Proposal: Use new JDK_EXPORT decorator instead of JNIEXPORT

Magnus Ihse Bursie magnus.ihse.bursie at
Thu Dec 13 10:37:52 UTC 2018

On 2018-12-12 13:17, David Holmes wrote:
> Okay I went away and did some homework ...
> Let me back up a bit and see if I have the evolution of this correctly.
> The native implementation of Java methods should be declared and 
> defined with JNIEXPORT and JNICALL.
> JNIEXPORT controls the export visibility so they can be linked.
> JNICALL controls the calling convention and is needed so that the 
> JVM's calling convention matches the native code. [This part was 
> unclear to me.]
Yes. And JNICALL is empty on all platforms except Windows 32, that's why 
we're only seeing issues about mismatch there.
> Other native methods exported from the same (or different) native 
> libraries may also be decorated with JNIEXPORT and JNICALL. But in 
> places there is a mismatch between the declaration in the header and 
> the definition leading to errors.
Yes; this mismatch has typically happened when the linking has not been 
done by simply including the relevant header file, but either 
duplicating the definition, or looking up the symbol dynamically. 
Otherwise things should basically work out.
> There are two main types of such native functions:
> a) those publicly defined in the various native APIs: JNI itself 
> (jni.h), JVM TI (jvmti.h), AWT (jawt.h) ...
> b) those intended only for use by other native code within the JDK 
> libraries (JLI_* - though I note Alan's comment re javafxpackager, 
> others??)
> and a possible third type are callback functions like the JPLISAgent 
> event handlers (e.g. eventHandlerVMInit).

I'm not sure I understand what the third type is, but if it is a 
publically defined API (which, at a first look, the JPLISAgent API seems 
to be), then it's part of catagory a, otherwise it's part of category b.

I must also state that my initial proposal didn't separate these two 
cases. I had in mind only the b cases -- I have no intention of changing 
public specifications, but I did not express this in my proposal. That 
might have been one source of confusion. I apologize for this omission.
> There is a view that no native method that isn't the native-half of a 
> Java method should use JNICALL. [Okay I can see how that makes sense - 
> regardless of the actual calling convention used marking such methods 
> as "must use the same calling convention as the VM native method call" 
> is somewhat meaningless. Yet obviously the public native APIs do 
> specify JNICALL so this is not a hard and fast rule. Further the 
> callbacks must also follow a convention known to the VM doing the 
> calling!]
Yes, or rather the rule is "only native half Java methods should use 
JNICALL, and also all public APIs where so is specified".

> Where we have (introduced?) a discrepancy between the definition and 
> declaration the approach has been (because of the previous view) to 
> remove JNICALL. [This should only relate to methods of kind (b) above.]
Actually, it's not so much as we have *removed* JNICALL, as that we have 
*introduced* JNIEXPORT. When I removed the map files, I also removed the 
.def files and /export command lines for Windows. As a result, I needed 
to add JNIEXPORT to a lot of places. Initially, I followed the rule of 
adding JNICALL to those calls -- but I can surely have missed a couple 
of places, since things will work fine anyway, as long as caller and 
callee agree on the calling convention; and a mismatch can only happen 
on Windows 32, which we do not build and test. So there is a risk that 
even with the initial patch, not all newly added JNIEXPORTs had JNICALL.

Then, it turned out, that a lot of this code did not compile with 
Windows 32. With no deeper analysis of the flaw, the blame was put on 
the absence or presence of JNICALL, and a series of patches were made 
where JNICALL was more or less arbitrarily added or removed, until 
things "worked". This should have been a warning sign, and I was 
increasingly uneasy about all these changes, but I hadn't spent enough 
time to realize what the core issue was or how to resolve it properly.

