unsafe and slice

Learn the array behind a slice with unsafe.

Slices and strings with unsafe

unsafe.String, unsafe.StringData, unsafe.Slice and unsafe.SliceData, added in Go 1.20, replaced old reflect.SliceHeader tricks. You can view a []byte as a string without copying:

b := []byte("merhaba")
s := unsafe.String(&b[0], len(b))
fmt.Println(s)  // merhaba

The other way: string → []byte

unsafe.Slice(unsafe.StringData(s), len(s)) gives a string's bytes as a slice without copying. Writing to this slice is forbidden; string constants may live in read-only memory and a write crashes the program. In most cases a plain []byte(s) copy is fast enough.

Exercise Zero-copy conversion

Declare b := []byte("gopher"). Create a string with unsafe.String(&b[0], len(b)) and print the string and its length.

Show example solution

package main

import (
    "fmt"
    "unsafe"
)

func main() {
    b := []byte("gopher")
    s := unsafe.String(&b[0], len(b))
    fmt.Println(s, len(s))
}

Test yourself

What is required for a string created without copying?
- The slice must be empty
- The string must be lower case
- GC must be disabled
- The underlying byte slice must not be modified later
Show answer

The underlying byte slice must not be modified later — Go strings are assumed immutable; if the bytes change, the same string shows different content and the program's behavior is undefined.

String without copying

b := []byte("go")
s := unsafe.____(&b[0], len(b))

Show answer

b := []byte("go")
s := unsafe.String(&b[0], len(b))

Go 1.20 unsafe functions

unsafe.String

String from bytes
unsafe.StringData

Pointer to a string's bytes
unsafe.Slice

Slice from a pointer
unsafe.SliceData

Pointer to a slice's first element

Next lesson: Summary - unsafe

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