linker
>> script.  This is a feature that most embedded toolchains (Keil, IAR, etc.)
>> support.
>>
>> Note that the attributes for the AVR here don't do what is needed, as far as I
>> can see - it looks like they declare the variable at the given address, but that
>> does not mean that there will be an absolute section allocated in the link.  In
>> other words, using the AVR "address"
>> attribute to put "Version" at address 0x0401000 will not actually put the
>> initialised data there, nor will it prevent the address being used by anything
>> else that is linked to that memory area.
>>
>> The example for the "address" attribute is :
>>
>> 	volatile int porta __attribute__((address (0x600)));
>>
>> The effect of this is very similar to the more common and portable version
>> used for other targets:
>>
>> 	#define porta *((volatile int *) 0x600)
>>
>> Henrique needs more than that here.
>>
>> AFAIK, Henrique, the only way to achieve your needs are a modified linker
>> script.  It's not hard to do that, but of course it looks hard the first time you
>> do it!  Post a copy of your current linker script and I can try to give you ideas
>> for modification.
>>
>>
> 
> Isn't this already possible by using a section:
> 
>    volatile int porta __attribute__((__section("section_600")));
> 
> and using an appropriate linker script?
> 
> SECTIONS
> {
>   .section_600 0x600: {
>    *(.section_600)
> ...
> 
> 
> Stefan
> 

Exactly, yes.  The point is that with gcc (and standard binutils) you
need to modify (or suppliment) the linker script to do this.  The OP is
used to a toolchain where you can specify this using an attribute in the
compiler alone, which is neater.