I have the following excerpt from an example given in a programming course, and I am not sure why a deadlock is occurring.
package main
import (
"fmt"
)
var (
Count int = 0
nFunctions int = 2
)
var sema = make(chan int)
func increment(ch chan int, nSteps int) {
for i := 0; i < nSteps; i++ {
<- sema
cnt := Count
Count = cnt + 1
ch <- 1
sema <- 1
}
return
}
func main() {
ch := make(chan int)
sema <- 1
go increment(ch, 1000)
go increment(ch, 1000)
for i := 0; i < nFunctions*1000; i++ {
<-ch
}
fmt.Printf("Count = %d\n", Count)
}
Strangely, the deadlock does not occur when I change the statement, in the main, from sema <- 1 to
go func () {
sema <- 1
}()
Any explanations for this much appreciated. The error message is :
fatal error: all goroutines are asleep - deadlock!
goroutine 1 [chan send]:
main.main()
Channels block both the sender and the receiver. If you send something, you are blocked until it is received. You could reduce your code even further, all you need is a channel and then write to that channel. Note that you could also use a buffered channel, which allows writes up to the buffer length without blocking. If the buffer is full however, it does still block.
Related
I am learning go routines and channels,here is a basic program which i tried on the topic ,I am getting error that fatal error: all goroutines are asleep - deadlock! and I also want to know why length of my channel is zero.
package main
import (
"fmt"
)
func main() {
mychannel := make(chan int)
go takeChn(mychannel)
fmt.Println("length", len(mychannel))
for res := range mychannel {
fmt.Println("firstFunc", res, len(mychannel))
}
}
func takeChn(c chan int) {
for i := 0; i < 10; i++ {
c <- (i + 1)
}
}
The length of the channel is zero, because it is an unbuffered channel. It does not store any elements.
Your program deadlocks because the for-loop never terminates. The range over the channel will terminate when the channel is closed, but you never close the channel.
If you close the channel at the end of the goroutine, the program will run without a deadlock.
Modifying the code as per Burak Serdar's input by closing the channel
package main
import (
"fmt"
)
func main() {
mychannel := make(chan int)
go takeChn(mychannel)
fmt.Println("length", len(mychannel))
for res := range mychannel {
fmt.Println("firstFunc", res, len(mychannel))
}
}
func takeChn(c chan int) {
for i := 0; i < 10; i++ {
c <- (i + 1)
}
close(c)
}
Output:
length 0
firstFunc 1 0
firstFunc 2 0
firstFunc 3 0
firstFunc 4 0
firstFunc 5 0
firstFunc 6 0
firstFunc 7 0
firstFunc 8 0
firstFunc 9 0
firstFunc 10 0
I just started to learn Go so please bear with me, I've tried to play around with Go routines and channels but are getting a deadlock somehow.
Here's the example
package main
import (
"fmt"
"sync"
)
func main() {
total := 2
var wg sync.WaitGroup
wg.Add(total)
ch := make(chan int)
for idx := 0; idx < total; idx++ {
fmt.Printf("Processing idx %d\n", idx)
go func(idx int) {
defer wg.Done()
ch <- idx
}(idx)
}
for val := range ch {
fmt.Println(val)
}
fmt.Println("Wait")
wg.Wait()
}
which throws the error
Processing idx 0
Processing idx 1
1
0
fatal error: all goroutines are asleep - deadlock!
range ch reads from the channel until it is closed.
How many times do you call close(ch)? When will the for val := range ch loop terminate?
When should you close the channel? You have a lot of options here, but one way to do it is to add another goroutine:
go func() {
wg.Wait()
close(ch)
}()
e.g., after spinning off all routines that will write-to-channel-then-call-wg.Done(), so that the channel is closed once all the writers are done writing. (You can run this goroutine as soon as you've increased the wg count to account for all writers.)
I am trying the fan in - fan out pattern with a factorial problem. But I am getting:
fatal error: all goroutines are asleep - deadlock!
and unable to identify the reason for deadlock.
