add a build tag 'ebitencbackend' and internal/cbackend

This enables to create a C archive with Ebiten without an actual driver
implementation. This enables Ebiten work on a platform Go doesn't support.
See also Hitsumabushi project: github.com/hajimehoshi/hitsumabushi.

The backend C API is not stable and might change later. Then, the build
tag 'ebitencbackend' is not documented on purpose.

Closes #1900

audio/internal/cbackend/context.go

@@ -0,0 +1,329 @@
+// Copyright 2021 The Ebiten Authors
+//
+// 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.
+
+//go:build ebitencbackend
+// +build ebitencbackend
+
+package cbackend
+
+import (
+	"io"
+	"runtime"
+	"sync"
+
+	"github.com/hajimehoshi/ebiten/v2/internal/cbackend"
+)
+
+type Context struct {
+	sampleRate      int
+	channelNum      int
+	bitDepthInBytes int
+}
+
+func NewContext(sampleRate, channelNum, bitDepthInBytes int) (*Context, chan struct{}, error) {
+	cbackend.OpenAudio(sampleRate, channelNum, bitDepthInBytes)
+	c := &Context{
+		sampleRate:      sampleRate,
+		channelNum:      channelNum,
+		bitDepthInBytes: bitDepthInBytes,
+	}
+	ready := make(chan struct{})
+	close(ready)
+	return c, ready, nil
+}
+
+func (c *Context) NewPlayer(r io.Reader) *Player {
+	cond := sync.NewCond(&sync.Mutex{})
+	p := &Player{
+		context:   c,
+		src:       r,
+		volume:    1,
+		cond:      cond,
+		onWritten: cond.Signal,
+	}
+	runtime.SetFinalizer(p, (*Player).Close)
+	return p
+}
+
+func (c *Context) Suspend() error {
+	// Do nothing so far.
+	return nil
+}
+
+func (c *Context) Resume() error {
+	// Do nothing so far.
+	return nil
+}
+
+func (c *Context) Err() error {
+	return nil
+}
+
+func (c *Context) oneBufferSize() int {
+	// TODO: This must be audio.oneBufferSize(p.context.sampleRate). Avoid the duplication.
+	return c.sampleRate * c.channelNum * c.bitDepthInBytes / 4
+}
+
+func (c *Context) MaxBufferSize() int {
+	// TODO: This must be audio.maxBufferSize(p.context.sampleRate). Avoid the duplication.
+	return c.oneBufferSize() * 2
+}
+
+type playerState int
+
+const (
+	playerStatePaused playerState = iota
+	playerStatePlaying
+	playerStateClosed
+)
+
+type Player struct {
+	context *Context
+	src     io.Reader
+	v       *cbackend.AudioPlayer
+	state   playerState
+	volume  float64
+	cond    *sync.Cond
+	err     error
+	buf     []byte
+
+	onWritten func()
+}
+
+func (p *Player) Pause() {
+	p.cond.L.Lock()
+	defer p.cond.L.Unlock()
+
+	if p.state == playerStateClosed {
+		return
+	}
+	if p.v == nil {
+		return
+	}
+
+	p.v.Pause()
+	p.state = playerStatePaused
+	p.cond.Signal()
+}
+
+func (p *Player) Play() {
+	p.cond.L.Lock()
+	defer p.cond.L.Unlock()
+
+	if p.state == playerStateClosed {
+		return
+	}
+
+	var runloop bool
+	if p.v == nil {
+		p.v = cbackend.CreateAudioPlayer(p.onWritten)
+		runloop = true
+	}
+
+	p.v.SetVolume(p.volume)
+	p.v.Play()
+
+	// Prepare the first data as soon as possible, or the audio can get stuck.
+	// TODO: Get the appropriate buffer size from the C++ side.
+	if p.buf == nil {
+		n := p.context.oneBufferSize()
+		if max := p.context.MaxBufferSize() - p.UnplayedBufferSize(); n > max {
+			n = max
+		}
+		p.buf = make([]byte, n)
+	}
+	n, err := p.src.Read(p.buf)
+	if err != nil && err != io.EOF {
+		p.setErrorImpl(err)
+		return
+	}
+	if n > 0 {
