affected/package: os

[athxx]

$ go version 1.17.6

Does this issue reproduce with the latest release?

yes

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

go env Output

set GO111MODULE=on
set GOARCH=amd64
set GOBIN=D:\Apps\gowin\bin
set GOCACHE=C:\Users\Administrator\AppData\Local\go-build
set GOENV=C:\Users\Administrator\AppData\Roaming\go\env
set GOEXE=.exe
set GOEXPERIMENT=
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMODCACHE=E:\Dev\gopath\pkg\mod
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows
set GOPATH=E:\Dev\gopath
set GOPRIVATE=
set GOPROXY=https://goproxy.io,direct
set GOROOT=D:\Apps\gowin
set GOSUMDB=sum.golang.org
set GOTMPDIR=
set GOTOOLDIR=D:\Apps\gowin\pkg\tool\windows_amd64
set GOVCS=
set GOVERSION=go1.17.6
set GCCGO=gccgo
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=E:\Dev\go\slm\svr\go.mod
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\ADMINI~1\AppData\Local\Temp\go-build2452781041=/tmp/go-build -gno-record-gcc-switches

What did you do?

I run this code times, when I try to run more than twice, it will leave many NUL characters in the document

package main
func main() {
	fPath := "./test.txt"
	f, _ := os.OpenFile(fPath, os.O_CREATE|os.O_RDWR, os.ModePerm)
	stat, _ := f.Stat()
	buf := make([]byte, stat.Size())
	f.Read(buf)
	f.Truncate(0)
	f.Sync()
	fmt.Println(string(buf))
	for i := 0; i < 10; i++ {
		f.WriteString(time.Now().GoString() + "\n")
	}
}

What did you expect to see?

it cleans all content and inserts new content

What did you see instead?

it remains many NUL characters

[ianlancetaylor]

I think that you are assuming that f.Truncate(0) resets the I/O position of the file to zero. It does not. After f.Truncate add a call to f.Seek(0, io.SeekStart). Then I think you will see the behavior that you expect.

[athxx]

@ianlancetaylor thanks