Mirror Mirror on ...

Of Mice (Unsafe) and Men (Reflect)
March 10, 2018
Golang Advanced Reflect Unsafe

About 12 minutes of reading.


While I was mentoring, I encouraged my pupils to break things so they understand how they work. Using reflect package seems easy, but understanding the mechanics is hard. So, this week, following my own advice, I’ve tried to create my own reflect package. Here is what I’ve learned.

Playing with Fire

Most of the articles on the subject I’ve read have (more or less) the following advice : “if you find yourself doing this in a real program, stop immediately and seek help. You are doing something wrong. You’ve been warned!”. Now, wait a minute, mister. That is hypocrisy!

If you take a look at the importers of reflect you will easily find that using the fmt implies you are using reflect. Using “unsafe” features in Golang is only for developers that develop the language itself? Maybe looking on importers of unsafe tells you otherwise.

unsafe - after that we reflect

I quote from the documentation : “unsafe.Pointer type allows a program to defeat the type system and read and write arbitrary memory. It should be used with extreme care”.

Let’s say you have type John which you are trying to convert to type Ivan. The documentation states that Ivan has to be smaller or equal with John (in terms of properties it has) and those to share the `equivalent memory layout.

Let’s code:

func TestIsJohnIvan(t *testing.T) {
    type John struct {
        Name   string
        Age    uint
        Powers uint

    type Ivan struct {
        givenName  string // yes, you can use private fields
        _          uint
        ThirdField uint
    john := John{Name: "John", Age: 40, Powers: 3}

    ivan := *(*Ivan)(unsafe.Pointer(&john))
    t.Logf("John as Ivan : GivenName %v ThirdField %d", ivan.givenName, ivan.ThirdField)

First observation is that you can violate access to private fields using this conversion. Secondly, as long as you respect the same number of fields and their types, you can omit properties. You can violate the second rule and get unexpected results, as below:

    type ShortIvan struct {
        Age uint
    smallIvan := *(*ShortIvan)(unsafe.Pointer(&john))
    t.Logf("Small Ivan (just powers) : %v", smallIvan.Age)

You would expect that age to be 40, but it’s not : it’s 5717318. Why? Because an uint is built by taking the required value from the Name property of John. The correct way to get a smaller Ivan is to omit the name property (observe that the third property is omitted too):

    type ShortIvan struct {
        _ string
        Age uint

What if you violate the first rule, which states that types have to have an equal amount of properties:

    type UpgradedIvan struct {
        //_ string // adding this at the beginning crashes
        Name    string
        Age     uint
        Powers  uint
        Address string // will get filled with the Name 
        Guns    uint   // will get filled with Age 
        //Say     string // adding yet another one will crash : "bad pointer in frame"
        //Data []byte // same adding this or more
        AFloat float32 // adding a different type seems safe
    chuckNorris := *(*UpgradedIvan)(unsafe.Pointer(&john))
    t.Logf("Chuck Ivan : %v", chuckNorris)

Well, it works, but with side effects : Address gets filled with same value as Name, Guns with Age and AFloat get a value of 4e-45. So, this the non-safety point that a developer should never touch. As long as we’re respecting the rules, it’s safe to play unsafe.

Also, upgrading John seems better (simpler) by using embedding:

    type EmbeddedJohn struct {
        Address string // will get filled with the Name ??? Weird huh
        Guns    uint   // will get filled with Age ???
    // convert John to EmbeddedJohn
    upgradedIvan := EmbeddedJohn{John: john}
    t.Logf("Upgraded Ivan : %v", upgradedIvan)

Surely, the bellow code is dangerous if it is misused. The code speaks for itself:

func TestAlteredPeople(t *testing.T) {
    type John struct {
        Name    string
        Age     int
        Altered bool

    john := John{Name: "John", Age: 30, Altered: false}

    ptrToJohn := unsafe.Pointer(&john)
    ptrToName := (*string)(unsafe.Pointer(uintptr(ptrToJohn) + unsafe.Offsetof(john.Name)))
    ptrToAge := (*int)(unsafe.Pointer(uintptr(ptrToJohn) + unsafe.Offsetof(john.Age)))
    ptrToAltered := (*bool)(unsafe.Pointer(uintptr(ptrToJohn) + unsafe.Offsetof(john.Altered)))

    *ptrToName = "Chuck"
    *ptrToAge = 100000
    *ptrToAltered = true

    t.Logf("Now John is %v", john)

Unsafe conclusions

The unsafe package is serving for Go compiler instead of Go runtime, because it has facilities for low-level programming including operations that violate the type system.

