Unable to call MapViewOfFile

[norrisjeremy]

I've stumbled across an issue when attempting to use the MapViewOfFile function.

It seems the code generator isn't correctly generating code to handle this function correctly?

Caused by: java.lang.invoke.WrongMethodTypeException: handle's method type (SegmentAllocator,MemorySegment,MemorySegment,int,int,int,long)MemorySegment but found (MemorySegment,MemorySegment,int,int,int,long)MemorySegment
        at java.base/java.lang.invoke.Invokers.newWrongMethodTypeException(Invokers.java:522)
        at java.base/java.lang.invoke.Invokers.checkExactType(Invokers.java:531)
        at com.jcraft.jsch.windowsapi.windows.win32.system.memory.Apis.MapViewOfFile(Apis.java:119)
        ... 11 more

This seems to indicate that Java is expecting a SegmentAllocator argument to be passed as the first argument to the generated MethodHandle?

[norrisjeremy]

Btw, here is a link to the full source for what I'm attempting to what I'm attempting to do: PageantFFMConnector.java.

I'm basically attempting to translate this older version that uses JNA over to the new FFM API: PageantConnector.java.

[norrisjeremy]

I believe I've resolved this issue with #9.
With the FFM API, it appears functions that return a struct value need to include a SegmentAllocator argument for the linker downcall so that the FFM API can allocate a MemorySegment for the struct value that is returned.

[norrisjeremy]

See Linker.downcallHandle() for additional details:

Moreover, if the function descriptor's return layout is a group layout, the resulting downcall method handle accepts an additional leading parameter of type SegmentAllocator, which is used by the linker runtime to allocate the memory region associated with the struct returned by the downcall method handle.

And also SegmentAllocator:

For instance, downcall method handles can accept an additional SegmentAllocator parameter if the underlying foreign function is known to return a struct by-value.
Effectively, the allocator parameter tells the linker where to store the return value of the foreign function.

And also JEP 454:

If a C function returns a by-value struct (not shown here) then a fresh memory segment must be allocated off-heap and returned to the Java client.
To achieve this, the method handle returned by downcallHandle requires an additional SegmentAllocator argument which the native linker uses to allocate a memory segment to hold the struct returned by the C function.

[norrisjeremy]

Also just to document for posterity, MapViewOfFile technically returns a pointer, not a struct.
But for whatever reason, Microsoft decided to modify it to return a single-element struct wrapping the pointer in the Win32 metadata (see microsoft/win32metadata#561 for the longwinded discussion that appears to have led to this decision).

[norrisjeremy]

I've also added a second commit to #9 to treat structs with the MetadataTypedef attribute the same way as structs with the NativeTypedef.

According to to the Projections doc for the win32metadata project, they are functionally equivalent, with the difference being that MetadataTypedef "represents metadata-only typedefs added to improve API usability".

With this additional change, the code generator no longer generates downcalls for MapViewOfFile that return this synthesized struct, but instead now correctly generate a downcall that returns a pointer, matching the underlying C function definition.