[lld] r207141 - [PECOFF] Define implicit symbols for exported ones.

[Rui Ueyama]

No, it does not seem writable. The following program crashes at the first
line in main().

__declspec(dllexport) void foo(void) {}
extern void (*_imp__foo)(void);

int main() {
  _imp__foo = 0;
  return 0;
}


On Thu, Apr 24, 2014 at 2:08 PM, Reid Kleckner <rnk at google.com> wrote:

> Can I use it do this?
>
> __declspec(dllimport) void hello(void);
> extern void (*__imp__hello)(void);
> void my_hello(void) { ... }
> int main() {
>   __imp__hello = my_hello;
> }
>
> I think the __imp_ pointers are writable.
>
>
> On Thu, Apr 24, 2014 at 1:57 PM, Rui Ueyama <ruiu at google.com> wrote:
>
>> On Thu, Apr 24, 2014 at 1:29 PM, Chandler Carruth <chandlerc at google.com>wrote:
>>
>>>
>>> On Thu, Apr 24, 2014 at 1:12 PM, Rui Ueyama <ruiu at google.com> wrote:
>>>
>>>> Using __imp_ symbols locally is I think not a good coding style. One
>>>> should just take an address using "&" operator rather than appending
>>>> __imp_ prefix. However, there are programs in the wild that depends
>>>> on this link.exe's behavior, so we need this feature.
>>>>
>>>
>>> I wonder if we should have a warning about this?
>>>
>>
>> Ideally, yes, but there's no good way to print a warning message when
>> some symbol is actually linked. In this patch I create __imp_ symbols
>> unconditionally for all dllexported symbols. Creating them is not visible
>> to user -- using them should be warned.
>>
>> I also wonder whether the cause in the wild is source code being shared
>>> between the DLL and the users of the DLL and being written "generically".
>>>
>>
>> The program I spotted in the wild is ffmpeg. That has /include:__imp_
>> *sym* where *sym* is a defined function in ffmpeg (/include is
>> equivalent to --undefined). Not sure why it has such option as it is
>> basically no-op to link.exe.
>>
>> I don't think writing code generically cannot really justify to use
>> __imp_ symbols, as you can just use "&", so I'm still wondering what this
>> feature is really for...
>>
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>>
>>
>
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