Introduce two more layers between device and endpoint:

device -> active config -> claimed interface -> endpoints.
Device can have at most one active config. A config can have
multiple claimed interfaces, each with a specific alt setting.
An interface with an alt setting defines a list of endpoints.

usb/interface.go

@@ -0,0 +1,127 @@
+// Copyright 2013 Google Inc.  All rights reserved.
+// Copyright 2016 the gousb Authors.  All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package usb
+
+import (
+	"fmt"
+)
+
+// InterfaceInfo contains information about a USB interface, extracted from
+// the descriptor.
+type InterfaceInfo struct {
+	// Number is the number of this interface, a zero-based index in the array
+	// of interfaces supported by the device configuration.
+	Number int
+	// AltSettings is a list of alternate settings supported by the interface.
+	AltSettings []InterfaceSetting
+}
+
+// String returns a human-readable descripton of the interface and it's
+// alternate settings.
+func (i InterfaceInfo) String() string {
+	return fmt.Sprintf("if=%d", i.Number)
+}
+
+// InterfaceSetting contains information about a USB interface with a particular
+// alternate setting, extracted from the descriptor.
+type InterfaceSetting struct {
+	// Number is the number of this interface, the same as in InterfaceInfo.
+	Number int
+	// Alternate is the number of this alternate setting.
+	Alternate int
+	// Class is the USB-IF class code, as defined by the USB spec.
+	Class Class
+	// SubClass is the USB-IF subclass code, as defined by the USB spec.
+	SubClass Class
+	// Protocol is USB protocol code, as defined by the USB spe.c
+	Protocol Protocol
+	// Endpoints has the list of endpoints available on this interface with
+	// this alternate setting.
+	Endpoints []EndpointInfo
+}
+
+// String returns a human-readable descripton of the particular
+// alternate setting of an interface.
+func (a InterfaceSetting) String() string {
+	return fmt.Sprintf("Interface %d alternate setting %d", a.Number, a.Alternate)
+}
+
+type Interface struct {
+	Setting InterfaceSetting
+
+	config *Config
+}
+
+func (i *Interface) String() string {
+	return fmt.Sprintf("%s,if=%d,alt=%d", i.config, i.Setting.Number, i.Setting.Alternate)
+}
+
+// Close releases the interface.
+func (i *Interface) Close() {
+	libusb.release(i.config.dev.handle, uint8(i.Setting.Number))
+	i.config.mu.Lock()
+	defer i.config.mu.Unlock()
+	delete(i.config.claimed, i.Setting.Number)
+}
+
+func (i *Interface) openEndpoint(epNum int) (*endpoint, error) {
+	var ep EndpointInfo
+	var found bool
+	for _, e := range i.Setting.Endpoints {
+		if e.Number == epNum {
+			debug.Printf("found ep %s in %s\n", e, i)
+			ep = e
+			found = true
+			break
+		}
+	}
+	if !found {
+		return nil, fmt.Errorf("%s does not have endpoint number %d. Available endpoints: %v", epNum, i.Setting.Endpoints)
+	}
+	return &endpoint{
+		InterfaceSetting: i.Setting,
+		Info:             ep,
+		h:                i.config.dev.handle,
+	}, nil
+}
+
+// InEndpoint prepares an IN endpoint for transfer.
+func (i *Interface) InEndpoint(epNum int) (*InEndpoint, error) {
+	ep, err := i.openEndpoint(epNum)
+	if err != nil {
+		return nil, err
+	}
+	if ep.Info.Direction != EndpointDirectionIn {
+		return nil, fmt.Errorf("%s is not an IN endpoint", ep)
+	}
+	return &InEndpoint{
+		endpoint: ep,
+	}, nil
+}
+
+// OutEndpoint prepares an OUT endpoint for transfer.
+func (i *Interface) OutEndpoint(epNum int) (*OutEndpoint, error) {
+	ep, err := i.openEndpoint(epNum)
+	if err != nil {
+		return nil, err
+	}
+	if ep.Info.Direction != EndpointDirectionOut {
+		return nil, fmt.Errorf("%s is not an OUT endpoint", ep)
+	}
+	return &OutEndpoint{
+		endpoint: ep,
+	}, nil
+}