Question

I’m trying to understand concurrency in Go. In particular, I wrote this thread-unsafe program:

package main

import "fmt"

var x = 1

func inc_x() { //test
  for {
    x += 1
  }
}

func main() {
  go inc_x()
  for {
    fmt.Println(x)
  }
}

I recognize that I should be using channels to prevent race conditions with x, but that’s not the point here. The program prints 1 and then seems to loop forever (without printing anything more). I would expect it to print an infinite list of numbers, possibly skipping some and repeating others due to the race condition (or worse — printing the number while it is being updated in inc_x).

My question is: Why does the program only print one line?

Just to be clear: I’m not using channels on purpose for this toy example.

Answer 1

There are a few things to keep in mind about Go’s goroutines:

1. They are not threads in the sense of Java’s or C++ threads
   • goroutines are more like greenlets
2. The go runtime multiplexes the goroutines across the system threads
   • the number of system threads is controlled by an environment variable GOMAXPROCS and defaults to 1 currently I think. This may change in the future
3. The way goroutines yield back to their current thread is controlled by several different constructs
   • the select statement can yield control back to the thread
   • sending on a channel can yield control back to the thread
   • doing IO operations can yield control back to the thread
   • runtime.Gosched() explicitly yields control back to the thread

The behavior you are seeing is because the main function never yields back to the thread and is instead involved in a busy loop and since there is only one thread the main loop has no place to run.