> That leaves those methods with JNIEXPORT only.
> That seems wrong to you because they are not "JNI methods", so you 
> want to replace with JDK_EXPORT to make it clear. [Ok - this seems 
> reasonable.]
Yes. And given the fact that we have a bunch of "non-JNI methods" that 
use the JNIEXPORT...JNICALL pattern, another way to interpret the 
JDK_EXPORT semantics is that this function is exported for usage *in the 
JDK*, but not for public consumption.
> If JNICALL is removed from those native methods and they are currently 
> linked by external applications, those applications may stop working. 
> But this only affects win32 (as its the only platform where JNICALL is 
> different to the default C/C++ calling convention?) so your position 
> is this is acceptable breakage - and would only affect 
> unsupported/undocumented APIs.
Yes. In fact, it's possible that at least some of the breakage that 
occurred was due to new *addition* of JNICALL, which was not present 
before. We might have had something like (I'm making this specific 
example up) a function "void ZIP_OpenFile(char* name)" with no 
decoration at all, and a "/export:ZIP_OpenFile" on the command line, and 
a ZIP_OpenFile entry in the unix map file. And I converted this to 
"JNIEXPORT void JNICALL ZIP_OpenFile(char* name)", which de facto -- 
although I didn't fully realize this at the time, changed the calling 
convention and name decoration on Windows 32. When something broke, 
perhaps because the user of ZIP_OpenFile did not include the proper 
header file with the JNICALL modifier, the solution was to remove the 

And of all the bug reports I've seen on this, the issues has been 
internal linking only, i.e. problems building the JDK, not complaints 
that external tools tried to use internal interfaces and failed. So yes, 
my position is if this should break things, tough shit. That, of 
courses, requires that we do not change public APIs, so we need to be 
careful not to mess with those.
> Does that sum it up?
Yep, I think so.
> We still need to be sure that we're only changing functions of type 
> (b) above.
Yes, definitely.
> And I note that this has been driven by the choice to remove JNICALL 
> where there was a discrepancy - that leads to the visibility of the 
> two kinds of methods. If it had been chosen to add JNICALL where 
> missing instead, then we may not have been having this conversation.
Actually, as I said, this has more been the result of a lot of added 
JNICALL where previously there was none.

An alternative course of action is the make sure that *all* calls use 
the JNIEXPORT...JNICALL pattern, even type b ones, and that we fix all 
parts of code to actually accept the decorated names on Windows 32. This 
will lead to a lot more code changes, like the fix for JDK-8214122 (JDWP 
is broken on 32 bit Windows: transport library missing onLoad entry). 
And all this special case handling will be needed only on Windows 32, 
which is a platform we do not want to spend to much time or effort on. 
And finally, I think doing so would make us miss out on an opportunity 
to make the semantics clearer.
> This will also need a CSR request due to the change in linking behaviour.
I'm not sure what you mean by this..? My entire intention here is that 
we should make no changes to documented interfaces; this is strictly an 
internal change, so no CSR should be needed. Also, I don't understand 
what you mean by "linking behavior"?