I would never use the above method of conversion, but investigation was needed because of what’s about to be described regarding reflect.

type Point struct {
    x, y int

func Extract(ptr unsafe.Pointer, size uintptr) []byte {
    out := make([]byte, size)
    for i := range out {
        out[i] = *((*byte)(unsafe.Pointer(uintptr(ptr) + uintptr(i))))
    return out

func TestExtract(t *testing.T) {
    p := Point{3, 4}
    mem := Extract(unsafe.Pointer(&p), unsafe.Sizeof(p))
    t.Logf("What's the Point? %v", mem)

Yes, you can extract the content of the memory, but what’s the point? Well, a friend of mine (with the same name and the same passion for Golang) might find this as a useful way to hide sensitive data, by reversing the extract into carefully filling it with secrets which comes from somewhere else.


If you kept in mind that unsafe is about the compiler and not the runtime, here is the proof:

func TestInTheBeginning(t *testing.T) {
    type r struct {
        sz  uintptr
        dt  uintptr
        _   uint32
        f   uint8
        _   uint8
        _   uint8
        knd uint8
        _   *struct{}
        c   *byte
        str int32
        w   int32
    type e struct {
        abracadabra *r
    t := func(p interface{}) *r {
        return (*(*e)(unsafe.Pointer(&p))).abracadabra
    p := Point{3, 4}
    v := t(&p)
    t.Logf("After looking in the mirror : %v %v %v %v %v %v", v.sz, v.dt, v.f, v.knd, v.str, v.w)

Once you run the above test, you will get the properties filled in with some values which seem pure magic. But there must be an explanation. We didn’t import reflect package. Also, the code is unreadable thus proving there is no magic convention like structs named in certain way or properties have some particular names.

So, what happen? Well, these data structures (e and r types) are known to the compiler which does it’s job and at the runtime we’re getting those results. To reinforce that truth, if we’re replacing that t function with it’s body v :=(*(*e)(unsafe.Pointer(&p))).abracadabra, it won’t work anymore. And even more, if we’re changing the parameter type of the t function from interface{} to *Point it will not work as expected.

If you look in reflect package, you will see that rtype struct looks exactly the same as our r struct, even if the properties are named different. Same goes for emptyInterface and our e struct - despite the fact that we are not using the word property - remember omitting properties in the unsafe example above?

Building your own reflection package

Can you build your own reflect package? So far my conclusion is yes, you can. At least for reading and writing the properties of structs it’s quite easy.

However, I’ve encountered some problems that I want to present here. First, the (long but minimal) code (mostly copy pasted from reflect):

import (
    "unsafe" // also for linkname

const (
    Invalid       Kind = iota

const (
    tflagUncommon  tflag = 1 << 0
    tflagExtraStar tflag = 1 << 1

const (
    kindMask = (1 << 5) - 1

type (
    Kind uint
    nameOff int32
    typeOff int32
    textOff int32
    tflag uint8
    name struct {
        bytes *byte
    uncommonType struct {
        pkgPath nameOff
        mcount  uint16
        _       uint16
        moff    uint32
        _       uint32
    rtype struct {
        size       uintptr
        ptrdata    uintptr
        hash       uint32
        tflag      tflag
        align      uint8
        fieldAlign uint8
        kind       uint8
        alg        *typeAlg
        gcdata     *byte
        str        nameOff
        ptrToThis  typeOff
    typeAlg struct {
        hash  func(unsafe.Pointer, uintptr) uintptr
        equal func(unsafe.Pointer, unsafe.Pointer) bool
    method struct {
        name nameOff
        mtyp typeOff
        ifn  textOff
        tfn  textOff
    structField struct {
        name       name
        typ        *rtype
        offsetAnon uintptr
    structType struct {
        rtype `reflect:"struct"`
        pkgPath name
        fields  []structField
    emptyInterface struct {
        typ  *rtype
        word unsafe.Pointer
    stringHeader struct {
        Data unsafe.Pointer
        Len  int
    ptrType struct {
        rtype `reflect:"ptr"`
        elem *rtype // pointer element (pointed at) type

func resolveReflectName(n name) nameOff {
    return nameOff(addReflectOff(unsafe.Pointer(n.bytes)))

func add(p unsafe.Pointer, x uintptr) unsafe.Pointer {
    return unsafe.Pointer(uintptr(p) + x)

func fnv1(x uint32, list ...byte) uint32 {
    for _, b := range list {
        x = x*16777619 ^ uint32(b)
    return x

func rtypeOff(section unsafe.Pointer, off int32) *rtype {
    return (*rtype)(add(section, uintptr(off)))

func typesByString(s string) []*rtype {
    sections, offset := typelinks()
    var ret []*rtype

    for offsI, offs := range offset {
        section := sections[offsI]
        i, j := 0, len(offs)
        for i < j {
            h := i + (j-i)/2
            if !(rtypeOff(section, offs[h]).String() >= s) {
                i = h + 1
            } else {
                j = h
        for j := i; j < len(offs); j++ {
            typ := rtypeOff(section, offs[j])
            if typ.String() != s {
            ret = append(ret, typ)
    return ret

func newName(n, tag string, exported bool) name {
    if len(n) > 1<<16-1 {
        panic("reflect.nameFrom: name too long: " + n)
    if len(tag) > 1<<16-1 {
        panic("reflect.nameFrom: tag too long: " + tag)

    var bits byte
    l := 1 + 2 + len(n)
    if exported {
        bits |= 1 << 0
    if len(tag) > 0 {
        l += 2 + len(tag)
        bits |= 1 << 1

    b := make([]byte, l)
    b[0] = bits
    b[1] = uint8(len(n) >> 8)
    b[2] = uint8(len(n))
    copy(b[3:], n)
    if len(tag) > 0 {
        tb := b[3+len(n):]
        tb[0] = uint8(len(tag) >> 8)
        tb[1] = uint8(len(tag))
        copy(tb[2:], tag)

    return name{bytes: &b[0]}

func (n name) isExported() bool {
    return (*n.bytes)&(1<<0) != 0

func (n name) name() (s string) {
    if n.bytes == nil {
        panic("no name")
    b := (*[4]byte)(unsafe.Pointer(n.bytes))

    hdr := (*stringHeader)(unsafe.Pointer(&s))
    hdr.Data = unsafe.Pointer(&b[3])
    hdr.Len = int(b[1])<<8 | int(b[2])
    return s

func (t *rtype) nameOff(off nameOff) name {
    return name{(*byte)(resolveNameOff(unsafe.Pointer(t), int32(off)))}

func (t *rtype) typeOff(off typeOff) *rtype {
    return (*rtype)(resolveTypeOff(unsafe.Pointer(t), int32(off)))

func (t *rtype) Kind() Kind { return Kind(t.kind & kindMask) }

func (t *rtype) String() string {
    s := t.nameOff(t.str).name()
    if t.tflag&tflagExtraStar != 0 {
        return s[1:]
    return s

func (t *uncommonType) methods() []method {
    if t.mcount == 0 {
        panic("zero methods")
    return (*[1 << 16]method)(add(unsafe.Pointer(t), uintptr(t.moff)))[:t.mcount:t.mcount]

func (t *rtype) uncommon() *uncommonType {
    if t.tflag&tflagUncommon == 0 {
        return nil
    if t.Kind() != Struct && t.Kind() != Ptr {
        panic("not struct or pointer")
    ptrToT := unsafe.Pointer(t)

    switch t.Kind() {
    case Struct:
        type u struct {
            u uncommonType
        return &(*u)(ptrToT).u
    case Ptr:
        type u struct {
            u uncommonType
        return &(*u)(ptrToT).u
        type u struct {
            u uncommonType
        return &(*u)(ptrToT).u

func (t *rtype) exportedMethods() []method {
    ut := t.uncommon()
    if ut == nil {
        return nil
    allMethods := ut.methods()
    allExported := true
    for _, method := range allMethods {
        name := t.nameOff(method.name)
        if !name.isExported() {
            allExported = false
    var methods []method
    if allExported {
        methods = allMethods
    } else {
        methods = make([]method, 0, len(allMethods))
        for _, m := range allMethods {
            name := t.nameOff(m.name)
            if name.isExported() {
                methods = append(methods, m)
        methods = methods[:len(methods):len(methods)]
    return methods