I am trying to concurrently calculate factorial for 100 numbers using the fan-in fan-out pattern.
package main
import (
"fmt"
)
func main() {
_inChannel := _inListener(generator())
for val := range _inChannel {
fmt.Print(val, " -- ")
}
}
func generator() chan int { // NEED TO CALCULATE FACTORIAL FOR 100 NUMBERS
ch := make(chan int) // CREATE CHANNEL TO INPUT NUMBERS
go func() {
for i := 1; i <= 100; i++ {
ch <- i
}
close(ch) // CLOSE CHANNEL WHEN ALL NUMBERS HAVE BEEN WRITTEM
}()
return ch
}
func _inListener(ch chan int) chan int {
rec := make(chan int) // CHANNEL RECEIVED FROM GENERATOR
go func() {
for num := range ch { // RECEIVE THE INPUT NUMBERS FROM GENERATOR
result := factorial(num) // RESULT IS A NEW CHANNEL CREATED
rec <- <-result // MERGE INTO A SINGLE CHANNEL; rec
close(result)
}
close(rec)
}()
return rec // RETURN THE DEDICATED CHANNEL TO RECEIVE ALL OUTPUTS
}
func factorial(n int) chan int {
ch := make(chan int) // MAKE A NEW CHANNEL TO OUTPUT THE RESULT
// OF FACTORIAL
total := 1
for i := n; i > 0; i-- {
total *= i
}
ch <- total
return ch // RETURN THE CHANNEL HAVING THE FACTORIAL CALCULATED
}
I have put in comments, so that it becomes easier to follow the code.
I'm no expert in channels. I've taking on this to try and get more familiar with go.
Another issue is the int isn't large enough to take all factorials over 20 or so.
As you can see, I added a defer close as well as a logical channel called done in the generator func. The rest of the changes probably aren't needed. With channels you need to make sure something is ready to take off a value on the channel when you put something on a channel. Otherwise deadlock. Also, using
go run -race main.go
helps at least see which line(s) are causing problems.
I hope this helps and isn't removed for being off topic.
I was able to remove the deadlock by doing this:
package main
import (
"fmt"
)
func main() {
_gen := generator()
_inChannel := _inListener(_gen)
for val := range _inChannel {
fmt.Print(val, " -- \n")
}
}
func generator() chan int { // NEED TO CALCULATE FACTORIAL FOR 100 NUMBERS
ch := make(chan int) // CREATE CHANNEL TO INPUT NUMBERS
done := make(chan bool)
go func() {
defer close(ch)
for i := 1; i <= 100; i++ {
ch <- i
}
//close(ch) // CLOSE CHANNEL WHEN ALL NUMBERS HAVE BEEN WRITTEM
done <- true
}()
// this function will pull off the done for each function call above.
go func() {
for i := 1; i < 100; i++ {
<-done
}
}()
return ch
}
func _inListener(ch chan int) chan int {
rec := make(chan int) // CHANNEL RECEIVED FROM GENERATOR
go func() {
for num := range ch { // RECEIVE THE INPUT NUMBERS FROM GENERATOR
result := factorial(num) // RESULT IS A NEW CHANNEL CREATED
rec <- result // MERGE INTO A SINGLE CHANNEL; rec
}
close(rec)
}()
return rec // RETURN THE DEDICATED CHANNEL TO RECEIVE ALL OUTPUTS
}
func factorial(n int) int {
// OF FACTORIAL
total := 1
for i := n; i > 0; i-- {
total *= i
}
return total // RETURN THE CHANNEL HAVING THE FACTORIAL CALCULATED
}
Here is my code at Go Playground
package main
import (
"fmt"
)
func sum_up(my_int int, cs chan int) {
my_sum := 0
for i := 0; i < my_int; i++ {
my_sum += i
}
cs <- my_sum
}
func main() {
my_channel := make(chan int)
for i := 2; i < 5; i++ {
go sum_up(i, my_channel)
}
for ele := range my_channel {
fmt.Println(ele)
}
//fatal error: all goroutines are asleep - deadlock!
fmt.Println("Done")
}
Which results in:
1
3
6
fatal error: all goroutines are asleep - deadlock!
And I don't understand what causes the error. My understanding is that in my function sum_up I am adding new values to my_channel. Why does the problem occur after I try to print out the values? Since I see 1,3,6 are printed it means that all goroutines have successfully finished.
Moreover, if the block that attempts to print the values of the channel
for ele := range my_channel {
fmt.Println(ele)
}
Is removed, then I don't get the error. So it is including the block that causes the error, but why?
