On 2/2/2022 4:17 AM, kinke wrote:
> On Wednesday, 2 February 2022 at 07:33:58 UTC, Basile B. wrote:
>> ```d
>> else version (CRuntime_Microsoft)
>> {
>>     ///
>>     shared FILE* stdin;  // = &__iob_func()[0];
>>     ///
>>     shared FILE* stdout; // = &__iob_func()[1];
>>     ///
>>     shared FILE* stderr; // = &__iob_func()[2];
>> }
>> ```
>>
>> shouldn't these variable declarations be `extern` ?
> 
> Nope, they are macros in the MSVC headers, no symbols are available. That's why druntime defines them and sets them up at startup, so they aren't available/linked with `-betterC`.

The trouble is -betterC does not link with druntime, so it can't find stderr. The solution is to make stderr a template, so that no linking with druntime is necessary.

On Wednesday, 2 February 2022 at 20:58:25 UTC, Walter Bright wrote:
> On 2/2/2022 4:17 AM, kinke wrote:
>> On Wednesday, 2 February 2022 at 07:33:58 UTC, Basile B. wrote:
>>> ```d
>>> else version (CRuntime_Microsoft)
>>> {
>>>     ///
>>>     shared FILE* stdin;  // = &__iob_func()[0];
>>>     ///
>>>     shared FILE* stdout; // = &__iob_func()[1];
>>>     ///
>>>     shared FILE* stderr; // = &__iob_func()[2];
>>> }
>>> ```
>>>
>>> shouldn't these variable declarations be `extern` ?
>> 
>> Nope, they are macros in the MSVC headers, no symbols are available. That's why druntime defines them and sets them up at startup, so they aren't available/linked with `-betterC`.
>
> The trouble is -betterC does not link with druntime, so it can't find stderr. The solution is to make stderr a template, so that no linking with druntime is necessary.

That's not a solution, but a hack. ;) - With at least 2 major problems:

* Template culling. If the compiler e.g. sees a stderr instantiation in druntime in the import graph, it may decide not to emit the stderr symbol into the betterC binary, assuming that druntime will be linked anyway.
* Potentially a DSO-local stderr symbol for each binary (executable or DLL/.so/.dylib), so that changing it in one DSO might not change it for the whole process.

There have been previous 'dummy-templatizations' in druntime/Phobos to work around missing symbols with -betterC. IMO, that's just patching over the cracks of the -betterC hack and pessimizing regular D, and in case it's not obvious, I'm definitely no fan of this. :)

On 2/3/2022 11:55 AM, kinke wrote:
> * Template culling. If the compiler e.g. sees a stderr instantiation in druntime in the import graph, it may decide not to emit the stderr symbol into the betterC binary, assuming that druntime will be linked anyway.
> * Potentially a DSO-local stderr symbol for each binary (executable or DLL/.so/.dylib), so that changing it in one DSO might not change it for the whole process.
> 
> There have been previous 'dummy-templatizations' in druntime/Phobos to work around missing symbols with -betterC. IMO, that's just patching over the cracks of the -betterC hack and pessimizing regular D, and in case it's not obvious, I'm definitely no fan of this. :)

"Header only" libraries work well for C++. We can make it work for D. In fact, we have to.

As for stderr, it should never be an instance of the iob itself. That instance resides only in the C stdlib. stderr should only be an indirect reference to it, hence multiple instances of stderr should work just fine.

With a bit of careful library design, we can make this work.