unsafe_slices.go

package main

import (
	"fmt"
	"unsafe"
)


// sliceHeader is the in memory structure of a slice
type sliceHeader struct {
	// Data is the pointer to the underlying array
	Data uintptr
	
	// Len and Cap are the len and cap of the slice
	Len  int
	Cap  int
}

func main() {
	// Create a slice of bytes, containing 5 elements
	x := []byte{1, 2, 3, 4, 5}

	// Make a copy of x as y
	y := x

	// Print out the address and contents of the two slices
	fmt.Printf("Before mutating x:\n")
	fmt.Printf("x Address: %p Contents: % x\n", &x, x)
	fmt.Printf("y Address: %p Contents: % x\n", &y, y)

	// Pass x to a func that mutates the slice
	mutate(x)

	// Print out the address and contents of the two slices after mutating
	fmt.Printf("\nAfter mutating x:\n")
	fmt.Printf("x Address: %p Contents: % x\n", &x, x)
	fmt.Printf("y Address: %p Contents: % x\n", &y, y)

	// Use unsafe to get a reference to the slice headers
	xHeader := (*sliceHeader)(unsafe.Pointer(&x))
	yHeader := (*sliceHeader)(unsafe.Pointer(&y))

	// Print out the contents of the headers
	fmt.Printf("\nContents of the slice headers:\n")
	fmt.Printf("x {Data: %#x, Len: %d, Cap: %d}\n", xHeader.Data, xHeader.Len, xHeader.Cap)
	fmt.Printf("y {Data: %#x, Len: %d, Cap: %d}\n", yHeader.Data, yHeader.Len, yHeader.Cap)

	// Use unsafe to get a reference to the underlying array
	array := (*[5]byte)(unsafe.Pointer(xHeader.Data))

	// Print out the contents of the array
	fmt.Printf("\nContents of array the data fields are pointing to:\n")
	fmt.Printf("array Address: %p Contents: % x\n", array, *array)
	
	// Change the array
	array[2] = 0xff

	// Print out the address and contents of the two slices after mutating
	fmt.Printf("\nAfter mutating the underlying array:\n")
	fmt.Printf("x Address: %p Contents: % x\n", &x, x)
	fmt.Printf("y Address: %p Contents: % x\n", &y, y)
	
	// Attempt to mutate the array in a function
	mutateArray(*array)
	
	// Print out the address and contents of the two slices after mutating
	fmt.Printf("\nAfter attempting to mutate array in mutateArray():\n")
	fmt.Printf("x Address: %p Contents: % x\n", &x, x)
	fmt.Printf("y Address: %p Contents: % x\n", &y, y)
}

// mutate changes the 3rd element (index 2) of a slice of bytes to 0
func mutate(b []byte) {
	fmt.Printf("\nAddress of the slice inside mutate()\n")
	fmt.Printf("b Address: %p\n", &b)
	b[2] = 0
}

// mutateArray changes the 3rd element (index 2) of a 5 element array of bytes to 0xAA
func mutateArray(a [5]byte) {
	fmt.Printf("\nAddress of the array inside mutateArray()\n")
	fmt.Printf("a Address: %p\n", &a)
	a[2] = 0xAA
}