[Mesa-dev] [PATCH 3/8] glapi/check_table: Remove 'extern "C"' block

[Emil Velikov]

On 4 December 2017 at 23:57, Dylan Baker <dylan at pnwbakers.com> wrote:
> Quoting Emil Velikov (2017-11-23 11:04:34)
>> On 20 November 2017 at 23:12, Dylan Baker <dylan at pnwbakers.com> wrote:
>> > This doesn't actually accomplish what it's meant to do, as extern C
>> > doesn't undefine __cplusplus, so the included headers define a template
>> > (because __cplusplus is defined), but then that code is in an 'extern
>> > "C"' block, and explosion.
>> >
>> The commit does sound a bit off, admittedly I don't fully grok what
>> you're trying to say.
>>
>> The extern "C" { } construct annotates anything within as if it were a
>> normal C code. Thus functions will have the C linkage (otherwise
>> they'll have the C++ mangling) and structs are using the C rules.
>> So if you have a C++ header further down the chain it will be
>> considered as C and hell will break loose.
>>
>> This patch is correct, just sent a fix for glapitable.h, while glapi.h
>> was handled with d38839a498b38b745f5bdf6b96fbafa2792670be.
>>
>> As-is "struct _glapi_table" will be interpret as C++ one, which may
>> not have the same guarantees about sizeof and pointer arithmetic that
>> we're using in the test.
>>
>> Hope the above provides some inspiration towards a better commit message.
>>
>> Thanks
>> Emil
>
> So here's the problem I run into:
>
> In file included from ../include/c99_compat.h:28:0,
>                  from ../src/util/macros.h:29,
>                  from ../src/mapi/glapi/glapi.h:47,
>                  from ../src/mapi/glapi/tests/check_table.cpp:28:
> ../include/no_extern_c.h:47:1: error: template with C linkage
>  template<class T> class _IncludeInsideExternCNotPortable;
>  ^~~~~~~~
> [54/93] Compiling C++ object 'src/gtest/gtest at sta/src_gtest-all.cc.o'.
>
> So, my commit message still makes sense to me, since you can't put a template in
> an extern "C" block, and that template is guarded by __cplusplus, which is still
> defined even in an extern "C" block.
>
This is one symptom, of the issue described before. The code in
c99_compat.h was explicitly added by Jose to make the issue obvious.

> Do you have a commit message in mind that is better?

Feel free to reuse as much or as little of my explanation. Borrowing
from analogous fixes also works ;-)
Some examples:
237dcb4aa7c39c59bfd225ae3d73caf709be216d
a177a13033cf9356eb26e8757055037a54268a18
36ad8265489fc66ab45b9b74dafa353a93bdb03b

I'm going through 4/8 - seems like there's a bunch of other bugs around :-\
Feel free to push the remainder of the series with the nitpicks addressed.

Thanks
Emil