How to check a channel is closed or not without reading it?
I know this answer is so late, I have wrote this solution, Hacking Go run-time, It's not safety, It may crashes:
import (
"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
}
There's no way to write a safe application where you need to know whether a channel is open without interacting with it.
The best way to do what you're wanting to do is with two channels -- one for the work and one to indicate a desire to change state (as well as the completion of that state change if that's important).
Channels are cheap. Complex design overloading semantics isn't.
[also]
<-time.After(1e9)
is a really confusing and non-obvious way to write
time.Sleep(time.Second)
Keep things simple and everyone (including you) can understand them.
In a hacky way it can be done for channels which one attempts to write to by recovering the raised panic. But you cannot check if a read channel is closed without reading from it.
Either you will
- eventually read the "true" value from it (
v <- c
) - read the "true" value and 'not closed' indicator (
v, ok <- c
) - read a zero value and the 'closed' indicator (
v, ok <- c
) (example) - will block in the channel read forever (
v <- c
)
Only the last one technically doesn't read from the channel, but that's of little use.