Using goroutines on loop iterator variables

Question

I was reading that goroutines on loop iterators often result in the last value in the loop assignment being used for every iteration. E.g. https://github.com/golang/go/wiki/CommonMistakes#using-goroutines-on-loop-iterator-variables

However, does this only happen for closures, e.g. with anonymous functions?

I can't seem to reproduce the issue with this example https://play.golang.org/p/lpZ-yD1mHu

When I use an anonymous function like this, it recreates the issue https://play.golang.org/p/mDa0Z6mUP8

It's not a problem, it is how it behaves (it was designed to behave like that deliberately). Whether it's a "problem" for you in your particular case only you can say. — zerkms
– zerkms, Commented Dec 1, 2017 at 1:03
I should have said behavior instead of problem - I've edited it to make it clearer it's not a "problem", but my misunderstanding of the behavior. — sager89
– sager89, Commented Dec 1, 2017 at 5:13

tkausl · Accepted Answer · 2017-12-01 01:03:56Z

6

However, is this only a problem for closures, e.g. with anonymous functions?

Yes. The difference between

go speak(c)

and

go func() {
    speak(c)
}()

is, the former calls speak with the value of c in a new goroutine. In the latter one, however, the anonymous function captures not the value of c but the variable c itself (by reference¹) and, at some later point in time, calls speak with whatever value c has at this point in time.

¹: There are no "references" in golang, I'm not sure how this is exactly implemented, but it is as if they reference the original variable.

answered Dec 1, 2017 at 1:03

tkausl

14.4k3 gold badges37 silver badges55 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Using goroutines on loop iterator variables

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related