why I can't convert pointer string to string in Go?

Question

This code parse command-line arguments. If I type "/netstat -c /etc/config -I eth0", it should is :" c /etc/config \n i eth0", but it's not . Terminal output is :

c configfile

c interface

./netstat -c /etc/config -i eth0
c configfile
c interface

Code is as below:

package main

import (
    "flag"
    "fmt"
)

type CmdSt struct {
    configPtr    string
    interfacePtr string
}

var cmdSt CmdSt

func usage() {

    cmdSt.configPtr = *flag.String("c", "configfile", "configure file to parse ")
    cmdSt.interfacePtr = *flag.String("i", "interface", "capture network interface")
    /*
        a := flag.String("c", "configfile", "configure file to parse ")
        b := flag.String("i", "interface", "capture network interface")
    */

    flag.Parse()

    fmt.Println("c", cmdSt.configPtr)
    fmt.Println("i", cmdSt.interfacePtr)
    /*
        fmt.Println("c:", *a)
        fmt.Println("i:", *b)
    */
}
func main() {
    usage()

}

Do not post screencasts of text; rather, copy and paste text itself. — leaf bebop
– leaf bebop, Commented Feb 21, 2019 at 2:45

ישו אוהב אותך · Accepted Answer · 2019-02-21 04:02:30Z

It is because you're still holding the default value before flag.Parse() is called:

// still holding the "configfile" as value
cmdSt.configPtr = *flag.String("c", "configfile", "configure file to parse ")
// still holding the "interface" as value
cmdSt.interfacePtr = *flag.String("i", "interface", "capture network interface")

// flag is parsed now, but both cmdSt.configPtr and cmdSt.interfacePtr still holding the default value because of the pointer.
flag.Parse()

You can solve the problem by using temporary variables:

// hold the value to temporary variables
a := flag.String("c", "configfile", "configure file to parse ")
b := flag.String("i", "interface", "capture network interface")

// parse the flag and save to the variables.
flag.Parse()

// now, point the value to the CmdSt struct
cmdSt.configPtr = *a
cmdSt.interfacePtr = *b


fmt.Println("c", cmdSt.configPtr)
fmt.Println("i", cmdSt.interfacePtr)

Myles · Accepted Answer · 2019-02-21 04:20:54Z

1

As an alternative to using temporary variables, You can populate your already initialized struct with the flag.StringVar() like below:

flag.StringVar(&cmdSt.configPtr, "c", "configfile", "configure file to parse ")
flag.StringVar(&cmdSt.interfacePtr, "i", "interface", "capture network interface")

Then you can call the value immediately.

https://play.golang.org/p/RkW7YKfE8t8

answered Feb 21, 2019 at 4:20

Myles

682 silver badges7 bronze badges

Comments

score 1 · Accepted Answer · 2019-02-21 06:01:09Z

1

The application dereferences the pointer returned from flag.String after the default is set and before the command line is parsed. The cmdSt fields are set to the default values as a result.

Fix by using the flag.xxxVar() functions. These functions store flag values into values allocated by the application.

flag.StringVar(&cmdSt.configPtr, "c", "configfile", "configure file to parse ")
flag.StringVar(&cmdSt.interfacePtr, "i", "interface", "capture network interface")
flag.Parse()

// cmdSt.configPtr and cmdSt.interfacePtr are now set to
// command flag value or default if the flag was 
// not specified.

edited Feb 21, 2019 at 6:01

answered Feb 21, 2019 at 4:21

user5728991

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