func (t *rtype) ptrTo() *rtype {
    if t.ptrToThis != 0 {
        return t.typeOff(t.ptrToThis)
    s := "*" + t.String()
    for _, tt := range typesByString(s) {
        p := (*ptrType)(unsafe.Pointer(tt))
        if p.elem != t {
        return &p.rtype
    var iptr interface{} = (*unsafe.Pointer)(nil)
    prototype := *(**ptrType)(unsafe.Pointer(&iptr))
    pp := *prototype
    pp.str = resolveReflectName(newName(s, "", false))
    pp.ptrToThis = 0
    pp.hash = fnv1(t.hash, '*')
    pp.elem = t
    return &pp.rtype

func TypeOf(i interface{}) *rtype {
    return (*(*emptyInterface)(unsafe.Pointer(&i))).typ.ptrTo()

Of course, to run tests, we have to create an empty.s file in the same folder and to add the linkname directives for two functions:

//go:linkname resolveTypeOff runtime.resolveTypeOff
func resolveTypeOff(rtype unsafe.Pointer, off int32) unsafe.Pointer

//go:linkname resolveNameOff runtime.resolveNameOff
func resolveNameOff(ptrInModule unsafe.Pointer, off int32) unsafe.Pointer

//go:linkname typelinks reflect.typelinks
func typelinks() (sections []unsafe.Pointer, offset [][]int32)

//go:linkname addReflectOff reflect.addReflectOff
func addReflectOff(ptr unsafe.Pointer) int32

On the Point struct declared above, we’re adding the followings:

func (p Point) AnotherMethod(scale int) int {
    return -1
func (p Point) Dist(scale int) int {
    return p.x*p.x*scale + p.y*p.y*scale
func (p Point) NoArgs() {
    println("NoArgs called.")
func (p Point) TotalDist(points ...Point) int {
    tot := 0
    for _, q := range points {
        dx := q.x - p.x
        dy := q.y - p.y
        tot += dx*dx + dy*dy
    return tot
func (p Point) NoArgsButReturn() string {
    return "something"

And finally, the test :

func TestMethod(t *testing.T) {
    p := Point{3, 4}
    pType := TypeOf(p)
    t.Logf("%v", pType)
    methods := pType.exportedMethods()
    for idx, method := range methods {
        name := pType.nameOff(method.name)
        typ := pType.typeOff(method.mtyp)
        t.Logf("%d : Method %q %v %v %v\n", idx, name.name(), typ, method.tfn, method.ifn)

When we run this test, we’re going to see that the methods signature are reported differently than what we’ve declared. This means we are not doing something that reflect package does.

Our version of TypeOf function doesn’t return an interface and also, that interface is built by calling toPtr() method of the rType. However, with that code added, the problem still doesn’t get fixed.

Adding the following code, fixes the test (the signatures are correct).

    type dummy struct{}
    func (d dummy) A() {}
    var _ = reflect.TypeOf(dummy{}).Method(0)

Seems the function func addReflectOff(ptr unsafe.Pointer) int32 which is implemented in the runtime package gets called from reflect package which creates reflectOffs structs for later lookups. We need to force the compiler to allow us to use the same functions as reflect does. Since we’re not using reflect anywhere, dead code removal does not allow us to initialize properly - so we need to force it.

Indeed, we’re importing reflect to write our own reflect, but we’re not using it in other than dumb init.

In the larger version (my own version of reflect), all Value.Call() tests were failing in a segmentation fault, with a reason (method types were zero) - the code being the same as in reflect package. For this reason I’ve presented you with this small test and it’s conclusions.


It took me four days to learn the internals and modify the reflect package for my needs, but in the end I’ve done it and later I will probably integrate it into the reflector package.

Probably the lack of documentation made things harder to understand and follow. Probably some things are never meant to be - that - public, due to some sort of programming language politics. Who knows but mostly who cares?

I encourage you to take my advice and break things so you can learn how they work, how other developers solved problems that you cannot think about while just reading the code.

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