NDK & JNI方式读写Android系统的GPIO

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NDK & JNI方式读写Android系统的GPIO

大家都知道Android系统是一种基于Linux的自由及开放源码的操作系统，所以读写GPIO也可以直接用Linux那一套export/unexport方法，本文将介绍如何使用NDK jni方式来读写Android系统的GPIO。

1. 软硬件环境：

Android Studio
RK3288 Android开发板

2. Android Studio安装NDK

打开Android Studio 中的SDK Manager，菜单Tools -> Android -> SDK Manager
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选中NDK，点击OK就可以安装NDK了，安装时间需要几分钟到十几分钟
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3. 创建Android工程

(1) 创建Android工程jnigpio，然后添加一个新的java类GPIOControl.java
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代码如下：

public class GPIOControl {    static {        System.loadLibrary("GPIOControl");    }    public final static native int exportGpio(int gpio);    public final static native int setGpioDirection(int gpio, int direction);    public final static native int readGpioStatus(int gpio);    public final static native int writeGpioStatus(int gpio, int value);    public final static native int unexportGpio(int gpio);}

(2) Build -> Rebuild Project
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(3) 生成jni的头文件

Android Studio Terminal中输入如下命令

cd app/src/main/java/
javah -d ../jni com.zhuang.jnigpio.GPIOControl

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生成jni头文件如下图：
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(4) jni目录下创建GPIOControl.c程序
代码如下：

//// Created by Gavin Zhuang on 05/12/2017.//#include <jni.h>#include <sys/stat.h>#include <sys/types.h>#include <fcntl.h>#include <stdio.h>#include <stdlib.h>#include <string.h>#include <fcntl.h>#include <sys/mman.h>#include <unistd.h>#include <android/log.h>#include "com_zhuang_jnigpio_GPIOControl.h"#define TAG "jni_gpio"#define LOGD(...) __android_log_print(ANDROID_LOG_DEBUG,TAG ,__VA_ARGS__)#define LOGI(...) __android_log_print(ANDROID_LOG_INFO,TAG ,__VA_ARGS__)#define LOGW(...) __android_log_print(ANDROID_LOG_WARN,TAG ,__VA_ARGS__)#define LOGE(...) __android_log_print(ANDROID_LOG_ERROR,TAG ,__VA_ARGS__)#define LOGF(...) __android_log_print(ANDROID_LOG_FATAL,TAG ,__VA_ARGS__)#define IN              0#define OUT             1#define LOW             0#define HIGH            1#define BUFFER_MAX    3#define DIRECTION_MAX 48/* * Class:     com_zhuang_jnigpio_GPIOControl * Method:    exportGpio * Signature: (I)I */JNIEXPORT jint JNICALL Java_com_zhuang_jnigpio_GPIOControl_exportGpio(JNIEnv *env, jobject instance, jint gpio){    char buffer[BUFFER_MAX];    int len;    int fd;    fd = open("/sys/class/gpio/export", O_WRONLY);    if (fd < 0) {        LOGE("Failed to open export for writing!\n");        return(0);    }    len = snprintf(buffer, BUFFER_MAX, "%d", gpio);    if (write(fd, buffer, len) < 0) {        LOGE("Fail to export gpio!\n");        return 0;    }    close(fd);    return 1;}/* * Class:     com_zhuang_jnigpio_GPIOControl * Method:    setGpioDirection * Signature: (II)I */JNIEXPORT jint JNICALL Java_com_zhuang_jnigpio_GPIOControl_setGpioDirection(JNIEnv *env, jobject instance, jint gpio, jint direction){    static const char dir_str[]  = "in\0out";    char path[DIRECTION_MAX];    int fd;    snprintf(path, DIRECTION_MAX, "/sys/class/gpio/gpio%d/direction", gpio);    fd = open(path, O_WRONLY);    if (fd < 0) {        LOGE("failed to open gpio direction for writing!\n");        return 0;    }    if (write(fd, &dir_str[direction == IN ? 0 : 3], direction == IN ? 2 : 3) < 0) {        LOGE("failed to set direction!\n");        return 0;    }    close(fd);    return 1;}/* * Class:     com_zhuang_jnigpio_GPIOControl * Method:    readGpioStatus * Signature: (I)I */JNIEXPORT jint JNICALL Java_com_zhuang_jnigpio_GPIOControl_readGpioStatus(JNIEnv *env, jobject instance, jint gpio){    char path[DIRECTION_MAX];    char value_str[3];    int fd;    snprintf(path, DIRECTION_MAX, "/sys/class/gpio/gpio%d/value", gpio);    fd = open(path, O_RDONLY);    if (fd < 0) {        LOGE("failed to open gpio value for reading!\n");        return -1;    }    if (read(fd, value_str, 3) < 0) {        LOGE("failed to read value!\n");        return -1;    }    close(fd);    return (atoi(value_str));}/* * Class:     com_zhuang_jnigpio_GPIOControl * Method:    writeGpioStatus * Signature: (II)I */JNIEXPORT jint JNICALL Java_com_zhuang_jnigpio_GPIOControl_writeGpioStatus(JNIEnv *env, jobject instance, jint gpio, jint value){    static const char values_str[] = "01";    char path[DIRECTION_MAX];    int fd;    snprintf(path, DIRECTION_MAX, "/sys/class/gpio/gpio%d/value", gpio);    fd = open(path, O_WRONLY);    if (fd < 0) {        LOGE("failed to open gpio value for writing!\n");        return 0;    }    if (write(fd, &values_str[value == LOW ? 0 : 1], 1) < 0) {        LOGE("failed to write value!\n");        return 0;    }    close(fd);    return 1;}/* * Class:     com_zhuang_jnigpio_GPIOControl * Method:    unexportGpio * Signature: (I)I */JNIEXPORT jint JNICALL Java_com_zhuang_jnigpio_GPIOControl_unexportGpio(JNIEnv *env, jobject instance, jint gpio){    char buffer[BUFFER_MAX];    int len;    int fd;    fd = open("/sys/class/gpio/unexport", O_WRONLY);    if (fd < 0) {        LOGE("Failed to open unexport for writing!\n");        return 0;    }    len = snprintf(buffer, BUFFER_MAX, "%d", gpio);    if (write(fd, buffer, len) < 0) {        LOGE("Fail to unexport gpio!");        return 0;    }    close(fd);    return 1;}

