Dear Ian, thanks for the inspiration and sorry for the late response. I
just got a chance to test your suggestion.
But, it turns out that the wrapping struct can cause the following error:
panic: runtime error: cgo argument has Go pointer to Go pointer
If my understanding of Cgo constraints correctly, the panic is to prevent a
potential error when GC moves the Go pointer,
although we don't manipulate the Go pointer (to the channel) from the C
side.
How could we avoid this?
On Monday, February 15, 2021 at 9:51:03 PM UTC+1 Ian Lance Taylor wrote:
> On Mon, Feb 15, 2021 at 12:39 PM changkun <h...@changkun.de> wrote:
> >
> > Thanks for the hint, but I have some follow-up questions:
> >
> >>
> >> Even if there were a way to do this, an atomic variable is not a good
> >> synchronization mechanism, because the other side has to poll the
> >> variable.
> >
> > Indeed, it the other side will be a spin loop to poll the atomic
> variable, which ...
> >
> >
> >>
> >> You can do this as a last resort, but it doesn't sound like
> >> you are at a last resort here. I suggest that you have your C
> >> function call a Go function to write a value on a channel.
> >
> > seems easy to write than this (?).
> >
> > Say a Go function foo is called from the C side, to be able to send to
> the corresponding channel, C must pass that channel to the Go function,
> which brings to the initial question: pass a channel from Go to C, is it
> supported at the moment (?)
> >
> > How could a Go function be able to send content to differently allocated
> channels in correspondingly invoked C functions?
>
> For example, on the Go side write
>
> type chanToUse struct {
> c chan int
> }
>
> //export MyGoFunction
> func MyGoFunction(u unsafe.Pointer, val int) {
> cu = (*chanToUse)(u)
> cu.c <- val
> }
>
> ch := make(chan int)
> ...
> C.MyCFunction(unsafe.Pointer(&chanToUse{ch}))
>
> and on the C side write
>
> void MyCFunction(void *goData) {
> ...
> MyGoFunction(goData, 0);
> }
>
> Yes, it's more work. But it's not a good idea to poll an atomic
> variable in Go. The Go scheduler reacts badly to busy loops.
>
> Ian
>
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