Sizeof and Alignof

Learn memory layout with Sizeof and Alignof.

Sizeof and Alignof

unsafe.Sizeof gives a type's size in bytes, and unsafe.Alignof the byte boundary it must be placed on in memory. On a 64-bit system int64 is 8 bytes and aligned to 8.

fmt.Println(unsafe.Sizeof(int16(0)), unsafe.Alignof(int16(0)))  // 2 2

Both are constants computed at compile time.

Field order and padding

Ordering fields from largest to smallest reduces padding:

type Kotu struct { a bool; b int64; c bool }  // 24 bytes
type Iyi  struct { b int64; a bool; c bool }  // 16 bytes

In slices holding millions of records this means real memory savings. The fieldalignment tool suggests this ordering.

Exercise See the padding

Define type A struct { x bool; y int64; z bool } and type B struct { y int64; x bool; z bool }. Print the unsafe.Sizeof of both on one line.

Show example solution

package main

import (
    "fmt"
    "unsafe"
)

type A struct {
    x bool
    y int64
    z bool
}

type B struct {
    y int64
    x bool
    z bool
}

func main() {
    fmt.Println(unsafe.Sizeof(A{}), unsafe.Sizeof(B{}))
}

Test yourself

On a 64-bit system, how many bytes is <code>struct{ a bool; b int64 }</code>?
- 10
- 16
- 9
- 8
Show answer

16 — Field b must be aligned to 8, so 7 bytes of padding follow a: 1 + 7 + 8 = 16.

Ask the alignment

var x int64
fmt.Println(unsafe.____(x))  // 8

Show answer

var x int64
fmt.Println(unsafe.Alignof(x))  // 8

Memory layout

Sizeof

Total bytes
Alignof

Alignment boundary
Padding

Empty bytes for alignment
Field ordering

Reduces padding

Next lesson: Offsetof

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