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

Martell Malone via llvm-dev
2015-08-13T06:19:01Z

>
> 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 ?



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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