unsafe.Pointer

Learn the universal pointer with unsafe.Pointer.

unsafe.Pointer

unsafe.Pointer is a generic pointer that any pointer type can convert to. Normally you can't turn a *int64 into a *float64; with unsafe.Pointer in between you can read the same memory as another type.

f := 1.0
bits := *(*uint64)(unsafe.Pointer(&f))
fmt.Printf("%x\n", bits)  // 3ff0000000000000

math.Float64bits does exactly this inside a safe function.

The uintptr trap

uintptr holds an address as a number; the garbage collector doesn't treat it as a pointer. Storing an address in a uintptr and converting it back later can touch invalid memory if the object moved or was collected. If arithmetic is needed, convert and compute in the same expression.

p2 := unsafe.Pointer(uintptr(p) + off)  // in one expression

Exercise Read the bits

Declare f := 2.0. Convert it to *uint64 through unsafe.Pointer and print the value in hex with fmt.Printf("%x\n", ...).

Show example solution

package main

import (
    "fmt"
    "unsafe"
)

func main() {
    f := 2.0
    bits := *(*uint64)(unsafe.Pointer(&f))
    fmt.Printf("%x\n", bits)
}

Test yourself

What is the correct way to convert <code>*T1</code> → <code>*T2</code>?
- T2(p)
- p.(T2)
- (*T2)(unsafe.Pointer(p))
- (*T2)(p)
Show answer

(*T2)(unsafe.Pointer(p)) — Different pointer types can't be converted directly; first convert to unsafe.Pointer, then to the target pointer type.

Generic pointer

x := int32(7)
p := (*uint32)(unsafe.____(&x))

Show answer

x := int32(7)
p := (*uint32)(unsafe.Pointer(&x))

Pointer kinds

*int

Typed pointer
unsafe.Pointer

Untyped pointer
uintptr

Number holding an address
math.Float64bits

Safe bit reading

Next lesson: Sizeof and Alignof

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