Possible bug in binutils for MinGW-w64 related to linking weak symbols

[Martin Storsjö]

On Thu, 19 Mar 2026, Martin Storsjö wrote:

> On Thu, 19 Mar 2026, Martin Storsjö wrote:
>
>> When you use a declaration of a weak symbol (which means that the symbol 
>> either may be defined, or may be undefined, when it essentially is a null 
>> pointer), then the symbol "bar" points at an absolute symbol with the 
>> address zero. The idea here is that if nothing else overrides the symbol, 
>> then it uses the fallback value of absolute zero (and in that case, the 
>> user should check if it is null before referencing it).
>
> Sorry, I misremembered one detail here. The default symbol it points at 
> isn't an absolute zero symbol, it's an undefined symbol.

Sorry again - I misinterpreted the output; what I said initially is indeed 
right - for a declaration (not definition) of a weak symbol, the default 
is an absolute null pointer.

And actually, using __attribute__((weak)) on the declaration when using 
the symbol isn't beneficial and consistent; it makes each object file 
using the symbol provide their own null pointer default for it. This means 
that depending on linking order, you either get a definition or a the null 
pointer default.

So if the declaration is weak, in translation units that use the symbol, 
then each of those uses also need to check if the symbol actually is 
nonnull.

It turns out that LLD (LLVM's linker) also does the same as GNU ld here; 
if the first object file is the one using it, we end up crashing in the 
same way here.

So IMO requiring the declarations to use weak isn't a fix, that makes it a 
gamble on which symbol the linker happens to see first. (However in 
practice, I haven't ever run into this combination in practice in code in 
the wild - using weak definitions in combinations with weak declarations 
doesn't seem to be common.)

We can avoid that gamble in LLD, by implementing preference between the 
different weak aliases like this though: 
https://github.com/mstorsjo/llvm-project/commit/lld-weak-prefer-nonnull

// Martin