Q: Weak symbol defaulting to an other symbol?

[Georg-Johann Lay]

Am 24.11.25 um 08:52 schrieb Jan Beulich:
> On 23.11.2025 12:54, Georg-Johann Lay via Binutils wrote:
>> In an assembler module, I would like to introduce a weak symbol like in:
>>
>>       .weak my_symbol
>>       my_symbol = __heap_start
>>       ;; use my_symbol...
>>
>> where __heap_start is defined in the linker script as
>>
>>       PROVIDE (__heap_start = .)
>>
>> The code will be part of a static library.
>>
>> The generated code when linking against the static lib is as
>> expected.  Though what doesn't work is to let the user adjust
>> my_symbol when they don't like the default, e.g. by
>>       --defsym my_symbol=0x12324
>> or by code like
>>
>>       .global my_symbol
>>       my_symbol = 0x12345
>>
>> in some module.
>>
>> Binutils version is 2.45.  So I already found out that overriding
>> the default doesn't work, and that's likely on purpose.
>>
>> In order to get a better understanding, I'd like to ask why this
>> doesn't work, and what's the thinko in my intended design? Having
>> a user-adjustable symbol that defaults to some symbol value seems
>> to be a useful feature.
> 
> The problem is the use of an equate in combination with an undefined
> (external) symbol. The assembler resolves the equate, as you will be
> able to see when you dump the generated object. The recorded relocation
> references __heap_start instead. See also how e.g. gcc's "alias"

Hi Jan,

Thank you for the explanation.

Johann

> attribute is only specified to work with defined symbols. (The test in
> the doc says "declared", but really means "defined", as can be easily
> checked by converting the function definition of __f() in the example
> to a function declaration.)
> 
> Jan