+		p.writeImpl(p.buf[:n])
+	}
+
+	if runloop {
+		go p.loop()
+	}
+	p.state = playerStatePlaying
+	p.cond.Signal()
+}
+
+func (p *Player) IsPlaying() bool {
+	p.cond.L.Lock()
+	defer p.cond.L.Unlock()
+
+	return p.state == playerStatePlaying
+}
+
+func (p *Player) Reset() {
+	p.cond.L.Lock()
+	defer p.cond.L.Unlock()
+
+	if p.state == playerStateClosed {
+		return
+	}
+	p.state = playerStatePaused
+
+	if p.v == nil {
+		return
+	}
+
+	p.v.Close(true)
+	p.v = nil
+	p.cond.Signal()
+}
+
+func (p *Player) Volume() float64 {
+	p.cond.L.Lock()
+	defer p.cond.L.Unlock()
+
+	if p.v == nil {
+		return p.volume
+	}
+	return p.v.Volume()
+}
+
+func (p *Player) SetVolume(volume float64) {
+	p.cond.L.Lock()
+	defer p.cond.L.Unlock()
+
+	p.volume = volume
+	if p.v == nil {
+		return
+	}
+	p.v.SetVolume(volume)
+}
+
+func (p *Player) UnplayedBufferSize() int {
+	if p.v == nil {
+		return 0
+	}
+	return p.v.UnplayedBufferSize()
+}
+
+func (p *Player) Err() error {
+	p.cond.L.Lock()
+	defer p.cond.L.Unlock()
+
+	return p.err
+}
+
+func (p *Player) Close() error {
+	runtime.SetFinalizer(p, nil)
+	return p.close(true)
+}
+
+func (p *Player) close(remove bool) error {
+	p.cond.L.Lock()
+	defer p.cond.L.Unlock()
+	return p.closeImpl(remove)
+}
+
+func (p *Player) closeImpl(remove bool) error {
+	if p.state == playerStateClosed {
+		return p.err
+	}
+
+	if p.v != nil {
+		p.v.Close(false)
+		p.v = nil
+	}
+	if remove {
+		p.state = playerStateClosed
+		p.onWritten = nil
+	} else {
+		p.state = playerStatePaused
+	}
+	p.cond.Signal()
+	return p.err
+}
+
+func (p *Player) setError(err error) {
+	p.cond.L.Lock()
+	defer p.cond.L.Unlock()
+	p.setErrorImpl(err)
+}
+
+func (p *Player) setErrorImpl(err error) {
+	if p.state != playerStateClosed && p.v != nil {
+		p.v.Close(true)
+		p.v = nil
+	}
+	p.err = err
+	p.state = playerStateClosed
+	p.cond.Signal()
+}
+
+func (p *Player) shouldWait() bool {
+	if p.v == nil {
+		return false
+	}
+	switch p.state {
+	case playerStatePaused:
+		return true
+	case playerStatePlaying:
+		return p.v.UnplayedBufferSize() >= p.context.MaxBufferSize()
+	}
+	return false
+}
+
+func (p *Player) waitUntilUnpaused() bool {
+	p.cond.L.Lock()
+	defer p.cond.L.Unlock()
+
+	for p.shouldWait() {
+		p.cond.Wait()
+	}
+	return p.v != nil && p.state == playerStatePlaying
+}
+
+func (p *Player) writeImpl(buf []byte) {
+	if p.state == playerStateClosed {
+		return
+	}
+	if p.v == nil {
+		return
+	}
+	p.v.Write(buf)
+}
+
+func (p *Player) loop() {
+	const readChunkSize = 4096
+
+	buf := make([]byte, readChunkSize)
+
+	for {
+		if !p.waitUntilUnpaused() {
+			return
+		}
+
+		p.cond.L.Lock()
+		n, err := p.src.Read(buf)
+		if err != nil && err != io.EOF {
+			p.setErrorImpl(err)
+			p.cond.L.Unlock()
+			return
+		}
+		if n > 0 {
+			p.writeImpl(buf[:n])
+		}
+
+		if err == io.EOF {
+			p.closeImpl(false)
+			p.cond.L.Unlock()
+			return
+		}
+		p.cond.L.Unlock()
+	}
+}

audio/oto.go

@@ -12,6 +12,9 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build !ebitencbackend
+// +build !ebitencbackend
+
 package audio
 
 import (
@@ -33,7 +36,7 @@ type contextProxy struct {
 	otoContext
 }
 
-// contextProxy implements context.
+// NewPlayer implements context.
 func (c *contextProxy) NewPlayer(r io.Reader) player {
 	return c.otoContext.NewPlayer(r)
 }

