Re: Constant at fixed address

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

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.