[Patch 4.6] In system.h, wrap include of C++ header in 'extern C++'

[Duncan Sands]

Hi,

>> If ENABLE_BUILD_WITH_CXX is defined, then GCC itself is built with C++,
>> and we want a C++ signature for functions.  If it is not defined, then
>> GCC itself is not built with C++, and we want (and must have) a C
>> signature.
>>
>> I suppose we would decide that fancy_abort always uses a C signature,
>> but that seems odd.
>>
>> Ian
>
> I guess the issue is when people care only about C plugins, yet fancy_abort
> get implicitly exported with a C++ linkage.
>
> I suspect this goes back to the eternal question: what do we consider as
> part of the public GCC public API (no, Basile, I am not suggesting to have
> the same discussion again.)

if the following are to hold

(1) fancy_abort is declared in system.h
(2) system.h should not be wrapped in extern "C" when included from a plugin,
(3) it should be valid to include it from plugins compiled as C or as C++,
(4) fancy_abort should use the same linkage as GCC, i.e. C when GCC built as C,
C++ when built as C++ (aka ENABLE_BUILD_WITH_CXX).

then something like the following seems inevitable:

#ifdef ENABLE_BUILD_WITH_CXX
#ifdef __cplusplus
extern void fancy_abort(const char *, int, const char *) ATTRIBUTE_NORETURN;
#else
extern void _Z11fancy_abortPKciS0_(const char *, int, const char *) 
ATTRIBUTE_NORETURN;
#endif
#else
#ifdef __cplusplus
extern "C" void fancy_abort(const char *, int, const char *) ATTRIBUTE_NORETURN;
#else
extern void fancy_abort(const char *, int, const char *) ATTRIBUTE_NORETURN;
#endif
#endif

That's pretty nasty.  But to avoid the nastiness one of (1) - (4) needs to be
dropped.  Which one?

Ciao, Duncan.