Using shm.GetImage().Reply()

[Jsewill]

This is not a bug, just a lack of understanding on my part. I have put xproto.GetImage() to good use in a project I'm working on, but I'd like to use shm.GetImage() to get an image from the X server via SHM, where should I start? I apologize if it is inappropriate for me to post this as an issue, and I'd be glad to delete it if needed.

[BurntSushi]

Honestly, I don't know (but I'd like to). I've never used shm before. If I were to investigate, I'd probably look for examples in C that use XCB. XGB should hopefully be a straight-forward translation from that.

[Jsewill]

Indeed, that was the first place I went looking. It seems I need to brush up on tmpfs/shm, especially as it relates to X11 in general. I'll continue looking into it and reply with any findings.

[BurntSushi]

@Jsewill yes, please do! I'm personally quite carious. Even if you're findings are, "I found this good C example but I can't translate it to Go. Can you help?" :-)

[Jsewill]

Well, I was able to use shm.NewSegId(), shm.CreateSegment().Reply(), and shm.GetImage().Reply() without error. This gets me a segment ID and file descriptor (I think), and has X create the image in that segment. I'm not really sure how to read it.

In my research earlier in the week, I came across an example in C from the VLC project: https://github.com/videolan/vlc/blob/master/modules/access/screen/xcb.c . This is the best example I have found yet. Perhaps I need to attach and detach the segment to and from X and my program, though I was hoping this was happening automatically.

As far as I know, SHM related stuff was explicitly left out of Golang's builtin, "syscall". Short of using a Go wrapper for the C shm related functions, do you have any insight regarding all this?

[Jsewill]

I've made some progress, but now I'm hitting a wall. The X server comes back with "BadIDChoice" when attempting to attach an SHM segment, whether I create the segment using shm.CreateSegment() or wrapped C functions. Any ideas?

[Jsewill]

After revisiting this, I figured out the proper arguements and their order, and with that X stops complaining.

The current process is as such:

• Using low-level C functions (shmget, shmat, etc) create an SHM segment
• Ask X to attach to that segment using shm.AttachChecked() from the xgb/shm package
• Make the shm.GetImage().Reply() calls
• Ask X to detach from the segment using shm.DetachChecked()
• Attach to the segment using shmat() and retrieve the data.

I still have a problem accessing the data from the segment yet, but I'll work through that when I have more time to work on it.

[sai-prasanna]

@Jsewill Can you post the code for this?

[sai-prasanna]

Here is working version, figured it out.

package main

/*
#include <sys/ipc.h>
#include <sys/shm.h>
*/
import "C"

import (
	"github.com/BurntSushi/xgb"
	"github.com/BurntSushi/xgb/shm"
	"github.com/BurntSushi/xgb/xproto"
        
        "fmt"
	"image"
	"time"
	"unsafe"
)

func main() {
	scr := newXgbShmScreen()
	for i := 0; i < 100; i++ {
		start := time.Now()
		scr.capture()
		elapsed := time.Since(start)
		fmt.Printf("%d\n", elapsed.Nanoseconds())
	}
	scr.Close()
}

type xgbShmScreen struct {
	conn    *xgb.Conn
	screen  *xproto.ScreenInfo
	shmId   int
	data    unsafe.Pointer
	shmSize int
	seg     shm.Seg
}

func newXgbShmScreen() *xgbShmScreen {
	var scr xgbShmScreen
	var err error
	scr.conn, err = xgb.NewConn()
	if err != nil || shm.Init(scr.conn) != nil {
		return nil
	}

	scr.screen = xproto.Setup(scr.conn).DefaultScreen(scr.conn)
	scr.shmSize = int(scr.screen.WidthInPixels) * int(scr.screen.HeightInPixels) * 4
	scr.shmId = int(C.shmget(C.IPC_PRIVATE, C.size_t(scr.shmSize), C.IPC_CREAT|0777))

	if scr.shmId == -1 {
		return nil
	}
	scr.data = C.shmat(C.int(scr.shmId), nil, 0)
	scr.seg, err = shm.NewSegId(scr.conn)
	if err != nil {
		return nil
	}

	shm.AttachChecked(scr.conn, scr.seg, uint32(scr.shmId), false)
	return &scr
}

func (scr *xgbShmScreen) capture() (imgData []byte, err error) {
	_, err = shm.GetImage(scr.conn, xproto.Drawable(scr.screen.Root), 0, 0, scr.screen.WidthInPixels, scr.screen.HeightInPixels, 0xffffffff, byte(xproto.ImageFormatZPixmap), scr.seg, 0).Reply()
	if err != nil {
		return nil, err
	}
	imgData = (*[1 << 30]byte)(scr.data)[:scr.shmSize:scr.shmSize]
	return imgData, nil
}

func (scr *xgbShmScreen) Close() {
	shm.Detach(scr.conn, scr.seg)
	C.shmctl(C.int(scr.shmId), C.IPC_RMID, nil)
	C.shmdt(scr.data)
	scr.conn.Close()
}

@BurntSushi

1. Is this repo accepting contribs? will give a pull req for this as an example.
2. Is xcb protocol frozen stable ? Would I be better off in long run using this or just using xcb via c go.