Summary - unsafe

Combine unsafe.Pointer and Sizeof to write a memory inspector.

unsafe summary

In this group you measured memory layout with Sizeof, Alignof and Offsetof; converted pointer types with unsafe.Pointer; and did zero-copy conversions with unsafe.String and unsafe.Slice.

Three rules: don't store a uintptr, don't modify the bytes of a zero-copy string, and look for a safe alternative first (e.g. math.Float64bits).

A tool that inspects a struct

Using the three functions together you can produce a layout report for a struct:

var h Hesap
fmt.Println("boyut:", unsafe.Sizeof(h))
fmt.Println("bakiye:", unsafe.Offsetof(h.Bakiye))

This report shows whether reordering fields in a frequently used struct would save memory.

Exercise Layout report

Define type Kayit struct { Aktif bool; ID int64; Puan int32 }. Print the struct's Sizeof and the Offsetof of ID and Puan on one line.

Show example solution

package main

import (
    "fmt"
    "unsafe"
)

type Kayit struct {
    Aktif bool
    ID    int64
    Puan  int32
}

func main() {
    var k Kayit
    fmt.Println(unsafe.Sizeof(k), unsafe.Offsetof(k.ID), unsafe.Offsetof(k.Puan))
}

Test yourself

Which is a constant computed at compile time?
- unsafe.String(p, n)
- uintptr(p)
- unsafe.Sizeof(x)
- unsafe.Pointer(&x)
Show answer

unsafe.Sizeof(x) — Sizeof, Alignof and Offsetof are computed at compile time; conversions happen at run time.

Address arithmetic

pb := (*int32)(unsafe.____(p, unsafe.Offsetof(k.b)))

Show answer

pb := (*int32)(unsafe.Add(p, unsafe.Offsetof(k.b)))

Rule and reason

Storing a uintptr

GC doesn't treat it as a pointer
Modifying the bytes

Strings are immutable
Order the fields

Padding shrinks
Safe alternative

Portable code

Next lesson: What is cgo?

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