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?
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.
You just need to put it in an infinate loop
package main
import (
"fmt"
"time"
"context"
)
// 3 seconds for example
var deadline = time.Second * 3
func main() {
c := make(chan string, 1)
go scan(c)
ctx, _ := context.WithTimeout(context.Background(), deadline)
select {
case <-ctx.Done():
// didnt type for deadline seconds
case <-c:
// did it in time
}
}
func scan(in chan string) {
var input string
_, err := fmt.Scanln(&input)
if err != nil {
panic(err)
}
in <- input
}
Related
The problem is that both the goOne and goTwo functions are sending values to the channels ch1 and ch2 respectively, but there is no corresponding receiver for these values in the main function. This means that the channels are blocked and the program is unable to proceed. As a result, the select statement in the main function is unable to read from the channels, so it always executes the default case.
package main
import (
"fmt"
"sync"
)
func main() {
var wg sync.WaitGroup
ch1 := make(chan string)
ch2 := make(chan string)
wg.Add(2)
go goOne(&wg, ch1)
go goTwo(&wg, ch2)
select {
case <-ch1:
fmt.Println(<-ch1)
close(ch1)
case <-ch2:
fmt.Println(<-ch2)
close(ch2)
default:
fmt.Println("Default Case")
}
wg.Wait()
}
func goTwo(wg *sync.WaitGroup, ch2 chan string) {
ch2 <- "Channel 2"
wg.Done()
}
func goOne(wg *sync.WaitGroup, ch1 chan string) {
ch1 <- "Channel 1"
wg.Done()
}
Output:
Default Case
fatal error: all goroutines are asleep - deadlock!
goroutine 1 \[semacquire\]:
sync.runtime_Semacquire(0xc000108270?)
/usr/local/go/src/runtime/sema.go:62 +0x25
sync.(\*WaitGroup).Wait(0x4b9778?)
/usr/local/go/src/sync/waitgroup.go:139 +0x52
main.main()
/home/nidhey/Documents/Go_Learning/goroutines/select.go:29 +0x2af
goroutine 6 \[chan send\]:
main.goOne(0x0?, 0x0?)
/home/nidhey/Documents/Go_Learning/goroutines/select.go:39 +0x28
created by main.main
/home/nidhey/Documents/Go_Learning/goroutines/select.go:14 +0xc5
goroutine 7 \[chan send\]:
main.goTwo(0x0?, 0x0?)
/home/nidhey/Documents/Go_Learning/goroutines/select.go:33 +0x28
created by main.main
/home/nidhey/Documents/Go_Learning/goroutines/select.go:15 +0x119\```
I'm looking for a different pattern such as select to handle the case when the channels are blocked.
To fix the issue, I've added a <-ch1 or <-ch2 in the main function after wg.Wait() to receive the values sent to the channels and unblock them
It's not entirely clear what you want to do. If you want to wait for both goroutines to complete their work and get the result of their work into the channel, you don't need a weight group (because it won't be reached).
You can do something like this.
package main
import (
"fmt"
"time"
)
func main() {
ch1 := make(chan string)
ch2 := make(chan string)
go goOne(ch1)
go goTwo(ch2)
for {
select {
case v := <-ch1:
fmt.Println("Done ch1:", v)
ch1 = nil
case v := <-ch2:
fmt.Println("Done ch2:", v)
ch2 = nil
case <-time.After(time.Second):
fmt.Println("I didn't get result so lets skip it!")
ch1, ch2 = nil, nil
}
if ch1 == nil && ch2 == nil {
break
}
}
}
func goTwo(ch chan string) {
ch <- "Channel 2"
}
func goOne(_ chan string) {
//ch1 <- "Channel 1"
}
UPD:
Imagine if we are having two api end points, API1 & API2 which are returning same data but are hosted on different regions. So what I want to do, I need to make API calls for both apis in two different function ie goroutines and as soon as any one api sends us response back, I want to process the data received. So for that Im check whcih api is fetching data first using select block.
