flag: define flag in multiple package does not working

[chinglinwen]

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

$ go version
go version go1.12.5 linux/amd64

Does this issue reproduce with the latest release?

yes

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

go env Output

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/wen/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/wen/gocode"
GOPROXY=""
GORACE=""
GOROOT="/home/wen/soft/go"
GOTMPDIR=""
GOTOOLDIR="/home/wen/soft/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build060836953=/tmp/go-build -gno-record-gcc-switches"

What did you do?

$ tree .
.
├── A
│   └── a.go
├── B
│   └── b.go
├── main.go
└── pkgflag

2 directories, 4 files

---

$ cat A/a.go 
package A

import (
	"flag"
	"fmt"
)

var A = flag.String("a", "", "")
var A2 string

func init() {
	flag.StringVar(&A2, "a2", "", "")
	flag.Parse()
	fmt.Printf("pkg a, A=%v, A2=%v\n", *A, A2)
}

---

$ cat B/b.go 
package B

import (
	"flag"
	"fmt"
)

var B = flag.String("b", "", "")
var B2 string

func init() {
	flag.StringVar(&B2, "b2", "", "")
	flag.Parse()
	fmt.Printf("pkg b, B=%v, B2=%v\n", *B, B2)
}

---

$ cat main.go 
package main

import (
	"flag"
	"fmt"
	"wen/pkgflag/A"
	"wen/pkgflag/B"
)

func main() {
	flag.Parse()

	fmt.Printf("pkg main, A=%v,A2=%v, B=%v,B2=%v\n", *A.A, A.A2, *B.B, B.B2)
}

What did you expect to see?

./pkgflag -b=hello is working, package B and package main can get the value hello of B.B

What did you see instead?

$ ./pkgflag -b=hello
flag provided but not defined: -b
Usage of ./pkgflag:
  -a string
    
  -a2 string
    
$ ./pkgflag -h
Usage of ./pkgflag:
  -a string
    
  -a2 string
    

[beoran]

I think you should not call flag.Parse() in the init() functions, but in the main() function of your application.

[robpike]

You can call flag.Parse in init if you like, although it's not a good idea for a number of reasons. But the important point is that your program should call it only once. And the best place is at the start of your main function.

[chinglinwen]

I remove the flag.Parse from package init, only main package do flag.Parse, it's just working.

$ ./pkgflag -b=aaa
pkg a, A=, A2=
pkg b, B=aaa, B2=
pkg main, A=,A2=, B=aaa,B2=

my bad, I've read some article that it can parse multiple times.

what I want is flag setting stick with package, so no need init variable(setting default secret etc) from main package.

It's working as expected ( without flag.parse in the imported package )

Thanks @beoran @robpike