通过JNA检索数组

Question

I've got an problem with getting information back from a memory block allocated by the called function. Specifically, there's function XGetWindowProperties in the X11 lib included with JNA. It returns the results via the last parameter - PointerByReference prop_return .

I know the actual memory area contains an array of Window . But how do I create those objects? I don't see any way to get it from the pointer itself and I can't get a Buffer from it, since I don't know the size of Window .

The real function is: int XGetWindowProperty(...., unsigned char **prop_return) and with the parameters I'm calling it with, it's actually int XGetWindowProperty(...., Window *prop_return[]) .

Answer 1

The size of the elements in the returned data is defined by the returned "format" from XGetWindowProperty(), with one annoying exception. Xlib always returns format 32 as an array of "long" even if "long" is 64-bit. Window would have format 32.

In a quick google search, you can use NativeLong.SIZE to determine this in JNA. So if (NativeLong.SIZE == 8) you have a buffer of 64-bit window IDs, otherwise of 32-bit window IDs.

The byte order of each value should match the CPU you're on.

Answer 2

You'll need to extract the data first as an array of primitive type (int or long as appropriate). you can then construct X11.Window objects as needed using the XIDs from the array.

It is possible to set up converters to handle some of this automatically, but ultimately it's simpler for a one-off to do the conversion explicitly.