package main
import (
"context"
"fmt"
"math/rand"
"time"
)
func main() {
regions := []string{
"Europe",
"America",
"Asia",
}
// Just for different results for each run
rand.Seed(time.Now().UnixNano())
rand.Shuffle(len(regions), func(i, j int) { regions[i], regions[j] = regions[j], regions[i] })
output := make(chan string)
ctx, cancel := context.WithTimeout(context.Background(), 2 * time.Second)
for i, region := range regions {
go func(ctx context.Context, region string, output chan <- string, i int) {
// Do call with context
// If context will be cancelled just ignore it here
timeout := time.Duration(i)*time.Second
fmt.Printf("Call %s (with timeout %s)\n", region, timeout)
time.Sleep(timeout) // Simulate request timeout
select {
case <-ctx.Done():
fmt.Println("Cancel by context:", region)
case output <- fmt.Sprintf("Answer from `%s`", region):
fmt.Println("Done:", region)
}
}(ctx, region, output, i)
}
select {
case v := <-output:
cancel() // Cancel all requests in progress (if possible)
// Do not close output chan to avoid panics: When the channel is no longer used, it will be garbage collected.
fmt.Println("Result:", v)
case <-ctx.Done():
fmt.Println("Timeout by context done")
}
fmt.Println("There is we already have result or timeout, but wait a little bit to check all is okay")
time.Sleep(5*time.Second)
}
Firstly you have a race condition in that your channel publishing goroutines will probably not have been started by the time you enter the select statement, and it will immediately fall through to the default.
But assuming you resolve this (e.g. with another form of semaphore) you're on to the next issue - your select statement will either get chan1 or chan2 message, then wait at the bottom of the method, but since it is no longer in the select statement, one of your messages won't have arrived and you'll be waiting forever.
You'll need either a loop (twice in this case) or a receiver for both channels running in their own goroutines to pick up both messages and complete the waitgroup.
But in any case - as others have queried - what are you trying to achieve?
You can try something like iterating over a single channel (assuming both channels return the same type of data) and then count in the main method how many tasks are completed. Then close the channel once all the tasks are done. Example:
package main
import (
"fmt"
)
func main() {
ch := make(chan string)
go goOne(ch)
go goTwo(ch)
doneCount := 0
for v := range ch {
fmt.Println(v)
doneCount++
if doneCount == 2 {
close(ch)
}
}
}
func goTwo(ch chan string) {
ch <- "Channel 2"
}
func goOne(ch chan string) {
ch <- "Channel 1"
}
There are two similar cases I would like to compare with you - the only difference is the way of handling values generation
the first case: values generation in one of case of select
package main
import (
"fmt"
"math/rand"
"time"
)
func main() {
generateValues := func(done <-chan interface{}) <-chan int {
values := make(chan int)
go func() {
defer fmt.Println("All values generated")
defer close(values)
for {
select {
case <-done:
fmt.Println("DONE")
return
case values <- rand.Int():
fmt.Println("Generated")
}
}
}()
return values
}
done := make(chan interface{})
values := generateValues(done)
for i := 0; i < 3; i++ {
fmt.Printf("Received value: %v\n", <-values)
}
fmt.Println("Closing the channel")
close(done)
time.Sleep(5 * time.Second)
}
Go playground: https://go.dev/play/p/edlOSqdZ9ys
the second case: value generation in default case
package main
import (
"fmt"
"math/rand"
"time"
)
func main() {
generateValues := func(done <-chan interface{}) <-chan int {
values := make(chan int)
go func() {
defer fmt.Println("All values generated")
defer close(values)
for {
select {
case <-done:
fmt.Println("DONE")
return
default:
values <- rand.Int()
fmt.Println("Generated")
}
}
}()
return values
}
done := make(chan interface{})
values := generateValues(done)
for i := 0; i < 3; i++ {
fmt.Printf("Received value: %v\n", <-values)
}
fmt.Println("Closing the channel")
close(done)
time.Sleep(5 * time.Second)
}
Go playground: https://go.dev/play/p/edlOSqdZ9ys
As you can see the second case seems leading to the situation that the 'Done' is not printed and calls related to 'defer' are not invoked. I believe we have here goroutine leaks, but cannot clearly explain it. I expected the same behaviour like in the first case.