audio/player_cbackend.go

@@ -0,0 +1,39 @@
+// Copyright 2021 The Ebiten Authors
+//
+// 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.
+
+//go:build ebitencbackend
+// +build ebitencbackend
+
+package audio
+
+import (
+	"io"
+
+	"github.com/hajimehoshi/ebiten/v2/audio/internal/cbackend"
+)
+
+func newContext(sampleRate, channelNum, bitDepthInBytes int) (context, chan struct{}, error) {
+	ctx, ready, err := cbackend.NewContext(sampleRate, channelNum, bitDepthInBytes)
+	return &contextProxy{ctx}, ready, err
+}
+
+// contextProxy is a proxy between cbackend.Context and context.
+type contextProxy struct {
+	*cbackend.Context
+}
+
+// NewPlayer implements context.
+func (c *contextProxy) NewPlayer(r io.Reader) player {
+	return c.Context.NewPlayer(r)
+}

audio/player_js.go

@@ -12,6 +12,9 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build !ebitencbackend
+// +build !ebitencbackend
+
 package audio
 
 import (

audio/player_notjs.go

@@ -12,8 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-//go:build !js
-// +build !js
+//go:build !js && !ebitencbackend
+// +build !js,!ebitencbackend
 
 package audio
 

internal/cbackend/cbackend.go

@@ -0,0 +1,243 @@
+// Copyright 2021 The Ebiten Authors
+//
+// 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.
+
+//go:build ebitencbackend
+// +build ebitencbackend
+
+package cbackend
+
+// #cgo !darwin LDFLAGS: -Wl,-unresolved-symbols=ignore-all
+// #cgo darwin LDFLAGS: -Wl,-undefined,dynamic_lookup
+//
+// #include <stdint.h>
+//
+// struct Gamepad {
+//   int id;
+//   char standard;
+//   int button_num;
+//   int axis_num;
+//   char button_pressed[32];
+//   float button_values[32];
+//   float axis_values[16];
+// };
+//
+// struct Touch {
+//   int id;
+//   int x;
+//   int y;
+// };
+//
+// // UI
+// void EbitenInitializeGame();
+// void EbitenGetScreenSize(int* width, int* height);
+// void EbitenBeginFrame();
+// void EbitenEndFrame();
+//
+// // Input
+// int EbitenGetGamepadNum();
+// void EbitenGetGamepads(struct Gamepad* gamepads);
+// int EbitenGetTouchNum();
+// void EbitenGetTouches(struct Touch* touches);
+//
+// // Audio
+// typedef void (*OnWrittenCallback)(int id);
+// void EbitenOpenAudio(int sample_rate, int channel_num, int bit_depth_in_bytes);
+// void EbitenCloseAudio();
+// int EbitenCreateAudioPlayer(OnWrittenCallback on_written_callback);
+// void EbitenAudioPlayerPlay(int id);
+// void EbitenAudioPlayerPause(int id);
+// void EbitenAudioPlayerWrite(int id, uint8_t* data, int length);
+// void EbitenAudioPlayerClose(int id, int immediately);
+// double EbitenAudioPlayerGetVolume(int id);
+// void EbitenAudioPlayerSetVolume(int id, double volume);
+// int EbitenAudioPlayerGetUnplayedBufferSize(int id);
+//
+// void EbitenAudioPlayerOnWrittenCallback(int id);
+// static int EbitenCreateAudioPlayerProxy() {
+//   return EbitenCreateAudioPlayer(EbitenAudioPlayerOnWrittenCallback);
+// }
+import "C"
+
+import (
+	"runtime"
+	"sync"
+	"unsafe"
+
+	"github.com/hajimehoshi/ebiten/v2/internal/driver"
+)
+
+type Gamepad struct {
+	ID            driver.GamepadID
+	Standard      bool
+	ButtonNum     int
+	AxisNum       int
+	ButtonPressed [32]bool
+	ButtonValues  [32]float64
+	AxisValues    [16]float64
+}
+
+type Touch struct {
+	ID driver.TouchID
+	X  int
+	Y  int
