Avoid includes within extern C namespace

[carlopi]

Line 244 of xxhash.h has a:

#if defined (__cplusplus)
extern "C" {
#endif

that is followed up by includes of system-provided headers, that are not guaranteed to work within an extern C linkage rules.

In particular, while compiling apache-arrow with Emscripten, I get:

/Users/carlo/emsdk/upstream/emscripten/cache/sysroot/include/emscripten/em_asm.h:151:1: error: templates must have C++ linkage
  151 | template<typename, typename = void> struct __em_asm_sig {};
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Users/carlo/duckdb-wasm/submodules/arrow/cpp/src/arrow/vendored/xxhash/xxhash.h:173:1: note: extern "C" language linkage specification begins here
  173 | extern "C" {
      | ^
In file included from /Users/carlo/duckdb-wasm/submodules/arrow/cpp/src/arrow/compute/kernels/vector_hash.cc:27:
In file included from /Users/carlo/duckdb-wasm/submodules/arrow/cpp/src/arrow/array/dict_internal.h:37:
In file included from /Users/carlo/duckdb-wasm/submodules/arrow/cpp/src/arrow/util/hashing.h:49:
In file included from /Users/carlo/duckdb-wasm/submodules/arrow/cpp/src/arrow/vendored/xxhash.h:18:
In file included from /Users/carlo/duckdb-wasm/submodules/arrow/cpp/src/arrow/vendored/xxhash/xxhash.h:3434:
In file included from /Users/carlo/emsdk/upstream/emscripten/cache/sysroot/include/compat/arm_neon.h:293:
In file included from /Users/carlo/emsdk/upstream/emscripten/cache/sysroot/include/emscripten.h:1:

that make compilation fail.

Current workaround, very much ad-hoc, is explicitly including <emscripten.h> (conditionally on EMSCRIPTEN being defined) before including xxhash.h. This works, but it's not great and solves only this specific case. PR is at duckdb/duckdb-wasm#1803.

Would it be possible to move the includes outside of the extern C namespace, given I believe not to be compliant with C++ compilers?

Thanks a lot.

[Cyan4973]

possibly ?
The "generic" headers (like <stddef.h>) can most likely be pushed outside,
but the "specific" ones (like <emscriptem.h>) are only included if some specific conditions are met, and it would be necessary to replicate these conditions outside of extern "C" block, which now becomes must less desirable for code maintenance.

To be investigated.

[Cyan4973]

What would make sense is to fold this objective into the planned effort to separate the source code into multiple small units.
There it will be easier to pay attention to #include outside of the extern "C" block

[carlopi]

Thanks for the reply!
Possibly a way out, altought very verbose, is closing and then reopening the extern C brackets, something like:

#if defined(__cplusplus)
extern `C`
#endif

// some code

#if something
#include <x>
#endif

// some more code

#if defined(__cplusplus)
}
#endif

to

#if defined(__cplusplus)
extern `C`
#endif

// some code

#if defined(__cplusplus)
}
#endif

#if something
#include <x>
#endif

#if define(__cplusplus)
extern `C`
#endif

// some more code

#if defined(__cplusplus)
}
#endif

unsure if doing this via macro or something would make sense.

[Cyan4973]

There are limits (one cannot generate macros with macros), but yeah, something like that is indeed possible.
It just will fit more naturally in a multi-units design, from which a single amalgamated xxhash.h could still be generated.

[t-mat]

As another possible solution, we'll be able to use the following inline extern C style

extern "C" void my_public_func(void);

We can also define it as a macro

#if defined(__cplusplus)
#define MY_PUBLIC_API extern "C"
#endif

MY_PUBLIC_API void my_public_func(void);

Msvc's dllexport is able to be supported in the same style.