Make the select statement wait for more than one channel at the same time

Question

To quote from the The Go Programming Language Specification :

A "select" statement chooses which of a set of possible communications will proceed. It looks similar to a "switch" statement but with the cases all referring to communication operations.

How can I wait for more than one channel in a case clause, so that only when both channels return the case is executed?

example :

select {

   case <-ch1, <-ch2 ... : //wait for ch1 and ch2 
        // do something 
   case  ..
}

Answer 1

There's no way to wait for multiple channels in the same select case, and there's also no way way to "fallthrough" in select cases, as the documentation describes:

• http://golang.org/ref/spec#Select_statements

You can easily do something like this via other means, though.

Without select:

This only works if c1 is guaranteed to return despite the availability of c2.

v1 := <-c1
v2 := <-c2

With looping:

This works despite the ordering, but c1 and c2 must only fire once, or it might misbehave:

var v1, v2 int
for i := 0; i < 2; i++ {
    select {
    case v1 = <-c1:
    case v2 = <-c2:
    }
}
// both v1 and v2 are set here

With goroutines:

This works either way, but you lose the ordering of results:

c3 := make(chan int, 2)
go func() { c3 <- (<-c1) }()
go func() { c3 <- (<-c2) }()

first := <-c3
second := <-c3

With sync.WaitGroup:

This works either way:

var wg sync.WaitGroup
var v1, v2 int
wg.Add(2)
go func() {
    v1 = <-c1
    wg.Done()
}()
go func() {
    v2 = <-c2
    wg.Done()
}()
wg.Wait()
// v1 and v2 are both set

And so on. There are certainly other ways. The best one depends on the details of what you're trying to achieve.