+}
+
+func InitializeGame() {
+	C.EbitenInitializeGame()
+}
+
+func ScreenSize() (int, int) {
+	var width, height C.int
+	C.EbitenGetScreenSize(&width, &height)
+	return int(width), int(height)
+}
+
+func BeginFrame() {
+	C.EbitenBeginFrame()
+}
+
+func EndFrame() {
+	C.EbitenEndFrame()
+}
+
+var cGamepads []C.struct_Gamepad
+
+func AppendGamepads(gamepads []Gamepad) []Gamepad {
+	n := int(C.EbitenGetGamepadNum())
+	if cap(cGamepads) < n {
+		cGamepads = append(cGamepads, make([]C.struct_Gamepad, n)...)
+	} else {
+		cGamepads = cGamepads[:n]
+	}
+	if n > 0 {
+		C.EbitenGetGamepads(&cGamepads[0])
+	}
+
+	for _, g := range cGamepads {
+		gamepad := Gamepad{
+			ID:        driver.GamepadID(g.id),
+			Standard:  g.standard != 0,
+			ButtonNum: int(g.button_num),
+			AxisNum:   int(g.axis_num),
+		}
+		for i := 0; i < gamepad.ButtonNum; i++ {
+			gamepad.ButtonPressed[i] = g.button_pressed[i] != 0
+			gamepad.ButtonValues[i] = float64(g.button_values[i])
+		}
+		for i := 0; i < gamepad.AxisNum; i++ {
+			gamepad.AxisValues[i] = float64(g.axis_values[i])
+		}
+
+		gamepads = append(gamepads, gamepad)
+	}
+	return gamepads
+}
+
+var cTouches []C.struct_Touch
+
+func AppendTouches(touches []Touch) []Touch {
+	n := int(C.EbitenGetTouchNum())
+	cTouches = cTouches[:0]
+	if cap(cTouches) < n {
+		cTouches = append(cTouches, make([]C.struct_Touch, n)...)
+	} else {
+		cTouches = cTouches[:n]
+	}
+	if n > 0 {
+		C.EbitenGetTouches(&cTouches[0])
+	}
+
+	for _, t := range cTouches {
+		touches = append(touches, Touch{
+			ID: driver.TouchID(t.id),
+			X:  int(t.x),
+			Y:  int(t.y),
+		})
+	}
+	return touches
+}
+
+func OpenAudio(sampleRate, channelNum, bitDepthInBytes int) {
+	C.EbitenOpenAudio(C.int(sampleRate), C.int(channelNum), C.int(bitDepthInBytes))
+}
+
+func CloseAudio() {
+	C.EbitenCloseAudio()
+}
+
+func CreateAudioPlayer(onWritten func()) *AudioPlayer {
+	id := C.EbitenCreateAudioPlayerProxy()
+	p := &AudioPlayer{
+		id: id,
+	}
+	onWrittenCallbacksM.Lock()
+	defer onWrittenCallbacksM.Unlock()
+	onWrittenCallbacks[id] = onWritten
+	return p
+}
+
+type AudioPlayer struct {
+	id C.int
+}
+
+func (p *AudioPlayer) Play() {
+	C.EbitenAudioPlayerPlay(p.id)
+}
+
+func (p *AudioPlayer) Pause() {
+	C.EbitenAudioPlayerPause(p.id)
+}
+
+func (p *AudioPlayer) Write(buf []byte) {
+	C.EbitenAudioPlayerWrite(p.id, (*C.uint8_t)(unsafe.Pointer(&buf[0])), C.int(len(buf)))
+	runtime.KeepAlive(buf)
+}
+
+func (p *AudioPlayer) Close(immediately bool) {
+	var i C.int
+	if immediately {
+		i = 1
+	}
+	C.EbitenAudioPlayerClose(p.id, i)
+
+	onWrittenCallbacksM.Lock()
+	defer onWrittenCallbacksM.Unlock()
+	delete(onWrittenCallbacks, p.id)
+}
+
+func (p *AudioPlayer) Volume() float64 {
+	return float64(C.EbitenAudioPlayerGetVolume(p.id))
+}
+
+func (p *AudioPlayer) SetVolume(volume float64) {
+	C.EbitenAudioPlayerSetVolume(p.id, C.double(volume))
+}
+
+func (p *AudioPlayer) UnplayedBufferSize() int {
+	return int(C.EbitenAudioPlayerGetUnplayedBufferSize(p.id))
+}
+
+var (
+	onWrittenCallbacks  = map[C.int]func(){}
+	onWrittenCallbacksM sync.Mutex
+)
+
+//export EbitenAudioPlayerOnWrittenCallback
+func EbitenAudioPlayerOnWrittenCallback(id C.int) {
+	onWrittenCallbacksM.Lock()
+	defer onWrittenCallbacksM.Unlock()
+	if c, ok := onWrittenCallbacks[id]; ok {
+		c()
+	}
+}