> Cheers,
> David
> -----
> On 12/12/2018 9:03 pm, Magnus Ihse Bursie wrote:
>> On 2018-12-11 23:47, David Holmes wrote:
>>> On 12/12/2018 12:34 am, Magnus Ihse Bursie wrote:
>>>> On 2018-12-11 00:23, David Holmes wrote:
>>>>> Hi Magnus,
>>>>> On 10/12/2018 11:19 pm, Magnus Ihse Bursie wrote:
>>>>>> I propose that we introduce a new define, available to all JDK 
>>>>>> native files (Hotspot included), called JDK_EXPORT. The behavior 
>>>>>> of this symbol will be very similar (as of now, in fact 
>>>>>> identical) to JNIEXPORT; however, the semantics will not.
>>>>>> Currently, we "mis-use" the JNIEXPORT define to mark a function 
>>>>>> for exporting from the library. The problem with this is that 
>>>>>> JNIEXPORT is part of the JNI interface, and is supposed to be 
>>>>>> used when C programs interact with Java. And, when doing this, 
>>>>>> the function should be fully decorated like this: "JNIEXPORT foo 
>>>>>> JNICALL".
>>>>> I've seen a lot of the emails on this issue and I don't fully 
>>>>> understand what has been going wrong. But the intent is obviously 
>>>>> the JNIEXPORT represents what is needed to export this function 
>>>>> for use by JNI, while JNICALL defines the calling convention. I 
>>>>> agree there may be some mistmatch when functions are actually not 
>>>>> intended for general export outside the JDK but are only for 
>>>>> internal JDK use.
>>>>>> We do have many such JNI exports in our native libraries, but we 
>>>>>> also have a lot of other, non-JNI exports, where one native 
>>>>>> library just provides an interface to other libraries. In these 
>>>>>> cases, we have still used JNIEXPORT for the functionality of 
>>>>>> getting the function exported, but we have not been consistent in 
>>>>>> our use of JNICALL. This has caused us way too much trouble for 
>>>>>> something that should Just Work<tm>.
>>>>> Are you suggesting that the interface between different libraries 
>>>>> in the JDK should not be a JNI interface? Is this because you 
>>>>> think the functions in these libraries are only for JDK internal 
>>>>> use or ... ??
>>>>>> I therefore propose that we define "JDK_EXPORT", with the same 
>>>>>> behavior as JNIEXPORT (that is, flagging the function for 
>>>>>> external visibility in the resulting native library), but which 
>>>>>> is *not* supposed to be exported to Java code using JNI, nor 
>>>>>> supposed to be decorated with 
>>>>> Just a clarification there. JNI functions are not exported to Java 
>>>>> code, they are exported to native code. Java code can declare 
>>>>> native methods and those native methods must be written as JNI 
>>>>> functions, but that's not what we are discussing. Libraries expose 
>>>>> a JNI interface (a set of functions in the library) that can be 
>>>>> called by application native code, using JNI.
>>>> We're apparently looking at "JNI" and "exporting" from two opposite 
>>>> sides here. :-) Just to make everything clear: If I have a Java class
>>>> class MyClass {
>>>>    public static void native myNativeFunc();
>>>> }
>>>> then I have one half of the JNI function, the Java half. This must 
>>>> be matched by a corresponding implementation in native, like this:
>>>> Java_MyClass_myNativeFunc(void) {
>>>> // ... do stuff
>>>> }
>>>> And this is the native half of the JNI function. Right? Let's leave 
>>>> aside which side is "exporting" to the other for now. :-)
>>>> This way of setting up native functions that can be called from 
>>>> Java is what I refer to as JNI. And when you declare a native JNI 
>>>> function, you *must* use both JNIEXPORT and JNICALL. Alright?
>>>> We do have a lot of those functions in our native libraries. And 
>>>> they are correct just the way they are.
>>> Yep all well and good. A function declared native in Java must have 
>>> an implementation as you describe. But not all native functions 
>>> exist to provide the native-half of a Java native function!
>>>> However, we also have *other* native functions, that are exported, 
>>>> not as JNI functions that should be called from Java, but as normal 
>>>> native library functions that should be called by other native 
>>>> code. Okay so far? And *those* functions have been problematic in 
>>>> how we decorate 
>>> But there are again two cases. Those functions exported from a 
>>> library that are expected to be called from external code using the 
>>> JNI interface mechanism - such as all the JNI functions and JVM TI 
>>> functions we export from the JVM - and those "exported" for access 
>>> between libraries within the JDK (such as all the JVM_* functions in 