A for-range on a empty channel will block until there are elements to read from the channel or until the channel is closed.
Here is a version that uses sync.WaitGroup to account for how many goroutines remain active. After all goroutines have finished, the channel is closed and the for-range loop exists.
https://play.golang.org/p/ZnLYxLMNdF
package main
import (
"fmt"
"sync"
)
func sum_up(my_int int, cs chan int, wg *sync.WaitGroup) {
my_sum := 0
for i := 0; i < my_int; i++ {
my_sum += i
}
cs <- my_sum
wg.Done()
}
func main() {
wg := &sync.WaitGroup{}
my_channel := make(chan int)
for i := 2; i < 5; i++ {
wg.Add(1)
go sum_up(i, my_channel, wg)
}
// Run a goroutine that will monitor how many sum_up are running.
go func(cs chan int, wg *sync.WaitGroup) {
wg.Wait()
close(cs)
}(my_channel, wg)
for ele := range my_channel {
fmt.Println(ele)
}
//fatal error: all goroutines are asleep - deadlock!
fmt.Println("Done")
}
When you use range on a channel, it will wait for values forever or until the channel is closed. It's deadlocking because when the last value is written to my_channel, it will wait forever for a value that will never come.
Here's a slightly modified variant that shows how to cleanly leave the range: https://play.golang.org/p/YDlM8EcRnx
for range chan exit when chan receive close signal. You must close(my_channel) somewhere, or loop will wait forever.
I am trying to understand, how golang channel works. I read a book about the go language and found the following example.
package main
import (
"fmt"
)
// Send the sequence 2, 3, 4, ... to returned channel
func generate() chan int {
ch := make(chan int)
go func() {
for i := 2; i <= 100 ; i++ {
ch <- i
}
}()
return ch
}
// Filter out input values divisible by 'prime', send rest to returned channel
func filter(in chan int, prime int) chan int {
out := make(chan int)
go func() {
for {
if i := <-in; i%prime != 0 {
out <- i
}
}
}()
return out
}
func sieve() chan int {
out := make(chan int)
go func() {
ch := generate()
for {
prime := <-ch
ch = filter(ch, prime)
out <- prime
}
}()
return out
}
func main() {
primes := sieve()
for {
fmt.Println(<-primes)
}
}
When I run this programm, I've got a deadlock, but when I change the generate function to
// Send the sequence 2, 3, 4, ... to returned channel
func generate() chan int {
ch := make(chan int)
go func() {
for i := 2; ; i++ {
ch <- i
}
}()
return ch
}
Then the programm will run the infinite loop, but not deadlock. Why do I get deadlock, when I remove the condition in for loop?
What do you mean with blocking principle?
You can see it illustrated in the blog post "The Nature Of Channels In Go "
for an unbuffered channel:
(Illustration from blog post "The Nature Of Channels In Go ", written by William Kennedy, Feb. 2014)
Unbuffered channels have no capacity and therefore require both goroutines to be ready to make any exchange.
When a goroutine attempts to write a resource to an unbuffered channel and there is no goroutine waiting to receive the resource, the channel will lock the goroutine and make it wait.
When a goroutine attempts to read from an unbuffered channel, and there is no goroutine waiting to send a resource, the channel will lock the goroutine and make it wait.
That is what happens in your case with your reader:
func main() {
primes := sieve()
for {
fmt.Println(<-primes)
}
}
since primes is never closed, main remains blocked.
It (main) is in step 3:
in step 3, the goroutine on the right places his hand into the channel or performs a read.
That goroutine is also locked in the channel until the exchange is complete.
The sender never calls close(primes).
Let's consider a simpler example:
func generate() chan int {
ch := make(chan int)
go func() {
for i := 2; /*i < 100*/; i++ {
ch <- i
}
}()
return ch
}
func main() {
for i := range generate() {
fmt.Println(i)
}
}
With the condition i < 100 uncommented, the goroutine spawned by generate stops after sending 98 numbers. However, it does not close the channel, so main has no way of knowing that no more numbers are going to be sent, and it just keeps blocking on the channel. Since main is now the only goroutine still in existence (the other one has returned), and it's blocking, you have a deadlock.