Number of elements in a channel

Using a buffered channel, how do measure how many elements are in the channel? For example, I'm creating and sending on a channel like this:

send_ch := make(chan []byte, 100)
// code
send_ch <- msg

I want to measure how many msgs are in the channel send_ch.

I'm aware that due to concurrency the measurement won't be exact, as pre-emption could occur between measurement and action (eg discussed in this video Google I/O 2012 - Go Concurrency Patterns). I'll be using this for flow control between producers and consumers ie once I've passed through a high watermark, changing some behaviour until I pass back through a low watermark.

http://golang.org/pkg/builtin/#len

> func len(v Type) int
> The len built-in function returns the length of v, according to its type: > > - Array: the number of elements in v.
> - Pointer to array: the number of elements in *v (even if v is nil).
> - Slice, or map: the number of elements in v; if v is nil, len(v) is zero.
> - String: the number of bytes in v.
> - Channel: the number of elements queued (unread) in the channel buffer; if v is nil, len(v) is zero.

package main

import "fmt"

func main() {
        c := make(chan int, 100)
        for i := 0; i < 34; i++ {
                c <- 0
        }
        fmt.Println(len(c))
}

will output:

34