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fmt does not format byte slices nicely #34916

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guysoffer opened this issue Oct 15, 2019 · 4 comments
Closed

fmt does not format byte slices nicely #34916

guysoffer opened this issue Oct 15, 2019 · 4 comments
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FrozenDueToAge WaitingForInfo Issue is not actionable because of missing required information, which needs to be provided.

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@guysoffer
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guysoffer commented Oct 15, 2019 •
edited by ALTree

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

go version go1.12.1 windows/amd64

Does this issue reproduce with the latest release?

reproduces on go playground

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

go env Output
C:\Users\gs082r>go env
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\gs082r\AppData\Local\go-build
set GOEXE=.exe
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOOS=windows
set GOPATH=C:\Users\gs082r\go
set GOPROXY=
set GORACE=
set GOROOT=C:\Go
set GOTMPDIR=
set GOTOOLDIR=C:\Go\pkg\tool\windows_amd64
set GCCGO=gccgo
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=
set CGO_CFLAGS=-g -O2
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-g -O2
set CGO_FFLAGS=-g -O2
set CGO_LDFLAGS=-g -O2
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -mthreads -fmessage-length=0 -fdebug-prefix-map=C:\Users\gs0
82r\AppData\Local\Temp\go-build177268958=/tmp/go-build -gno-record-gcc-switches

What did you do?

I'm trying to print a slice of bytes as an array of hex values.

What did you expect to see?

[0x0a 0x14 0x1e 0x28 0x32 0x3c 0x46 0x50 0x5a 0x64 0x65]

What did you see instead?

0x0a14

Please see playground link:
https://play.golang.org/p/gQIYJV6ogx4

The first line is ok for a slice of ints.. the %.2v is a sort of workaround, but not what I actually need.

dump from playground:

package main

import (
	"fmt"
)

func main() {
	slcInt8 := [...]int8{10,20,30,40,50,60,70,80,90,100,101}
	fmt.Printf("%#.2x\n", slcInt8)	
	slcByte := [...]byte{10,20,30,40,50,60,70,80,90,100,101}
	fmt.Printf("%#.2x\n", slcByte)	
	slcUint8 := [...]byte{10,20,30,40,50,60,70,80,90,100,101}
	fmt.Printf("%#.2x\n", slcUint8)	
	
	fmt.Printf("%#x\n", slcInt8)	
	fmt.Printf("%#x\n", slcByte)	
	fmt.Printf("%#x\n", slcUint8)	
	
	fmt.Printf("%#.2v\n", slcInt8)	
	fmt.Printf("%#.2v\n", slcByte)	
	fmt.Printf("%#.2v\n", slcUint8)		
}

output:

[0x0a 0x14 0x1e 0x28 0x32 0x3c 0x46 0x50 0x5a 0x64 0x65]
0x0a14
0x0a14
[0xa 0x14 0x1e 0x28 0x32 0x3c 0x46 0x50 0x5a 0x64 0x65]
0x0a141e28323c46505a6465
0x0a141e28323c46505a6465
[11]int8{10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 101}
[11]uint8{0x0a, 0x14, 0x1e, 0x28, 0x32, 0x3c, 0x46, 0x50, 0x5a, 0x64, 0x65}
[11]uint8{0x0a, 0x14, 0x1e, 0x28, 0x32, 0x3c, 0x46, 0x50, 0x5a, 0x64, 0x65}
@ianlancetaylor
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I'm sorry, I don't understand what you are asking for. It seems that using %#.2x gives you exactly what you want.

Whatever we think of the behavior of the fmt package (and I think it is making a reasonable choice here) we can't change it now. It would break too many existing programs.

@ianlancetaylor ianlancetaylor added the WaitingForInfo Issue is not actionable because of missing required information, which needs to be provided. label Oct 15, 2019
@martisch
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martisch commented Oct 15, 2019 •
edited

fmt.Printf("[%# x]", slcByte) will output:

[0x0a 0x14 0x1e 0x28 0x32 0x3c 0x46 0x50 0x5a 0x64 0x65]

@guysoffer
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@ianlancetaylor

slcByte := [...]byte{10,20,30,40,50,60,70,80,90,100,101}
fmt.Printf("%#.2x\n", slcByte)

gives output:
0x0a14

does this look like expected/reasonable behaviour ?

@martisch thanks ! this is exactly what I was looking for !

@martisch
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martisch commented Oct 16, 2019 •
edited

The current behavior looks to me as documented in package fmt. Note that []byte often formats the way a string would be formatted. fmt.Printf("%#.2x\n",string([]byte{10,20,30,40,50,60,70,80,90,100,101})) gives: 0x0a14

That is why []int behaves differently. We can not change it without breaking backwards compatibility. Since there is a way to format the slcByte the way wanted I am closing the issue.

If the behavior should be changed please open a new issue with a concrete proposal for Go 2 how the behavior of fmt should be changed and how the gained advantage is worth breaking the backwards compatibility.

@golang golang locked and limited conversation to collaborators Oct 15, 2020
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@martisch @ianlancetaylor @gopherbot @guysoffer