“Select” statement inside of goroutine with for loop

Could somebody please explain, why if goroutine has endless for loop and select inside of the loop, a code in the loop is run only one time?

for

select

package main

import (
"time"
)

func f1(quit chan bool)
go func()
for
println("f1 is working...")
time.Sleep(1 * time.Second)

select
case <-quit:
println("stopping f1")
break


()


func main()
quit := make(chan bool)
f1(quit)
time.Sleep(4 * time.Second)



Output:

f1 is working...
Program exited.


But if "select" is commented out:

package main

import (
"time"
)

func f1(quit chan bool)
go func()
for
println("f1 is working...")
time.Sleep(1 * time.Second)

//select
//case <-quit:
//println("stopping f1")
//break
//

()


func main()
quit := make(chan bool)
f1(quit)
time.Sleep(4 * time.Second)



Output:

f1 is working...
f1 is working...
f1 is working...
f1 is working...
f1 is working...


https://play.golang.org/p/MxKy2XqQlt8

for

break

select

for

case

select

1 Answer
1

A select statement without a default case is blocking until a read or write in at least one of the case statements can be executed. Accordingly, your select will block until a read from the quit channel is possible (either of a value or the zero value if the channel is closed). The language spec provides a concrete description of this behavior, specifically:

select

default

case

select

quit

If one or more of the communications [expressed in the case statements] can proceed, a single one that can proceed is chosen via a uniform pseudo-random selection. Otherwise, if there is a default case, that case is chosen. If there is no default case, the "select" statement blocks until at least one of the communications can proceed.

case

Other code issues

break

select

However, even if you did close the quit channel to signal shutdown of your program, your implementation would likely not have the desired effect. A (unlabelled) call to break will terminate execution of the inner-most for, select or switch statement within the function. In this case, the select statement will break and the for loop will run through again. If quit was closed, it will run forever until something else stops the program, otherwise it will block again in select (Playground example)

quit

break

for

select

switch

select

for

quit

select

quit

As noted in the comments, the call to time.Sleep blocks for a second on each iteration of the loop, so any attempt to stop the program by closing quit will be delayed for up to approximately a second before the goroutine checks quit and escapes. It is unlikely this sleep period must complete in its entirety before the program stops.

time.Sleep

quit

quit

More idiomatic Go would block in the select statement on receiving from two channels:

select

quit

time.After

Timer

Resolution

A resolution to fix your immediate issue with minimal changes to your code would be:

quit

select

quit

for

break

return

f1

Depending on your circumstances, you may find it more idiomatic Go to use a context.Context to signal termination, and to use a sync.WaitGroup to wait for the goroutine to finish before returning from main.

context.Context

sync.WaitGroup

main

package main

import (
"fmt"
"time"
)

func f1(quit chan bool)
go func()
for
println("f1 is working...")
time.Sleep(1 * time.Second)

select
case <-quit:
fmt.Println("stopping")
return
default:


()


func main()
quit := make(chan bool)
f1(quit)
time.Sleep(4 * time.Second)
close(quit)
time.Sleep(4 * time.Second)



Working example

(Note: I have added an additional time.Sleep call in your main method to avoid this returning immediately after the call to close and terminating the program.)

time.Sleep

main

close

To fix the additional issue regarding the blocking sleep preventing immediate quit, move the sleep to a timer in the select block. Modifying your for loop as per this playground example from the comments does exactly this:

select

for

for
println("f1 is working...")

select
case <-quit:
println("stopping f1")
return
case <-time.After(1 * time.Second):
// repeats loop




Related literature

time.Sleep()

quit

select

quit

time.After

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