Golang Unsafe Type Conversions and Memory Access

Illustration composed from MariaLetta/free-gophers-pack, original gopher by Renee French.

Warning! You should avoid using unsafe in your applications, unless you 100% sure what you are doing. It's called unsafe for a reason.

Go is a strongly typed, you have to convert variable from one type to another to use in different parts of application. But sometimes you need to step around this type safety. It could be needed to optimization of bottle necks in high load systems, where every tick counts. Unsafe operation potentially could safe a lot of allocations. Unsafe also allows to hack into any struct field, including slices, strings, maps etc.

Change string example

String are immutable in Go. If you want to change a string, usually you have to allocate a new one. Let's hack Go a little bit with unsafe package, and actually change the string!

package main

import (
	"fmt"
	"reflect"
	"time"
	"unsafe"
)

func main() {
	a := "Hello. Current time is " + time.Now().String()

	fmt.Println(a)

	stringHeader := (*reflect.StringHeader)(unsafe.Pointer(&a))
	*(*byte)(unsafe.Pointer(stringHeader.Data + 5)) = '!'

	fmt.Println(a)
}

Result:

Hello. Current time is 2020-03-14 21:40:38.36328248 +0300 +03 m=+0.000037994
Hello! Current time is 2020-03-14 21:40:38.36328248 +0300 +03 m=+0.000037994

String is changed!

Code snippet - https://play.golang.org/p/QJ6hIMz6_gg

What's going on in this code? Following steps:

Convert a string pointer to
```
unsafe.Pointer
```
. Any pointer could be converted to
```
unsafe.Pointer
```
and vise versa.
```
unsafe.Pointer
```
also could be converted to
```
uintptr
```
- address in integer form.
Convert
```
unsafe.Pointer
```
to
```
reflect.StringHeader
```
pointer, to get Data pointer.
```
reflect.StringHeader
```
type reflects string internal runtime struct. Under the hood string is represented by length value and
```
uintptr
```
pointer to memory with the data.
Move the pointer 5 bytes forward. With
```
uintptr
```
it is possible to do pointer arithmetic.
Convert new
```
uintptn
```
to
```
unsafe.Pointer
```
, and next to
```
byte
```
pointer. This byte is actually part of a string.
Assign new value to byte by pointer. String is changed!

By the way, following code will produce segmentation violation error. Go compiler understands that string is constant and put it in memory along with other constants, where changes are forbidden. That's why I used

time.Now()

in the first example.

package main

import (
	"fmt"
	"reflect"
	"unsafe"
)

func main() {
	a := "Hello. Have a nice day!"

	fmt.Println(a)

	stringHeader := (*reflect.StringHeader)(unsafe.Pointer(&a))
	*(*byte)(unsafe.Pointer(stringHeader.Data + 5)) = '!'

	fmt.Println(a)
}

Result:

Hello. Have a nice day!
unexpected fault address 0x4c3e31
fatal error: fault
[signal SIGSEGV: segmentation violation code=0x2 addr=0x4c3e31 pc=0x48cf33]

Code snippet - https://play.golang.org/p/ix70l38QvA4

Package unsafe

https://golang.org/pkg/unsafe/
Package consist of only 3 functions and single usable type.

func Alignof(x ArbitraryType) uintptr
func Offsetof(x ArbitraryType) uintptr
func Sizeof(x ArbitraryType) uintptr
type Pointer *ArbitraryType

type ArbitraryType
// represents the type of an arbitrary Go expression,
// not actually part of a package

Offsetof

gives offset of field in struct.

Sizeof

is a size of variable in memory, referenced memory not included.

Alignof

gives information regarding the alignment of variable address.

Some examples of using

unsafe.Offsetof

and

unsafe.Sizeof

functions:

Unsafe array iteration and type conversion on the fly with zero allocations.
Getting information about the actual size of structs in memory.
Changing struct field directly in memory, with struct pointer and field offset.

package main

import (
	"fmt"
	"unsafe"
)

type Bytes400 struct {
	val [100]int32
}

type TestStruct struct {
	a [9]int64
	b byte
	c *Bytes400
	d int64
}

func main() {
	array := [10]uint64{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}

	var sum int8

	// Unsafe array iteration
	sizeOfUint64 := unsafe.Sizeof(array[0])
	for i := uintptr(0); i < 10; i++ {
		sum += *(*int8)(unsafe.Pointer(uintptr(unsafe.Pointer(&array)) + sizeOfUint64*i))
	}
	fmt.Println(sum)

	// Size of struct and offsets of struct fields
	t := TestStruct{b: 42}
	fmt.Println(unsafe.Sizeof(t))
	fmt.Println(unsafe.Offsetof(t.a), unsafe.Offsetof(t.b), unsafe.Offsetof(t.c), unsafe.Offsetof(t.d))

	fmt.Println(unsafe.Sizeof(Bytes400{}))

	// Change struct field t.b value
	*(*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(&t)) + unsafe.Offsetof(t.b)))++
	fmt.Println(t.b)
}

Result:

Code snippet - https://play.golang.org/p/cSmAUdu3sS9

Pointer arithmetic and garbage collector

unsafe.Pointer

can be converted to a

uintptr

and vise versa. Only way to perform pointer arithmetic is with usage of

uintptr

. General rule is that

uintptr

is an integer value without pointer semantics. It is not safe to use

uintptr

as an only pointer to object. Garbage collector unaware of it and object memory could be reused. It is best to convert it back to

unsafe.Pointer

right away, after arithmetic operations are done.

Benchmarks of unsafe string to []byte conversion

package main

import (
	"reflect"
	"unsafe"
)

func SafeBytesToString(bytes []byte) string {
	return string(bytes)
}

func SafeStringToBytes(s string) []byte {
	return []byte(s)
}

func UnsafeBytesToString(bytes []byte) string {
	sliceHeader := (*reflect.SliceHeader)(unsafe.Pointer(&bytes))

	return *(*string)(unsafe.Pointer(&reflect.StringHeader{
		Data: sliceHeader.Data,
		Len:  sliceHeader.Len,
	}))
}

func UnsafeStringToBytes(s string) []byte {
	stringHeader := (*reflect.StringHeader)(unsafe.Pointer(&s))
	return *(*[]byte)(unsafe.Pointer(&reflect.SliceHeader{
		Data: stringHeader.Data,
		Len:  stringHeader.Len,
		Cap:  stringHeader.Len,
	}))
}

go test -bench=.

Benchmark results:

BenchmarkSafeBytesToString-8       257141380       4.50 ns/op
BenchmarkSafeStringToBytes-8       227980887       5.38 ns/op
BenchmarkUnsafeBytesToString-8     1000000000      0.305 ns/op
BenchmarkUnsafeStringToBytes-8     1000000000      0.274 ns/op

Unsafe conversion is more than 10 times faster! But this conversion only allocates new header, data remains the same. So if you change slice after conversion, string will also change. The following test will pass:

func TestUnsafeBytesToString(t *testing.T) {
	bs := []byte("Test")
	str := UnsafeBytesToString(bs)
	if str != "Test" {
		t.Fail()
	}

	// Test string mutation
	bs[0] = 't'
	if str != "test" {
		t.Fail()
	}
}

If you really need to speed up conversions or hack inside go structures - use unsafe package. In other cases try to avoid it, it is not hard to mess something up with it.

Thanks for reading and have a nice safe day!

Repo with code samples:
https://github.com/kochetkov-av/go-unsafe-type-conversions