How to cancel fmt.Scanf after a certain timeout?

Question

I have a very simple command line utility and at the end of it I'm waiting for a user to hit the enter key to end the program:

fmt.Scanf("\n") // wait for hitting the enter key to end the program

Now I want to change it and if the enter key hasn't been hit for some time, I want to cancel Scanf and do something else. Is it possible to cancel waiting for user's input?

Answer 1

You can simply create a channel and launch a goroutine which does the fmt.Scanf("\n") and then writes something to the channel. Then select between that channel and time.After(3 * time.Second).

Here's a solution:

package main

import (
    "fmt"
    "time"
    "os"
)

func main() {
    // Rest of the program...

    ch := make(chan int)

    go func() {
        fmt.Scanf("\n")
        ch <- 1
    }()

    select {
    case <-ch:
        fmt.Println("Exiting.")
        os.Exit(0)
    case <-time.After(3 * time.Second):
        fmt.Println("Timed out, exiting.")
    }
}

NOTE: as pointed out by @Fabian and @YotKay in the comments, this will "leak" a goroutine (meaning it will remain running until the process exits) if the timeout expires and the user does not enter anything. This is still fine in a situation in which you have to either wait for input or exit the program, but not advisable in other cases, since it's not possible to "cancel" goroutines from the outside.