The unsafe package

Learn the risks and usage of the unsafe package.

The unsafe package

The unsafe package lets you deliberately step around Go's type safety: convert pointers between types and learn a type's size and alignment in memory.

var n int64
fmt.Println(unsafe.Sizeof(n))  // 8

Its name is a warning: code using it may not be portable across Go versions and architectures.

When is it used?

Real use cases are narrow: zero-copy conversions (e.g. []byte ↔ string), system calls, sharing data with C, and optimizing very hot code paths. In the standard library, runtime, reflect and syscall use unsafe; application code almost never needs it.

Exercise Print the sizes

Use unsafe.Sizeof to print the sizes of an int8, an int64 and a float32 with one fmt.Println.

Show example solution

package main

import (
    "fmt"
    "unsafe"
)

func main() {
    var a int8
    var b int64
    var c float32
    fmt.Println(unsafe.Sizeof(a), unsafe.Sizeof(b), unsafe.Sizeof(c))
}

Test yourself

Which is true for code that uses unsafe?
- It is always safer
- It turns off the garbage collector
- It only runs on Windows
- It may fall outside the Go compatibility promise
Show answer

It may fall outside the Go compatibility promise — The Go 1 compatibility promise does not cover programs that use unsafe; they may break when the compiler or runtime changes.

Learn the size

var x int32
fmt.Println(unsafe.____(x))  // 4

Show answer

var x int32
fmt.Println(unsafe.Sizeof(x))  // 4

unsafe tools

unsafe.Pointer

Converts to any pointer
unsafe.Sizeof

Size in bytes
unsafe.Alignof

Alignment
unsafe.Offsetof

Field offset

Next lesson: unsafe.Pointer

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