internal/graphicsdriver/opengl/gl/package_notwindows.go

@@ -6,7 +6,7 @@
 package gl
 
 // #cgo darwin LDFLAGS: -framework OpenGL
-// #cgo linux freebsd openbsd pkg-config: gl
+// #cgo linux,!ebitencbackend freebsd,!ebitencbackend openbsd,!ebitencbackend pkg-config: gl
 //
 // #ifndef APIENTRY
 //   #define APIENTRY

internal/graphicsdriver/opengl/gl/procaddr_notwindows.go

@@ -21,9 +21,11 @@ package gl
 #cgo darwin CFLAGS: -DTAG_DARWIN
 #cgo darwin LDFLAGS: -framework OpenGL
 #cgo linux freebsd openbsd CFLAGS: -DTAG_POSIX
-#cgo linux freebsd openbsd pkg-config: gl
+#cgo linux,!ebitencbackend freebsd,!ebitencbackend openbsd,!ebitencbackend pkg-config: gl
 #cgo egl CFLAGS: -DTAG_EGL
-#cgo egl pkg-config: egl
+#cgo egl,!ebitencbackend pkg-config: egl
+#cgo !darwin ebitencbackend LDFLAGS: -Wl,-unresolved-symbols=ignore-all
+#cgo darwin ebitencbackend LDFLAGS: -Wl,-undefined,dynamic_lookup
 // Check the EGL tag first as it takes priority over the platform's default
 // configuration of WGL/GLX/CGL.
 #if defined(TAG_EGL)