Could someone please help in understanding the difference between them?
In the second case, the generating goroutine is not likely to receive the done message. Since the default case is always enabled, after the main goroutine receives the last value, the generating goroutine makes another round, falls into the default case, and blocks waiting to write to the values channel. While it is waiting there, the main goroutine closes the done channel and terminates.
This doesn't mean there doesn't exist an execution path where the generating goroutine doesn't receive the done channel. For this to happen, immediately after sending the last value, the generating goroutine must be preempted by the main goroutine that runs until it closes the done channel. Then if the generating goroutine is scheduled, it can receive the done signal. However, this sequence of events is highly unlikely.
I am writing code to control external LEDs over UDP, and need to keep a frame rate (e.g. 60Hz) that is as constant as possible. Too much jitter will look bad. I've written a simple test using time.NewTicker and the results are not ideal. I'm wondering if there is a different way to execute code on a more accurate interval. This test was run on macOS, but needs to run on Windows and Linux too. For what it's worth, it needs to sync to audio, so maybe there is an audio ticker API on each OS it could potentially sync with?
package main
import (
"encoding/csv"
"fmt"
"log"
"os"
"strconv"
"time"
)
var f *os.File
var testTimeSeconds = 30
func appendToCsv(t time.Time) {
w := csv.NewWriter(f)
defer w.Flush()
records := []string{strconv.FormatInt(t.UnixMicro(), 10)}
w.Write(records)
}
func init() {
var err error
f, err = os.Create("newTicker.csv")
if err != nil {
log.Fatal(err)
}
w := csv.NewWriter(f)
defer w.Flush()
records := []string{"timestamps"}
w.Write(records)
}
func main() {
usPerFrame := 16666
ticker := time.NewTicker(time.Duration(usPerFrame) * time.Microsecond)
defer ticker.Stop()
done := make(chan bool)
go func() {
time.Sleep(time.Duration(testTimeSeconds) * time.Second)
done <- true
}()
for {
select {
case <-done:
fmt.Println("Done!")
return
case t := <-ticker.C:
appendToCsv(t)
}
}
}
UPDATE:
I ran another test comparing the first method with the method in #jochen's answer, still not very accurate.
One idea would be to just use time.Sleep() instead of a ticker. This takes the channel send/receive out of the loop and may lead to more accurate timing. To do this, you could run a function like the following in a separate goroutine:
func ticker(step time.Duration, done <-chan struct{}) {
next := time.Now().Add(step)
for {
time.Sleep(time.Until(next))
appendToCsv(time.Now())
select { // check whether `done` was closed
case <-done:
return
default:
// pass
}
next = next.Add(step)
}
}
I recently started learning go and I am really impressed with all the features. I been playing with go routines and term-ui and facing some trouble. I am trying to exit this code from console after I run it but it just doesn't respond. If I run it without go-routine it does respond to my q key press event.
Any help is appreciated.
My code
package main
import (
"fmt"
"github.com/gizak/termui"
"time"
"strconv"
)
func getData(ch chan string) {
i := 0
for {
ch <- strconv.Itoa(i)
i++
time.Sleep(time.Second)
if i == 20 {
break
}
}
}
func Display(ch chan string) {
err := termui.Init()
if err != nil {
panic(err)
}
defer termui.Close()
termui.Handle("/sys/kbd/q", func(termui.Event) {
fmt.Println("q captured")
termui.Close()
termui.StopLoop()
})
for elem := range ch {
par := termui.NewPar(elem)
par.Height = 5
par.Width = 37
par.Y = 4
par.BorderLabel = "term ui example with chan"
par.BorderFg = termui.ColorYellow
termui.Render(par)
}
}
func main() {
ch := make(chan string)
go getData(ch)
Display(ch)
}
This is possibly the answer you are looking for. First off, you aren't using termui correctly. You need to call it's Loop function to start the Event loop so that it can actually start listening for the q key. Loop is called last because it essentially takes control of the main goroutine from then on until StopLoop is called and it quits.
