Reusing a Go channel causes deadlock

Question

I'm new to golang (whith Java concurrency background). Consider this peace of code :

package main

import "fmt"

func sendenum(num int, c chan int) {
    c <- num
}

func main() {
    c := make(chan int)
    go sendenum(0, c)
    x, y := <-c, <-c
    fmt.Println(x, y)
}

When I run this code , I get this error

fatal error: all goroutines are asleep - deadlock!

goroutine 1 [chan receive]:
main.main()
    /home/tarrsalah/src/go/src/github.com/tarrsalah/stackoverflow/chan_dead_lock.go:12 +0x90
exit status 2

I know, adding another go sendenum(0, c) statement fix the issue, ... but

When and Where the deadlock happened ?

Answer 1

After it receives the 0, main keeps on waiting on the receiving end of c for another value to arrive (to put in the y variable), but it never will, as the goroutine running main is the only one left to live.

When you add another go sendenum(0, c) , it actually gets a value on the second channel receive, puts it into the y variable, prints x and y out and the program finishes succesfully.

Answer 2

It's not that "reusing" a channel is a problem. It's just a simple deadlock happening because the code prescribes two reads, but only one write to the same channel. The second read can never happen, hence the deadlock.