It seems from testing that libffi does not reliably handle by-value unions.

Others are reporting similar experiences: https://github.com/libffi/libffi/issues/33

When running java/foreign/nested/TestNested.java crashes were observed when passing the values with the following layouts:

union U{ int f0; double f1; short f2[2]; long long f3; };

And:

union U{ float f0; struct { short f0; char f1; } f1; char f2; char f3; };

(as well as derivatives)