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Re: Patch fixed strings in .hex file

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On 16/01/2024 15:42, pozz wrote:
> Il 16/01/2024 13:51, David Brown ha scritto:
>> On 16/01/2024 13:19, pozz wrote:
>>> In one project I have many quasi-fixed strings that I'd like to keep 
>>> in non volatile memory (Flash) to avoid losing precious RAM space.
>>>
>>> static const char s1[] = "/my/very/long/string/of/01020304";
>>> static const char s2[] = "/another/string/01020304";
>>> ...
>>>
>>> Substring "01020304" is a serial number that changes during 
>>> production with specific device. It has the same length in bytes 
>>> (it's a simple hex representation of a 32-bits integer).
>>>
>>> Of course it's too difficult and slow to rebuild the firmware during 
>>> production passing to the compiler the real serial number. I think a 
>>> better solution is to patch the .hex file generated by the compiler.
>>>
>>> I'm wondering how to detect the exact positions (addresses) of serial 
>>> numbers to fix.
>>>
>>> The build system is gcc, so I could search for s1 in the elf file. Do 
>>> you know of a tool that returns the address of a symbol in the elf or 
>>> map file?
>>>
>>> Could you suggest a better approach?
>>>
>>
>> In the source code, put the serial number in as "PQRXYZ" or some other 
>> distinct string of characters.  Generate bin files, not hex (or 
>> convert with objcopy).  Then do a simple search for the special string 
>> to find its position and replace it with the serial number using a 
>> simple Python script or your other favourite tool (awk, sed, perl, 
>> whatever).
> 
> I thought about this approach, but is it so difficult to have the same 
> exact sequence of bytes somewhere else in the output?

Try it and see.

> 
> 
>> Oh, and in the source code, don't forget to make the string "volatile".
> 
> Why?
> 

If you have :

	static const char s1[] = "PQRXYZ";

and your code later does, say :

	const int last_digit = s1[5] - '0';

the compiler will optimise it to :

	const int last_digit = '*';

i.e., it will calculate 'Z' - '0' at compile time - and if I remember by 
ASCII codes correctly, that matches '*'.

You will be messing with the string behind the compiler's back.  Make it 
volatile.  "volatile const" might be unusual, but it is useful in 
exactly this kind of circumstance.

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Patch fixed strings in .hex file pozz <pozzugno@gmail.com> - 2024-01-16 13:19 +0100
  Re: Patch fixed strings in .hex file David Brown <david.brown@hesbynett.no> - 2024-01-16 13:51 +0100
    Re: Patch fixed strings in .hex file pozz <pozzugno@gmail.com> - 2024-01-16 15:42 +0100
      Re: Patch fixed strings in .hex file David Brown <david.brown@hesbynett.no> - 2024-01-16 16:36 +0100
        Re: Patch fixed strings in .hex file pozz <pozzugno@gmail.com> - 2024-01-16 16:57 +0100
      Re: Patch fixed strings in .hex file dalai lamah <antonio12358@hotmail.com> - 2024-01-16 16:47 +0100
        Re: Patch fixed strings in .hex file David Brown <david.brown@hesbynett.no> - 2024-01-16 18:50 +0100
  Re: Patch fixed strings in .hex file Herbert Kleebauer <klee@unibwm.de> - 2024-01-16 15:07 +0100
    Re: Patch fixed strings in .hex file pozz <pozzugno@gmail.com> - 2024-01-16 15:42 +0100
  Re: Patch fixed strings in .hex file Grant Edwards <invalid@invalid.invalid> - 2024-01-16 15:24 +0000
    Re: Patch fixed strings in .hex file David Brown <david.brown@hesbynett.no> - 2024-01-16 16:38 +0100
      Re: Patch fixed strings in .hex file Grant Edwards <invalid@invalid.invalid> - 2024-01-16 18:32 +0000
    Re: Patch fixed strings in .hex file pozz <pozzugno@gmail.com> - 2024-01-16 17:01 +0100
  Re: Patch fixed strings in .hex file David Brown <david.brown@hesbynett.no> - 2024-01-16 16:39 +0100
    Re: Patch fixed strings in .hex file Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2024-01-17 20:19 +0200
  Re: Patch fixed strings in .hex file Stefan Reuther <stefan.news@arcor.de> - 2024-01-16 17:44 +0100
    Re: Patch fixed strings in .hex file Grant Edwards <invalid@invalid.invalid> - 2024-01-16 18:39 +0000
    Re: Patch fixed strings in .hex file Michael Schwingen <news-1513678000@discworld.dascon.de> - 2024-01-16 19:30 +0000
  Re: Patch fixed strings in .hex file Hans-Bernhard Bröker <HBBroeker@gmail.com> - 2024-01-16 19:35 +0100
    Re: Patch fixed strings in .hex file pozz <pozzugno@gmail.com> - 2024-01-17 08:45 +0100
      Re: Patch fixed strings in .hex file pozz <pozzugno@gmail.com> - 2024-01-17 09:07 +0100
      Re: Patch fixed strings in .hex file David Brown <david.brown@hesbynett.no> - 2024-01-17 11:27 +0100
        Re: Patch fixed strings in .hex file pozz <pozzugno@gmail.com> - 2024-01-17 12:54 +0100
          Re: Patch fixed strings in .hex file David Brown <david.brown@hesbynett.no> - 2024-01-17 13:54 +0100
          Re: Patch fixed strings in .hex file Stefan Reuther <stefan.news@arcor.de> - 2024-01-17 17:39 +0100
            Re: Patch fixed strings in .hex file David Brown <david.brown@hesbynett.no> - 2024-01-17 18:28 +0100
            Re: Patch fixed strings in .hex file Grant Edwards <invalid@invalid.invalid> - 2024-01-17 19:00 +0000
            Re: Patch fixed strings in .hex file pozz <pozzugno@gmail.com> - 2024-01-19 09:10 +0100
      Re: Patch fixed strings in .hex file Hans-Bernhard Bröker <HBBroeker@gmail.com> - 2024-01-18 18:49 +0100
  Re: Patch fixed strings in .hex file pozz <pozzugno@gmail.com> - 2024-01-17 09:57 +0100
  Re: Patch fixed strings in .hex file Don Y <blockedofcourse@foo.invalid> - 2024-01-17 10:03 -0700
  Re: Patch fixed strings in .hex file Paul Rubin <no.email@nospam.invalid> - 2024-01-17 21:42 -0800

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