Question re: Usage of JNICALL with varargs function with 32bit compilers

[Thomas Stüfe]

Hi Steve,

I always saw this as a harmless warning, safe to ignore, since both caller
and callee are forced back to the same convention so it all works out in
the end. Or can you envision a scenario where this would be harmfull?

Thank god Microsoft abandoned this calling convention circus with 64bit.

Cheers, Thomas


On Tue, Jun 18, 2019 at 1:31 PM Steve Groeger <GROEGES at uk.ibm.com> wrote:

> Hi all,
>
> I had a query regarding the use of JNICALL with varargs functions, which I
> am hoping someone can comment on.
>
> As explained on MSDN at [1], all functions declared as __stdcall that have
> variable parameters (varargs) will be forced back to __cdecl by the MSVC.
> In such case, keeping JNICALL for varargs functions/function pointers
> turns out be inappropriate.
> e.g.
>
> include/jni.h
>     jobject (JNICALL *NewObject)
>       (JNIEnv *env, jclass clazz, jmethodID methodID, ...);
>
> According to [2], stdcall does not support variadic calls in C on 32-bit
> x86 targets.
>
> When compiling a JNI native (calling NewObject) with Clang, it ends up
> with the warning as follows:
> ...\include\jni.h:277:14: warning: stdcall calling convention ignored on
> variadic function [-Wignored-attributes]
>     jobject (JNICALL *NewObject)
>              ^
>
> So basically the compiler is saying it can't do what was requested, as
> such should we stop asking for JNICALL on varargs functions.
> Doing so will only improve the portability of code to a wider set of
> compilers.
>
> Comments?
>
> [1] https://msdn.microsoft.com/en-us/library/zxk0tw93.aspx
> [2] https://clang.llvm.org/docs/AttributeReference.html#stdcall
>
> Thanks
> Steve Groeger
> IBM Runtime Technologies
> Hursley, Winchester
> Tel: (44) 1962 816911  Mobex: 279990  Mobile: 07718 517 129
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>
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> Unless stated otherwise above:
> IBM United Kingdom Limited - Registered in England and Wales with number
> 741598.
> Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU
>
>