https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78804
--- Comment #18 from Ian Lance Taylor <ian at airs dot com> --- How could the size of that struct possibly be 12? Can you figure out what is causing that to happen? There are exactly 64 bits specified, and the fields are all packed. The size ought to be 8. I don't see what could cause it to be 12. Also, I should have said that you need to compile with -m64bit-doubles.