[llvm-dev] [lld] Alias in COFF short import library.

[Rui Ueyama via llvm-dev]

newfun.obj seems broken -- dumpbin couldn't read.

On Thu, Aug 13, 2015 at 3:19 PM, Martell Malone <martellmalone at gmail.com>
wrote:

> If you want to define an alias symbol "bar" to "foo" (which is an
>> extension you want to provide), one way is to create an object file that
>> defines "bar" and "__imp_bar" as aliases to "foo" and "__imp_foo",
>> respectively, and add that object file to the import library. As a result,
>> the import library file defines four symbols, {,__imp_}{foo,bar}.
>>
>> Do you know how to define symbol aliases in COFF? If you don't, take a
>> look a member of oldnames.lib (which comes with MSVC). That library file
>> defines bunch of aliases, and you can do the same thing.
>>
> Thanks for pointing me in the right direction :)
>
> After looking at oldnames.lib I done a little more reading and I assume I
> do it like this then for the alias.
>
> The import table points to the functions like this.
> __imp_oldfu -> newfun.obi
> oldfu -> newfun.obj
> I have attached the 2 files named here.
>
> I assume this is what you meant by Weak Externals ?
>

Yes, that's what I meant.

>
>
>
> On Thu, Aug 13, 2015 at 6:36 AM, Rui Ueyama <ruiu at google.com> wrote:
>
>> Hmm, that's not correct too. All symbols listed in the archive header
>> must be defined by some member in the archive. In other words, the header
>> is created just by extracting all external symbols from members. It's not
>> doable to rename symbol A to B by writing B to the archive file header.
>>
>> By aliasing, I was suggesting adding an extra object file to an archive
>> file containing a short import file. The extra object file is to define
>> aliases to symbols defined by the import library.
>>
>> Say, you are creating a DLL which defines a function symbol "foo". You
>> need to create an import library that defines "foo" and "__imp_foo". This
>> is what normal linkers do.
>>
>> If you want to define an alias symbol "bar" to "foo" (which is an
>> extension you want to provide), one way is to create an object file that
>> defines "bar" and "__imp_bar" as aliases to "foo" and "__imp_foo",
>> respectively, and add that object file to the import library. As a result,
>> the import library file defines four symbols, {,__imp_}{foo,bar}.
>>
>> Do you know how to define symbol aliases in COFF? If you don't, take a
>> look a member of oldnames.lib (which comes with MSVC). That library file
>> defines bunch of aliases, and you can do the same thing.
>>
>> On Thu, Aug 13, 2015 at 2:25 PM, Martell Malone <martellmalone at gmail.com>
>> wrote:
>>
>>> The header of libuser32b.a says that it defines MessageBoxB and
>>>> __imp_MessageBoxB, but the import library file in the archive actually
>>>> defines MessageBoxA (not B). So the archive file is broken. You may want to
>>>> fix the header and try again.
>>>
>>> Yes this is how I done the alias.
>>>
>>> If you consider this invalid then look at libuser32.a
>>> The header defines MessageBoxA and __imp_MessageBoxB.
>>> This should be valid yes ?
>>>
>>>
>>> On Thu, Aug 13, 2015 at 6:19 AM, Rui Ueyama <ruiu at google.com> wrote:
>>>
>>>> The header of libuser32b.a says that it defines MessageBoxB and
>>>> __imp_MessageBoxB, but the import library file in the archive actually
>>>> defines MessageBoxA (not B). So the archive file is broken. You may want to
>>>> fix the header and try again.
>>>>
>>>>
>>>> On Thu, Aug 13, 2015 at 2:04 PM, Martell Malone <
>>>> martellmalone at gmail.com> wrote:
>>>>
>>>>> Hi Rui,
>>>>>
>>>>> I have finished my tool to generate short import libs for mingw-w64.
>>>>>
>>>>> It works fine with lld for actual symbols.
>>>>> But it seems lld segfaults when I try to use and an alias as we
>>>>> discussed before.
>>>>>
>>>>> I have attached 3 import libraries.
>>>>> They all only have 1 import function.
>>>>>
>>>>> libuser32orig.a -> Works fine and only has MessageBoxA
>>>>>
>>>>> libuser32b.a -> the symbol table has MessageBoxB and __imp_MessageBoxB
>>>>> but the Import object has MessageBoxA.user32.dll.
>>>>>
>>>>> This causes a segfault in lld but should link correctly right ?
>>>>>
>>>>> Additionally I have attached libuser32.a
>>>>> This is the same as libuser32b.a but the symbol table has MessageBoxA
>>>>> and __imp_MessageBoxB rather then MessageBoxB and __imp_MessageBoxB
>>>>>
>>>>> This also segfaults I assume due to the same bug.
>>>>>
>>>>> The reason I created this iteration is because we should be able to in
>>>>> the import library just add extra __imp_{alias} to the symbol tables that
>>>>> point to the original import of MessageBoxA.
>>>>>
>>>>> Other then alias support the only difference between my new tool and
>>>>> lib.exe is that I don't bother to create .debug sections as we don't need
>>>>> them.
>>>>>
>>>>> Kind Regards
>>>>> Martell
>>>>>
>>>>
>>>>
>>>
>>
>
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