(5) 修改项目下的gradle.properties修改app下的build.gradle

在项目下的gradle.properties添加：

android.useDeprecatedNdk=true

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在defaultConfig中添加ndk内容：

ndk{        moduleName "GPIOControl" //so文件: lib+moduleName+.so        ldLibs "log", "z", "m" // 添加android日志引用        abiFilters "armeabi", "armeabi-v7a", "x86" //cpu的类型    }

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(6) Rebuild Project 生成so文件
so文件在app>build>intermediates>ndk>debug>lib>目录下，如图：
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(7) 方法调用

    // 定义GPIO输入输出方式和高低电平    public final static int GPIO_DIRECTION_IN = 0;    public final static int GPIO_DIRECTION_OUT = 1;    public final static int GPIO_VALUE_LOW = 0;    public final static int GPIO_VALUE_HIGH = 1;

例如，输出高低电平变化信号（如：IO口接LED电路，闪烁发光）
调用流程跟直接linux下文件方式控制gpio类似，先export该管脚，然后设定输入输出方式，最后不用的时候要unexport该管脚。

mStartButton.setOnClickListener(new View.OnClickListener() {            @Override            public void onClick(View v) {                String gpioPinStr = mGPIOEdit.getEditableText().toString();                if (TextUtils.isEmpty(gpioPinStr)){                    return;                }                mStartButton.setEnabled(false);                final int gpioPin = Integer.valueOf(gpioPinStr);                Thread thread = new Thread(new Runnable() {                    @Override                    public void run() {                        mIsFlashing = true;                        try {                            GPIOControl.exportGpio(gpioPin);                            GPIOControl.setGpioDirection(gpioPin, GPIOControl.GPIO_DIRECTION_OUT);                            boolean flag = true;                            while (mIsFlashing){                                GPIOControl.writeGpioStatus(gpioPin, flag ? GPIOControl.GPIO_VALUE_HIGH : GPIOControl.GPIO_VALUE_LOW);                                flag = !flag;                                Thread.sleep(1000);                            }                            GPIOControl.unexportGpio(gpioPin);                        } catch (Exception ex){                        }                    }                });                thread.start();            }        });

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读取gpio电平状态，按键按下为高电平，松开为低电平，如下是读取该管脚的日志：

12-05 10:09:22.624: W/CallSettingFragment(2551): GpioRead value; 262; 112-05 10:09:22.724: W/CallSettingFragment(2551): GpioRead value; 262; 112-05 10:09:22.824: W/CallSettingFragment(2551): GpioRead value; 262; 112-05 10:09:22.925: W/CallSettingFragment(2551): GpioRead value; 262; 012-05 10:09:23.025: W/CallSettingFragment(2551): GpioRead value; 262; 012-05 10:09:23.126: W/CallSettingFragment(2551): GpioRead value; 262; 012-05 10:09:23.228: W/CallSettingFragment(2551): GpioRead value; 262; 0

注意事项：该控制GPIO需要su权限，可以试着用adb shell去修改 /sys/class/gpio/export 和 /sys/class/gpio/unexport 的读写权限。如：chmod 666 /sys/class/export

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