xgb-image: add support for the MIT-SHM extension

prototypes/xgb-image.go

@@ -4,10 +4,13 @@ import (
 	"encoding/binary"
 	"github.com/BurntSushi/xgb"
 	"github.com/BurntSushi/xgb/render"
+	"github.com/BurntSushi/xgb/shm"
 	"github.com/BurntSushi/xgb/xproto"
 	"log"
 	"os"
+	"reflect"
 	"time"
+	"unsafe"
 
 	"image"
 	_ "image/gif"
@@ -15,6 +18,10 @@ import (
 	_ "image/png"
 )
 
+// #include <sys/ipc.h>
+// #include <sys/shm.h>
+import "C"
+
 func F64ToFixed(f float64) render.Fixed { return render.Fixed(f * 65536) }
 func FixedToF64(f render.Fixed) float64 { return float64(f) / 65536 }
 
@@ -182,13 +189,15 @@ func main() {
 	_ = xproto.CreateGC(X, cid, xproto.Drawable(pixid),
 		xproto.GcGraphicsExposures, []uint32{0})
 
-	// Load data into the pixmap. Also see xgbutil/xgraphics/xsurface.go.
-	// We're being lazy and resolve the 1<<16 limit of requests by sending
-	// a row at a time. The encoding is also done inefficiently.
 	bounds := img.Bounds()
-
 	Lstart := time.Now()
 
+	if err := shm.Init(X); err != nil {
+		log.Println("MIT-SHM unavailable")
+
+		// We're being lazy and resolve the 1<<16 limit of requests by sending
+		// a row at a time. The encoding is also done inefficiently.
+		// Also see xgbutil/xgraphics/xsurface.go.
 		row := make([]byte, bounds.Dx()*4)
 		for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
 			for x := bounds.Min.X; x < bounds.Max.X; x++ {
@@ -201,11 +210,63 @@ func main() {
 				0, int16(y),
 				0, 32, row)
 		}
+	} else {
+		rep, err := shm.QueryVersion(X).Reply()
+		if err != nil {
+			log.Fatalln(err)
+		}
+		if rep.PixmapFormat != xproto.ImageFormatZPixmap ||
+			!rep.SharedPixmaps {
+			log.Fatalln("MIT-SHM configuration unfit")
+		}
+
+		shmSize := bounds.Dx() * bounds.Dy() * 4
+
+		// As a side note, to clean up unreferenced segments (orphans):
+		//  ipcs -m | awk '$6 == "0" { print $2 }' | xargs ipcrm shm
+		shmID := int(C.shmget(C.IPC_PRIVATE,
+			C.size_t(shmSize), C.IPC_CREAT|0777))
+		if shmID == -1 {
+			// TODO: We should handle this case by falling back to PutImage,
+			// if only because the allocation may hit a system limit.
+			log.Fatalln("memory allocation failed")
+		}
+
+		dataRaw := C.shmat(C.int(shmID), nil, 0)
+		defer C.shmdt(dataRaw)
+		defer C.shmctl(C.int(shmID), C.IPC_RMID, nil)
+
+		data := *(*[]byte)(unsafe.Pointer(&reflect.SliceHeader{
+			Data: uintptr(dataRaw), Len: shmSize, Cap: shmSize}))
+		for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
+			row := data[y*bounds.Dx()*4:]
+			for x := bounds.Min.X; x < bounds.Max.X; x++ {
+				r, g, b, a := img.At(x, y).RGBA()
+				encoding.PutUint32(row[x*4:],
+					(a>>8)<<24|(r>>8)<<16|(g>>8)<<8|(b>>8))
+			}
+		}
+
+		segid, err := shm.NewSegId(X)
+		if err != nil {
+			log.Fatalln(err)
+		}
+
+		// Need to have it attached on the server before we unload the segment.
+		c := shm.AttachChecked(X, segid, uint32(shmID), true /* RO */)
+		if err := c.Check(); err != nil {
+			log.Fatalln(err)
+		}
+
+		_ = shm.PutImage(X, xproto.Drawable(pixid), cid,
+			uint16(bounds.Dx()), uint16(bounds.Dy()), 0, 0,
+			uint16(bounds.Dx()), uint16(bounds.Dy()), 0, 0,
+			32, xproto.ImageFormatZPixmap,
+			0 /* SendEvent */, segid, 0 /* Offset */)
+	}
 
 	log.Println("uploading took", time.Now().Sub(Lstart))
 
-	// TODO: SHM, for an example see https://github.com/BurntSushi/xgb/issues/28
-
 	var scale float64 = 1
 	for {
 		ev, xerr := X.WaitForEvent()