Check if Go channel is closed without touching/reading it.

isChanClosed.go

package main 

import (
	"fmt"
	"unsafe"
	"reflect"
)


func isChanClosed(ch interface{}) bool {
	if reflect.TypeOf(ch).Kind() != reflect.Chan {
		panic("only channels!")
	}
	
	// get interface value pointer, from cgo_export 
	// typedef struct { void *t; void *v; } GoInterface;
	// then get channel real pointer
	cptr := *(*uintptr)(unsafe.Pointer(
		unsafe.Pointer(uintptr(unsafe.Pointer(&ch)) + unsafe.Sizeof(uint(0))),
	))
	
	// this function will return true if chan.closed > 0
	// see hchan on https://github.com/golang/go/blob/master/src/runtime/chan.go 
	// type hchan struct {
	// qcount   uint           // total data in the queue
	// dataqsiz uint           // size of the circular queue
	// buf      unsafe.Pointer // points to an array of dataqsiz elements
	// elemsize uint16
	// closed   uint32
	// **
	
	cptr += unsafe.Sizeof(uint(0))*2
	cptr += unsafe.Sizeof(unsafe.Pointer(uintptr(0)))
	cptr += unsafe.Sizeof(uint16(0))
	return *(*uint32)(unsafe.Pointer(cptr)) > 0
}

func main() {
	ch := make(chan int64)
	fmt.Println(isChanClosed(ch)) // -> false
	close(ch)
	fmt.Println(isChanClosed(ch)) // -> true
}