alignment.go

package main

import (
	"fmt"
	"unsafe"
)

type Example struct {
	Field1 int64
	Field2 int32
	Field3 int16
	Field4 byte
}

type BadExample struct {
	Field1 byte
	Field2 int64
	Field3 int16
	Field4 int32
}

func padding(offsetField1, offsetField2, sizeField1 uintptr) uintptr {
	return offsetField2 - (offsetField1 + sizeField1)
}

func main() {
	example := Example{}
	fmt.Println("Size of Example struct:", unsafe.Sizeof(example))
	fmt.Println("Alignment of Field1:", unsafe.Alignof(example.Field1))
	fmt.Println("Alignment of Field2:", unsafe.Alignof(example.Field2))
	fmt.Println("Alignment of Field3:", unsafe.Alignof(example.Field3))
	fmt.Println("Alignment of Field4:", unsafe.Alignof(example.Field4))
	fmt.Println("Offset of Field1:", unsafe.Offsetof(example.Field1))
	fmt.Println("Offset of Field2:", unsafe.Offsetof(example.Field2))
	fmt.Println("Offset of Field3:", unsafe.Offsetof(example.Field3))
	fmt.Println("Offset of Field4:", unsafe.Offsetof(example.Field4))
	fmt.Println(
		"Padding after Field1:",
		padding(
			unsafe.Offsetof(example.Field1),
			unsafe.Offsetof(example.Field2),
			unsafe.Sizeof(example.Field1),
		),
	)
	fmt.Println(
		"Padding after Field2:",
		padding(
			unsafe.Offsetof(example.Field2),
			unsafe.Offsetof(example.Field3),
			unsafe.Sizeof(example.Field2),
		),
	)
	fmt.Println(
		"Padding after Field3:",
		padding(
			unsafe.Offsetof(example.Field3),
			unsafe.Offsetof(example.Field4),
			unsafe.Sizeof(example.Field3),
		),
	)

	fmt.Println()

	bad := BadExample{}
	fmt.Println("Size of BadExample struct:", unsafe.Sizeof(bad))
	fmt.Println("Alignment of Field1:", unsafe.Alignof(bad.Field1))
	fmt.Println("Alignment of Field2:", unsafe.Alignof(bad.Field2))
	fmt.Println("Alignment of Field3:", unsafe.Alignof(bad.Field3))
	fmt.Println("Alignment of Field4:", unsafe.Alignof(bad.Field4))
	fmt.Println("Offset of Field1:", unsafe.Offsetof(bad.Field1))
	fmt.Println("Offset of Field2:", unsafe.Offsetof(bad.Field2))
	fmt.Println("Offset of Field3:", unsafe.Offsetof(bad.Field3))
	fmt.Println("Offset of Field4:", unsafe.Offsetof(bad.Field4))
	fmt.Println(
		"Padding after Field1:",
		padding(
			unsafe.Offsetof(bad.Field1),
			unsafe.Offsetof(bad.Field2),
			unsafe.Sizeof(bad.Field1),
		),
	)
	fmt.Println(
		"Padding after Field2:",
		padding(
			unsafe.Offsetof(bad.Field2),
			unsafe.Offsetof(bad.Field3),
			unsafe.Sizeof(bad.Field2),
		),
	)
	fmt.Println(
		"Padding after Field3:",
		padding(
			unsafe.Offsetof(bad.Field3),
			unsafe.Offsetof(bad.Field4),
			unsafe.Sizeof(bad.Field3),
		),
	)
}