Added ListDevices() and exposed more device capabilities

examples/list_devices/list_devices.go

@@ -0,0 +1,26 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+
+	"github.com/blackjack/webcam"
+)
+
+func main() {
+	flag.Parse()
+
+	devices, err := webcam.ListDevices()
+	if err != nil {
+		panic(err.Error())
+	}
+	if len(devices) == 0 {
+		fmt.Printf("No valid video devices found in %q\n", webcam.VIDEO4LINUX_DIR)
+	} else {
+		fmt.Println("Video devices found:")
+		for devPath, name := range devices {
+			fmt.Printf("  %q located in %s\n", name, devPath)
+		}
+	}
+}
+

v4l2.go

@@ -203,18 +203,15 @@ type v4l2_control struct {
 	value int32
 }
 
-func checkCapabilities(fd uintptr) (supportsVideoCapture bool, supportsVideoStreaming bool, err error) {
-
-	caps := &v4l2_capability{}
+func checkCapabilities(fd uintptr) (caps *v4l2_capability, err error) {
 
+	caps = &v4l2_capability{}
 	err = ioctl.Ioctl(fd, VIDIOC_QUERYCAP, uintptr(unsafe.Pointer(caps)))
 
 	if err != nil {
-		return
+		return nil, err
 	}
 
-	supportsVideoCapture = (caps.capabilities & V4L2_CAP_VIDEO_CAPTURE) != 0
-	supportsVideoStreaming = (caps.capabilities & V4L2_CAP_STREAMING) != 0
 	return
 
 }

webcam.go

@@ -5,8 +5,13 @@ package webcam
 
 import (
 	"errors"
+	"fmt"
 	"golang.org/x/sys/unix"
+	"os"
+	"path"
+	"path/filepath"
 	"reflect"
+	"strings"
 	"unsafe"
 )
 
@@ -16,6 +21,7 @@ type Webcam struct {
 	bufcount  uint32
 	buffers   [][]byte
 	streaming bool
+	capabilities *v4l2_capability
 }
 
 type ControlID uint32
@@ -38,26 +44,48 @@ func Open(path string) (*Webcam, error) {
 		return nil, err
 	}
 
-	supportsVideoCapture, supportsVideoStreaming, err := checkCapabilities(fd)
-
+	caps, err := checkCapabilities(fd)
 	if err != nil {
 		return nil, err
 	}
 
-	if !supportsVideoCapture {
-		return nil, errors.New("Not a video capture device")
+	w := &Webcam{
+		fd: uintptr(fd),
+		bufcount: 256,
+		capabilities: caps,
 	}
 
-	if !supportsVideoStreaming {
+	// Makes sure it supports some form of video capture capability.
+	if !w.SupportsVideoCapture() {
+		return nil, errors.New("Not a video capture device")
+	}
+	if !w.SupportsVideoStreaming() {
 		return nil, errors.New("Device does not support the streaming I/O method")
 	}
 
-	w := new(Webcam)
-	w.fd = uintptr(fd)
-	w.bufcount = 256
 	return w, nil
 }
 
+func (w *Webcam) SupportsVideoCapture() bool {
+	return (w.capabilities.capabilities & V4L2_CAP_VIDEO_CAPTURE) != 0
+}
+
+func (w *Webcam) SupportsVideoStreaming() bool {
+	return (w.capabilities.capabilities & V4L2_CAP_STREAMING) != 0
+}
+
+func (w *Webcam) Card() string {
+	return CToGoString(w.capabilities.card[:])
+}
+
+func (w *Webcam) Driver() string {
+	return CToGoString(w.capabilities.driver[:])
+}
+
+func (w *Webcam) BusInfo() string {
+	return CToGoString(w.capabilities.bus_info[:])
+}
+
 // Returns image formats supported by the device alongside with
 // their text description
 // Not that this function is somewhat experimental. Frames are not ordered in
@@ -281,10 +309,56 @@ func (w *Webcam) SetAutoWhiteBalance(val bool) error {
 	return setControl(w.fd, V4L2_CID_AUTO_WHITE_BALANCE, v)
 }
 
+
 func gobytes(p unsafe.Pointer, n int) []byte {
 
 	h := reflect.SliceHeader{uintptr(p), n, n}
 	s := *(*[]byte)(unsafe.Pointer(&h))
 
 	return s
 }
+
+// VIDEO4LINUX_DIR path to kernel known list of videos devices.
+var VIDEO4LINUX_DIR string = "/sys/class/video4linux"
+
+// ListDevices enumerates video devices present in the system. It returns a map of
+// of path names to the "human readable" device name (the "card name").
+func ListDevices() (devices map[string]string, err error) {
+	devices = make(map[string]string)
+	if _, err = os.Stat(VIDEO4LINUX_DIR); err != nil {
+		if os.IsNotExist(err) {
+			// No devices present, make error nil and return an empty list.
+			err = nil
+		}
+		return
+	}
+	err = filepath.Walk(VIDEO4LINUX_DIR, func(_ string, info os.FileInfo, err error) error {
+		if err != nil {
+			return fmt.Errorf("failure accessing %q: %v", VIDEO4LINUX_DIR, err)
+		}
+		if info.IsDir() { return nil }  // Root directory.
+		if !strings.HasPrefix(info.Name(), "video") && !strings.HasPrefix(info.Name(), "subdev") {
+			return nil
+		}
+		devPath := path.Join("/dev", info.Name())
+		w, err := Open(devPath)
+		if err != nil{
+			return fmt.Errorf("Failed to open device %q: %v", devPath, err)
+		}
+		defer w.Close()
+
+		// For some reason the kernel creates more than one path per actual physical device,
+		// one of which has no supported formats and can't be used for streaming.
+		formats := w.GetSupportedFormats()
+		if len(formats) == 0 {
+			return nil
+		}
+		devices[devPath] = w.Card()
+		return nil
+	})
+	if err != nil {
+		return nil, err
+	}
+	return
+}
+