>>> libjvm).
>>> I think it is only the second group that should be addressed by your 
>>> JDK_EXPORT proposal - though I'm not completely clear exactly how to 
>>> identify them.
>> Alright, now I think we're on the same page again. :)
>> Yes, this is what I'm saying. I'm not proposing to modify public APIs.
>> You identify external APIs by the fact that they are documented. And 
>> if you are unsure, you ask Jon. ;-)
>>>> them. My proposal is that we *refrain* from using JNIEXPORT for 
>>>> those functions, and instead use JDK_EXPORT as name for the macro 
>>>> that decorates them as exported. That way, we can clearly see that 
>>>> a function like this:
>>>> JDK_EXPORT void
>>>> JLI_ReadEnv(char* env);
>>>> is correctly declared, and will be exported to other native 
>>>> libraries, but not to Java.
>>> The issue is not whether it is "exported to Java"** but whether it 
>>> is exported using the JNI mechanism such that other native code 
>>> calls it using the JNI mechanism.
>>> ** There is no way to write a native method declaration in Java such 
>>> that it would be linked to the JLI_ReadEnv function. The naming is 
>>> all wrong, as is the signature.
>>>> Just to clarify, this has nothing to do with if this is a 
>>>> officially supported API or not. In general though, I assume that 
>>>> most (if not all?) of our exported functions (apart from the JNI_* 
>>>> stuff) is supposed to be consumed by other libraries in the JDK, 
>>>> and is not a public API.
>>> I think it varies library by library. You may need native 
>>> application code that can call directly into native JDK libraries. 
>>> JLI is the Java Launcher Interface - I think it was introduced to 
>>> make it easier for other launchers to be created. Native agents may 
>>> need access to libmanagement or libjdwp functions. Native graphics 
>>> code may need access to the JDK graphics library. Some of these 
>>> accesses may be unsupported and undocumented, but I don't think you 
>>> can just cut them all off.
>> If they are unsupported and undocumented, I sure as h*ll can cut them 
>> all off! :-)
>> Besides, and let me re-iterate this, the only change that will happen 
>> by removing JNICALL, is on Windows 32. No other platform will be 
>> affected. There is no official support for Windows 32 anymore. 
>> There's some low-effort community work done on keeping Windows 32 
>> alive, but it's not backed by any major player. Right now, they are 
>> taking a lot of hits *due to the fact* that we have not sorted this 
>> out, and waste a lot of their precious effort trying to fix this 
>> piecemeal. If we do fix things according to this proposal, then it's 
>> at least clear how things *should* work. If it turns out that there's 
>> some code out there in the wild, running on Windows 32, that uses an 
>> undocumented and unsupported function call, and it breaks -- well, 
>> sucks to be them. They'll just have to do what everyone does who uses 
>> an undocumented interface that suddenly changes: update their code.
>> /Magnus
>>> David
>>>> /Magnus
>>>>>> JNICALL. All current instances of JNIEXPORT which is not pure JNI 
>>>>>> native functions should be changed to use JDK_EXPORT instead.
>>>>>> I further propose that this macro should reside in a new file 
>>>>>> "jdk.h", placed in the new directory 
>>>>>> src/java.base/share/native/include/internal. This header file 
>>>>>> path will automatically be provided to all native libraries, but 
>>>>>> not copied to the JDK being built. (The existence of a 
>>>>>> "include/internal" directory with this behavior has been 
>>>>>> discussed before. There are more files that ought to be moved 
>>>>>> there, if/when it is created.) I believe in many cases the 
>>>>>> #include "jni.h" can be just modified to #include "#jdk.h", since 
>>>>>> most native code will not require "jni.h" unless actually doing 
>>>>>> JNI calls -- most have included this file to get the JNIEXPORT 
>>>>>> macro, which would explain the pervasive use of #include "jni.h" 
>>>>>> in our code base.
>>>>> jni.h also defines all of the types used by the JNI. Those types 
>>>>> are pervsive to the native code used throughout the JDK.
>>>>>> Thoughts?
>>>>> I think we need to understand the problems on Windows that 
>>>>> prompted all this. Then I think we need to look at exactly how 
>>>>> jni.h and JNIEXPORT etc are being used and understand whether this 
>>>>> is truly an exported interface or not.
>>>>> Cheers,
>>>>> David
>>>>>> /Magnus

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