In order to stop the goroutines, it is common to have a "stop" channel. Usually it is a chan struct{} to save memory because you don't ever have to put anything in it. Wherever you want the goroutine to possibly stop and shutoff (or do something else perhaps), you use a select statement with the channels you are using. This select is ordered, so it will take from them in order unless they block, in which case it tries the next one, so the stop channel usually goes first. The stop channel normally blocks, but to get it to take this path, simply close()ing it will cause this path to be chosen in the select. So we close() it in the q keyboard handler.
package main
import (
"fmt"
"github.com/gizak/termui"
"strconv"
"time"
)
func getData(ch chan string, stop chan struct{}) {
i := 0
for {
select {
case <-stop:
break
case ch <- strconv.Itoa(i):
}
i++
time.Sleep(time.Second)
if i == 20 {
break
}
}
}
func Display(ch chan string, stop chan struct{}) {
for {
var elem string
select {
case <-stop:
break
case elem = <-ch:
}
par := termui.NewPar(elem)
par.Height = 5
par.Width = 37
par.Y = 4
par.BorderLabel = "term ui example with chan"
par.BorderFg = termui.ColorYellow
termui.Render(par)
}
}
func main() {
ch := make(chan string)
stop := make(chan struct{})
err := termui.Init()
if err != nil {
panic(err)
}
defer termui.Close()
termui.Handle("/sys/kbd/q", func(termui.Event) {
fmt.Println("q captured")
close(stop)
termui.StopLoop()
})
go getData(ch, stop)
go Display(ch, stop)
termui.Loop()
}
This is an easy way to read a key from the console
reader := bufio.NewReader(os.Stdin)
// ...
func readKey() rune {
char, _, err := reader.ReadRune()
if err != nil {
fmt.Println("Error reading key: ", err)
}
return char
}
// ...
fmt.Println("Checking keyboard input...")
loop:
for {
keyb := readKey()
switch keyb {
case 'x':
fmt.Println("x key pressed, exiting loop")
break loop
}
}
However the issue is the application always waits for a key to be read. What if you want to wait only 5 seconds for a key to be read, and if no key is read, continue the application?
I'm thinking that I must pull in a dependency maybe, such as ncurses or a unit (module) that turbopascal had which was called crt and had a readkey function. But is a dependency really necessary or is there an easy way to do it without? Possibly even some defer() tricks, I don't know.
You don't need external dependencies to achieve this.
You can use a channel and set a timeout on it.
Here's documentation info about that: https://gobyexample.com/timeouts
The key part is making the input go through the channel in a separate goroutine, so that the main thread does not block waiting. You can then decide how long to wait to receive the input through the channel by setting a timeout in the select clause.
And here's a working sample using your post as a base:
package main
import (
"bufio"
"os"
"log"
"fmt"
"time"
)
var reader = bufio.NewReader(os.Stdin)
func readKey(input chan rune) {
char, _, err := reader.ReadRune()
if err != nil {
log.Fatal(err)
}
input <- char
}
func main() {
input := make(chan rune, 1)
fmt.Println("Checking keyboard input...")
go readKey(input)
select {
case i := <-input:
fmt.Printf("Input : %v\n", i)
case <-time.After(5000 * time.Millisecond):
fmt.Println("Time out!")
}
}
Probably the most "go-isch" way to do this is using a goroutine and channels. You start two goroutines, one which waits for input, and one which sleeps until after the timeout period. You then use a select statement in your main goroutine to check what event happened first (the input, or the timeout). Example code:
package main
import (
"fmt"
"time"
)
func waitForInput(didInput chan<- bool) {
// Wait for a valid input here
didInput <- true
}
func main() {
didInput := make(chan bool, 1)
timeout := make(chan bool, 1)
go func() {
time.Sleep(5 * time.Second)
timeout <- true
}()
go waitForInput(didInput)
select {
case <-didInput:
fmt.Println("")
// Continue your application here
case <-timeout:
// Input timed out, quit your application here
}
}