本文以編譯LCD背光的庫檔案為例，生成的庫檔案為 lights.mini6410.so.

原始檔的目錄為：Android-2.3.4/device/samsung/crespo/liblight/    light.c   Android.mk

light.c

/*
 * Copyright (C) 2008 The Android Open Source Project
 *
 * 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.
 */

#define LOG_TAG "lights"
#include <cutils/log.h>
#include <stdint.h>
#include <string.h>
#include <errno.h>
#include <fcntl.h>
#include <pthread.h>
#include <sys/ioctl.h>
#include <sys/types.h>
#include <hardware/lights.h>

static pthread_once_t g_init = PTHREAD_ONCE_INIT;
static pthread_mutex_t g_lock = PTHREAD_MUTEX_INITIALIZER;

char const *const LCD_FILE = "/sys/class/backlight/pwm-backlight.0/brightness";

static int write_int(char const *path, int value)
{
	int fd;
	static int already_warned;

	already_warned = 0;

	LOGV("write_int: path %s, value %d", path, value);
	fd = open(path, O_RDWR);

	if (fd >= 0) {
		char buffer[20];
		int bytes = sprintf(buffer, "%d\n", value);
		int amt = write(fd, buffer, bytes);
		close(fd);
		return amt == -1 ? -errno : 0;
	} else {
		if (already_warned == 0) {
			LOGE("write_int failed to open %s\n", path);
			already_warned = 1;
		}
		return -errno;
	}
}

static int rgb_to_brightness(struct light_state_t const *state)
{
	int color = state->color & 0x00ffffff;

	return ((77*((color>>16) & 0x00ff))
		+ (150*((color>>8) & 0x00ff)) + (29*(color & 0x00ff))) >> 8;
}

static int set_light_backlight(struct light_device_t *dev,
			struct light_state_t const *state)
{
	int err = 0;
	int brightness = rgb_to_brightness(state);

	pthread_mutex_lock(&g_lock);
	err = write_int(LCD_FILE, brightness);

	pthread_mutex_unlock(&g_lock);
	return err;
}

static int close_lights(struct light_device_t *dev)
{
	LOGV("close_light is called");
	if (dev)
		free(dev);

	return 0;
}

static int open_lights(const struct hw_module_t *module, char const *name,
						struct hw_device_t **device)
{
	int (*set_light)(struct light_device_t *dev,
		struct light_state_t const *state);

	LOGV("open_lights: open with %s", name);

	if (0 == strcmp(LIGHT_ID_BACKLIGHT, name))
		set_light = set_light_backlight;
	else
		return -EINVAL;

	pthread_mutex_init(&g_lock, NULL);

	struct light_device_t *dev = malloc(sizeof(struct light_device_t));
	memset(dev, 0, sizeof(*dev));

	dev->common.tag = HARDWARE_DEVICE_TAG;
	dev->common.version = 0;
	dev->common.module = (struct hw_module_t *)module;
	dev->common.close = (int (*)(struct hw_device_t *))close_lights;
	dev->set_light = set_light;

	*device = (struct hw_device_t *)dev;

	return 0;
}

static struct hw_module_methods_t lights_module_methods = {
	.open =  open_lights,
};

const struct hw_module_t HAL_MODULE_INFO_SYM = {
	.tag = HARDWARE_MODULE_TAG,
	.version_major = 1,
	.version_minor = 0,
	.id = LIGHTS_HARDWARE_MODULE_ID,
	.name = "lights Module",
	.author = "Google, Inc.",
	.methods = &lights_module_methods,
};
 

char const *const LCD_FILE = "/sys/class/backlight/pwm-backlight.0/brightness"; 是核心驅動載入後生成的節點檔案，應用層傳遞brightness引數到節點，控制亮度。

Android.mk

# Copyright (C) 2008 The Android Open Source Project
#
# 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.

#ifneq ($(filter crespo crespo4g,$(TARGET_DEVICE)),)

LOCAL_PATH:= $(call my-dir)
# HAL module implemenation, not prelinked and stored in
# hw/<COPYPIX_HARDWARE_MODULE_ID>.<ro.board.platform>.so
include $(CLEAR_VARS)

LOCAL_SRC_FILES := lights.c

LOCAL_PRELINK_MODULE := false
LOCAL_MODULE_PATH := $(TARGET_OUT_SHARED_LIBRARIES)/hw

LOCAL_SHARED_LIBRARIES := liblog

LOCAL_MODULE := lights.$(TARGET_BOARD_PLATFORM)
#LOCAL_MODULE := lights.default
#LOCAL_MODULE_TAGS := optional
LOCAL_MODULE_TAGS := eng
include $(BUILD_SHARED_LIBRARY)

#endif
 

編譯的時候遇到的問題：

1、編譯之後沒有生成lights.mini6410.so，原因是Android.mk中的這一句沒有執行時判斷不通過：ifneq ($(filter crespo crespo4g,$(TARGET_DEVICE)),)，將其遮蔽就可以生成所需的檔案了。

2、在out/target/product/mini6410/system/lib/目錄下沒有產生 lights.mini6410.so檔案， 是由於LOCAL_MODULE_TAGS的值得原因， 當LOCAL_MODULE_TAGS := optional時，編譯生成的庫檔案到了out/target/product/mini6410/symbols/system/lib/目錄下；當LOCAL_MODULE_TAGS := eng 時，編譯生成的庫檔案到了out/target/product/mini6410/system/lib/目錄下，android2.3版本是這樣，也許高階的版本不是這樣，有待驗證。

 Android.mk中的LOCAL_MODULE的名字的取法也是有講究的，應該是對應的 xxx.xxx，前一個給功能，後一個為板子的名稱或者default ，名字例如mini6410,，和out/target/product/mini6410一致。