internal/uidriver/cbackend/input.go

@@ -0,0 +1,271 @@
+// Copyright 2021 The Ebiten Authors
+//
+// 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.
+
+//go:build ebitencbackend
+// +build ebitencbackend
+
+package cbackend
+
+import (
+	"sync"
+	"time"
+
+	"github.com/hajimehoshi/ebiten/v2/internal/cbackend"
+	"github.com/hajimehoshi/ebiten/v2/internal/driver"
+)
+
+type Input struct {
+	gamepads []cbackend.Gamepad
+	touches  []cbackend.Touch
+
+	m sync.Mutex
+}
+
+func (i *Input) update(context driver.UIContext) {
+	i.m.Lock()
+	defer i.m.Unlock()
+
+	i.gamepads = i.gamepads[:0]
+	i.gamepads = cbackend.AppendGamepads(i.gamepads)
+
+	i.touches = i.touches[:0]
+	i.touches = cbackend.AppendTouches(i.touches)
+
+	for idx, t := range i.touches {
+		x, y := context.AdjustPosition(float64(t.X), float64(t.Y), deviceScaleFactor)
+		i.touches[idx].X = int(x)
+		i.touches[idx].Y = int(y)
+	}
+}
+
+func (i *Input) AppendInputChars(runes []rune) []rune {
+	return nil
+}
+
+func (i *Input) AppendGamepadIDs(gamepadIDs []driver.GamepadID) []driver.GamepadID {
+	i.m.Lock()
+	defer i.m.Unlock()
+
+	for _, g := range i.gamepads {
+		gamepadIDs = append(gamepadIDs, g.ID)
+	}
+	return gamepadIDs
+}
+
+func (i *Input) AppendTouchIDs(touchIDs []driver.TouchID) []driver.TouchID {
+	i.m.Lock()
+	defer i.m.Unlock()
+
+	for _, t := range i.touches {
+		touchIDs = append(touchIDs, t.ID)
+	}
+	return touchIDs
+}
+
+func (i *Input) CursorPosition() (x, y int) {
+	return 0, 0
+}
+
+func (i *Input) GamepadSDLID(id driver.GamepadID) string {
+	return ""
+}
+
+func (i *Input) GamepadName(id driver.GamepadID) string {
+	return ""
+}
+
+func (i *Input) GamepadAxisValue(id driver.GamepadID, axis int) float64 {
+	i.m.Lock()
+	defer i.m.Unlock()
+
+	for _, g := range i.gamepads {
+		if g.ID != id {
+			continue
+		}
+		if axis < 0 {
+			return 0
+		}
+		if g.AxisNum <= axis {
+			return 0
+		}
+		if len(g.AxisValues) <= axis {
+			return 0
+		}
+		return g.AxisValues[axis]
+	}
+	return 0
+}
+
+func (i *Input) GamepadAxisNum(id driver.GamepadID) int {
+	i.m.Lock()
+	defer i.m.Unlock()
+
+	for _, g := range i.gamepads {
+		if g.ID != id {
+			continue
+		}
+		return g.AxisNum
+	}
+	return 0
+}
+
+func (i *Input) GamepadButtonNum(id driver.GamepadID) int {
+	i.m.Lock()
+	defer i.m.Unlock()
+
+	for _, g := range i.gamepads {
+		if g.ID != id {
+			continue
+		}
+		return g.ButtonNum
+	}
+	return 0
+}
+
+func (i *Input) IsGamepadButtonPressed(id driver.GamepadID, button driver.GamepadButton) bool {
+	i.m.Lock()
+	defer i.m.Unlock()
+
+	for _, g := range i.gamepads {
+		if g.ID != id {
+			continue
+		}
+		if button < 0 {
+			return false
+		}
+		if g.ButtonNum <= int(button) {
+			return false
+		}
+		if len(g.ButtonPressed) <= int(button) {
+			return false
+		}
+		return g.ButtonPressed[button]
+	}
+	return false
+}
+
+func (i *Input) IsKeyPressed(key driver.Key) bool {
+	return false
+}
+
+func (i *Input) IsMouseButtonPressed(button driver.MouseButton) bool {
+	return false
+}
+
+func (i *Input) IsStandardGamepadButtonPressed(id driver.GamepadID, button driver.StandardGamepadButton) bool {
+	i.m.Lock()
+	defer i.m.Unlock()
+
+	for _, g := range i.gamepads {
+		if g.ID != id {
+			continue
+		}
+		if !g.Standard {
+			return false
+		}
+		if button < 0 {
+			return false
+		}
+		if g.ButtonNum <= int(button) {
+			return false
+		}
+		if len(g.ButtonPressed) <= int(button) {
+			return false
+		}
+		return g.ButtonPressed[button]
+	}
+	return false
+}
+
+func (i *Input) IsStandardGamepadLayoutAvailable(id driver.GamepadID) bool {
+	i.m.Lock()
+	defer i.m.Unlock()
+
+	for _, g := range i.gamepads {
+		if g.ID != id {
+			continue
+		}
+		return g.Standard
+	}
+	return false
+}
+
+func (i *Input) StandardGamepadAxisValue(id driver.GamepadID, axis driver.StandardGamepadAxis) float64 {
+	i.m.Lock()
+	defer i.m.Unlock()
+
+	for _, g := range i.gamepads {
+		if g.ID != id {
+			continue
+		}
+		if !g.Standard {
+			return 0
+		}
+		if axis < 0 {
+			return 0
+		}
+		if g.AxisNum <= int(axis) {
+			return 0
+		}
+		if len(g.AxisValues) <= int(axis) {
+			return 0
+		}
+		return g.AxisValues[axis]
+	}
+	return 0
+}
+
+func (i *Input) StandardGamepadButtonValue(id driver.GamepadID, button driver.StandardGamepadButton) float64 {
+	i.m.Lock()
+	defer i.m.Unlock()
+
+	for _, g := range i.gamepads {
+		if g.ID != id {
+			continue
+		}
+		if !g.Standard {
+			return 0
+		}
+		if button < 0 {
+			return 0
+		}
+		if g.ButtonNum <= int(button) {
+			return 0
+		}
+		if len(g.ButtonValues) <= int(button) {
+			return 0
+		}
+		return g.ButtonValues[button]
+	}
+	return 0
+}
+
+func (i *Input) TouchPosition(id driver.TouchID) (x, y int) {
+	i.m.Lock()
+	defer i.m.Unlock()
+
+	for _, t := range i.touches {
+		if t.ID == id {
+			return t.X, t.Y
+		}
+	}
+	return 0, 0
+}
+
+func (i *Input) VibrateGamepad(id driver.GamepadID, duration time.Duration, strongMagnitude float64, weakMagnitude float64) {
+}
+
+func (i *Input) Wheel() (xoff, yoff float64) {
+	return 0, 0
+}

