Cgo: using syscall.Select

[Juliusz Chroboczek]

I'm interfacing with a C library that expects to do its own I/O, but
wants to be called after a file descriptor is ready for read. My code
currently looks roughly like this:

var fdset syscall.FdSet
var bits = unsafe.Sizeof(fdset.Bits[0]) * 8
fdset.Bits[uintptr(fd)/bits] |= (1 << (fd % bits))
var ctv C.struct_timeval
C.gettimeout(&ctv)
tv := syscall.Timeval{int64(ctv.tv_sec), int64(ctv.tv_usec)}
n, err := syscall.Select(int(fd + 1), &fdset, nil, nil, &tv)
if n < 0 {
return err
}
rc, err := C.dostuff(fd)
if(rc < 0) {
return err
}

I'm bothered by two things:

- the way I access the syscall.FdSet feels like an unportable hack;
- I'd much rather hook into Go's scheduler then burn a thread on
sleeping in select.

Is the above the correct way to interface with the C library, or is
there a better way?

[Ian Lance Taylor]

If you do this in Go, you should use golang.org/x/sys/unix package
rather than the syscall package. But since you have to call C anyhow,
I would suggest just doing it in C.

There isn't any way to hook into Go's scheduler for this. Go's
scheduler provides no mechanism for waiting for data without reading
the data.

Ian

[Juliusz Chroboczek]

> If you do this in Go, you should use golang.org/x/sys/unix package
> rather than the syscall package.

What's the advantage? (In this particular case, not in general.)

> But since you have to call C anyhow, I would suggest just doing it in C.

Yeah, I guess it's simpler.

> There isn't any way to hook into Go's scheduler for this. Go's
> scheduler provides no mechanism for waiting for data without reading
> the data.

I suppose it makes sense, the operation might be difficult to implement in
a hypothetical scheduler that's layered over mainframe-style async operations.

Thanks for your reply,

-- Juliusz

[Ian Lance Taylor]

On Thu, May 17, 2018 at 9:56 AM, Juliusz Chroboczek <j...@irif.fr> wrote:
>> If you do this in Go, you should use golang.org/x/sys/unix package
>> rather than the syscall package.
>
> What's the advantage? (In this particular case, not in general.)

The advantage in this particular case is likely minimal. The main
advantage is portability. For example, Go runs on Solaris, but the
syscall package does not support syscall.Select on Solaris. The
golang.org/x/sys/unix package does.

Ian

[Сергей Камардин]

You could also take a look at https://godoc.org/github.com/mailru/easygo/netpoll

четверг, 17 мая 2018 г., 3:08:29 UTC+3 пользователь Juliusz Chroboczek написал:

[Juliusz Chroboczek]

> You could also take a look
> at https://godoc.org/github.com/mailru/easygo/netpoll

I don't have a net.Conn, just a raw file descriptor.

-- Juliusz

[Sergey Kamardin]

There are methods which are using raw file descriptors (such that https://godoc.org/github.com/mailru/easygo/netpoll#NewDesc).

суббота, 19 мая 2018 г., 2:52:27 UTC+3 пользователь Juliusz Chroboczek написал: