Yes, the cgo behavior has changed, but why is this a bug? You can't refer to the bitfield either way. The type alignments mean that the fields will continue to be at the correct offsets.

var st C.MyStruct
C.fill_my_struct(&st)

Size of st is possibly differ to C's one.

Try printing out the sizes and field offsets using the reflect package. They should be the same as the C sizes and offsets available using C sizeof and offsetof.

Sorry, I mis-read description in this issue. What I expected is cgo generate _.

Yes, you are correct - the sizes and offsets are the same.

The main source of confusion was a difference in number of fields reported with:

var s C.struct_foobar
t := reflect.TypeOf(s)
for i := 0; i < t.NumField(); i++ {
	fmt.Println(i, ":", t.Field(i), ",", t.Field(i).Offset)
}

// go1.11.4
// 0 : {a main main._Ctype_uchar  0 [0] false} , 0
// 1 : {d main [3]main._Ctype_ushort  2 [1] false} , 2

// go1.11.3
// 0 : {a main main._Ctype_uchar  0 [0] false} , 0
// 1 : {_ main [1]uint8  1 [1] false} , 1
// 2 : {d main [3]main._Ctype_ushort  2 [2] false} , 2

Thanks.