internal/uidriver/cbackend/ui.go

@@ -0,0 +1,142 @@
+// Copyright 2021 The Ebiten Authors
+//
+// 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.
+
+//go:build ebitencbackend
+// +build ebitencbackend
+
+package cbackend
+
+import (
+	"runtime"
+	"time"
+
+	"github.com/hajimehoshi/ebiten/v2/internal/cbackend"
+	"github.com/hajimehoshi/ebiten/v2/internal/driver"
+	"github.com/hajimehoshi/ebiten/v2/internal/graphicsdriver/opengl"
+)
+
+const deviceScaleFactor = 1
+
+func init() {
+	runtime.LockOSThread()
+}
+
+type UserInterface struct {
+	input Input
+}
+
+var theUserInterface UserInterface
+
+func Get() *UserInterface {
+	return &theUserInterface
+}
+
+func (u *UserInterface) Run(context driver.UIContext) error {
+	cbackend.InitializeGame()
+	for {
+		w, h := cbackend.ScreenSize()
+		context.Layout(float64(w), float64(h))
+
+		cbackend.BeginFrame()
+		u.input.update(context)
+
+		if err := context.UpdateFrame(); err != nil {
+			return err
+		}
+
+		cbackend.EndFrame()
+	}
+}
+
+func (*UserInterface) RunWithoutMainLoop(context driver.UIContext) {
+	panic("cbackend: RunWithoutMainLoop is not implemented")
+}
+
+func (*UserInterface) DeviceScaleFactor() float64 {
+	return deviceScaleFactor
+}
+
+func (*UserInterface) IsFocused() bool {
+	return true
+}
+
+func (*UserInterface) ScreenSizeInFullscreen() (int, int) {
+	return 0, 0
+}
+
+func (*UserInterface) ResetForFrame() {
+}
+
+func (*UserInterface) CursorMode() driver.CursorMode {
+	return driver.CursorModeHidden
+}
+
+func (*UserInterface) SetCursorMode(mode driver.CursorMode) {
+}
+
+func (*UserInterface) CursorShape() driver.CursorShape {
+	return driver.CursorShapeDefault
+}
+
+func (*UserInterface) SetCursorShape(shape driver.CursorShape) {
+}
+
+func (*UserInterface) IsFullscreen() bool {
+	return false
+}
+
+func (*UserInterface) SetFullscreen(fullscreen bool) {
+}
+
+func (*UserInterface) IsRunnableOnUnfocused() bool {
+	return false
+}
+
+func (*UserInterface) SetRunnableOnUnfocused(runnableOnUnfocused bool) {
+}
+
+func (*UserInterface) FPSMode() driver.FPSMode {
+	return driver.FPSModeVsyncOn
+}
+
+func (*UserInterface) SetFPSMode(mode driver.FPSMode) {
+}
+
+func (*UserInterface) ScheduleFrame() {
+}
+
+func (*UserInterface) IsScreenTransparent() bool {
+	return false
+}
+
+func (*UserInterface) SetScreenTransparent(transparent bool) {
+}
+
+func (*UserInterface) SetInitFocused(focused bool) {
+}
+
+func (*UserInterface) Vibrate(duration time.Duration, intensity float64) {
+}
+
+func (*UserInterface) Input() driver.Input {
+	return &theUserInterface.input
+}
+
+func (*UserInterface) Window() driver.Window {
+	return nil
+}
+
+func (*UserInterface) Graphics() driver.Graphics {
+	return opengl.Get()
+}

uidriver_cbackend.go

@@ -0,0 +1,27 @@
+// Copyright 2021 The Ebiten Authors
+//
+// 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.
+
+//go:build ebitencbackend
+// +build ebitencbackend
+
+package ebiten
+
+import (
+	"github.com/hajimehoshi/ebiten/v2/internal/driver"
+	"github.com/hajimehoshi/ebiten/v2/internal/uidriver/cbackend"
+)
+
+func uiDriver() driver.UI {
+	return cbackend.Get()
+}

uidriver_glfw.go

@@ -12,8 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-//go:build !android && !js && !ios
-// +build !android,!js,!ios
+//go:build !android && !js && !ios && !ebitencbackend
+// +build !android,!js,!ios,!ebitencbackend
 
 package ebiten
 

uidriver_js.go

@@ -12,6 +12,9 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build !ebitencbackend
+// +build !ebitencbackend
+
 package ebiten
 
 import (

uidriver_mobile.go

@@ -12,8 +12,9 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-//go:build android || ios
+//go:build (android || ios) && !ebitencbackend
 // +build android ios
+// +build !ebitencbackend
 
 package ebiten