javah doesn't generate the right prototypes for stub files
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src/share/tools/javah/javah.c

The prototypes of the user's native methods are not declared correctly
in the stub files. A mismatch between the prototype and the actual function
could cause problems on some platforms:

Old:

/* Stubs for class java_lang_Object */
/* SYMBOL: "java_lang_Object/getClass()Ljava/lang/Class;",
Java_java_lang_Object_getClass_stub */
 stack_item * _Optlink _Export Java_java_lang_Object_getClass_stub(stack_item
*_P_,struct execenv *_EE_) {
        extern void* java_lang_Object_getClass(void *);
        _P_[0].p = java_lang_Object_getClass(_P_[0].p);
        return _P_ + 1;
}

New, with proper prototypes:

/* Stubs for class java_lang_Object */
/* SYMBOL: "java_lang_Object/getClass()Ljava/lang/Class;",
Java_java_lang_Object_getClass_stub */
struct Hjava_lang_Object;
struct Hjava_lang_Class;
 stack_item * _Optlink Java_java_lang_Object_getClass_stub(stack_item
*_P_,struct execenv *_EE_) {
        extern struct Hjava_lang_Class *java_lang_Object_getClass(struct
Hjava_lang_Object *);
        _P_[0].p = java_lang_Object_getClass(_P_[0].p);
        return _P_ + 1;
}