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encoding/binary: Read and Write should be functionally equivalent #54492

QianChenglong opened this issue Aug 17, 2022 · 1 comment


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What version of Go are you using (go version)?

$ go version
go version go1.19 darwin/arm64

Does this issue reproduce with the latest release?


What operating system and processor architecture are you using (go env)?

go env Output
$ go env

What did you do?

func Test_struct(t *testing.T) {
	type S1 struct {
		age   uint8
		Class uint8
	s1 := &S1{
		age:   255,
		Class: 1,

	buf := new(bytes.Buffer)
	err := binary.Write(buf, binary.BigEndian, s1)
	assert.Nil(t, err)
	fmt.Printf("%#X\n", buf)

	s2 := new(S1)
	err = binary.Read(buf, binary.BigEndian, s2)
	assert.Nil(t, err)
	fmt.Printf("%#v\n", s2)

What did you expect to see?

Read should work as Write OR Write panic as Read

What did you see instead?

GOROOT=/opt/homebrew/Cellar/go/1.19/libexec #gosetup
GOPATH=/Users/chenglong/go #gosetup
/opt/homebrew/Cellar/go/1.19/libexec/bin/go test -c -o /private/var/folders/8r/llxlqhb519540gxrx8crdkx80000gn/T/GoLand/___1Test_struct_in_binary_test_go.test -gcflags all=-N -l /Users/chenglong/src/go-test/0.go/2.library/encoding/binary/binary_test.go #gosetup
/opt/homebrew/Cellar/go/1.19/libexec/bin/go tool test2json -t /private/var/folders/8r/llxlqhb519540gxrx8crdkx80000gn/T/ /Users/chenglong/Library/Application Support/JetBrains/Toolbox/apps/Goland/ch-0/222.3345.118/ --listen= --headless=true --api-version=2 --check-go-version=false --only-same-user=false exec /private/var/folders/8r/llxlqhb519540gxrx8crdkx80000gn/T/GoLand/___1Test_struct_in_binary_test_go.test -- -test.v -test.paniconexit0 ^\QTest_struct\E$
=== RUN Test_struct
--- FAIL: Test_struct (432.25s)
panic: reflect: reflect.Value.SetUint using value obtained using unexported field [recovered]
panic: reflect: reflect.Value.SetUint using value obtained using unexported field

goroutine 4 [running]:
testing.tRunner.func1.2({0x1042d37e0, 0x140000550a0})
/opt/homebrew/Cellar/go/1.19/libexec/src/testing/testing.go:1396 +0x374
/opt/homebrew/Cellar/go/1.19/libexec/src/testing/testing.go:1399 +0x560
panic({0x1042d37e0, 0x140000550a0})
/opt/homebrew/Cellar/go/1.19/libexec/src/runtime/panic.go:890 +0x26c
/opt/homebrew/Cellar/go/1.19/libexec/src/reflect/value.go:257 +0xf0
/opt/homebrew/Cellar/go/1.19/libexec/src/reflect/value.go:247 +0x68
reflect.Value.SetUint({0x1042d39e0, 0x14000018be2, 0x1a8}, 0xff)
/opt/homebrew/Cellar/go/1.19/libexec/src/reflect/value.go:2331 +0x30
encoding/binary.(*decoder).value(0x1400007bd38, {0x1042d39e0, 0x14000018be2, 0x1a8})
/opt/homebrew/Cellar/go/1.19/libexec/src/encoding/binary/binary.go:650 +0x220
encoding/binary.(*decoder).value(0x1400011bd38, {0x1042e7100, 0x14000018be2, 0x199})
/opt/homebrew/Cellar/go/1.19/libexec/src/encoding/binary/binary.go:625 +0x660
encoding/binary.Read({0x10430ae28, 0x140000673e0}, {0x10430c3a8, 0x104459d68}, {0x1042cc5c0, 0x14000018be2})
/opt/homebrew/Cellar/go/1.19/libexec/src/encoding/binary/binary.go:326 +0x10a0
/Users/chenglong/src/go-test/0.go/2.library/encoding/binary/binary_test.go:28 +0x1a0
testing.tRunner(0x14000003ba0, 0x104309dc8)
/opt/homebrew/Cellar/go/1.19/libexec/src/testing/testing.go:1446 +0x174
created by testing.(*T).Run
/opt/homebrew/Cellar/go/1.19/libexec/src/testing/testing.go:1493 +0x540

Debugger finished with the exit code 0

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That is how the language works: you can read unexported fields, but you can't write them.

@ianlancetaylor ianlancetaylor closed this as not planned Won't fix, can't repro, duplicate, stale Aug 17, 2022
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