Android 串口接收不到数据或者扫描不到串口的记录

1.程序正常，但是找不到串口

adb root && adb remount && adb shell

cd dev

setenforce 0

2.接收不到数据

修改serialport.c文件，加入两句代码：

ioctl(fd, 0x544d);

ioctl(descriptor, 0x544e);

具体加的位置如下图所示： 

/*
 * Copyright 2009-2011 Cedric Priscal
 *
 * 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.
 */

#include <termios.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>
#include <jni.h>

#include "SerialPort.h"

#include "android/log.h"

static const char *TAG = "serial_port";
#define LOGI(fmt, args...) __android_log_print(ANDROID_LOG_INFO,  TAG, fmt, ##args)
#define LOGD(fmt, args...) __android_log_print(ANDROID_LOG_DEBUG, TAG, fmt, ##args)
#define LOGE(fmt, args...) __android_log_print(ANDROID_LOG_ERROR, TAG, fmt, ##args)

static speed_t getBaudrate(jint baudrate) {
    switch (baudrate) {
        case 0:
            return B0;
        case 50:
            return B50;
        case 75:
            return B75;
        case 110:
            return B110;
        case 134:
            return B134;
        case 150:
            return B150;
        case 200:
            return B200;
        case 300:
            return B300;
        case 600:
            return B600;
        case 1200:
            return B1200;
        case 1800:
            return B1800;
        case 2400:
            return B2400;
        case 4800:
            return B4800;
        case 9600:
            return B9600;
        case 19200:
            return B19200;
        case 38400:
            return B38400;
        case 57600:
            return B57600;
        case 115200:
            return B115200;
        case 230400:
            return B230400;
        case 460800:
            return B460800;
        case 500000:
            return B500000;
        case 576000:
            return B576000;
        case 921600:
            return B921600;
        case 1000000:
            return B1000000;
        case 1152000:
            return B1152000;
        case 1500000:
            return B1500000;
        case 2000000:
            return B2000000;
        case 2500000:
            return B2500000;
        case 3000000:
            return B3000000;
        case 3500000:
            return B3500000;
        case 4000000:
            return B4000000;
        default:
            return -1;
    }
}

/*
 * Class:     android_serialport_SerialPort
 * Method:    open
 * Signature: (Ljava/lang/String;II)Ljava/io/FileDescriptor;
 */
JNIEXPORT jobject JNICALL Java_android_1serialport_1api_SerialPort_open
        (JNIEnv *env, jclass thiz, jstring path, jint baudrate, jint stopBits, jint dataBits,
         jint parity, jint flowCon, jint flags) {
    int fd;
    speed_t speed;
    jobject mFileDescriptor;

    /* Check arguments */
    {
        speed = getBaudrate(baudrate);
        if (speed == -1) {
            /* TODO: throw an exception */
            LOGE("Invalid baudrate");
            return NULL;
        }
    }

    /* Opening device */
    {
        jboolean iscopy;
        const char *path_utf = (*env)->GetStringUTFChars(env, path, &iscopy);
        LOGD("Opening serial port %s with flags 0x%x", path_utf, O_RDWR | flags);
        fd = open(path_utf, O_RDWR | flags);
        LOGD("open() fd = %d", fd);
        (*env)->ReleaseStringUTFChars(env, path, path_utf);
        if (fd == -1) {
            /* Throw an exception */
            LOGE("Cannot open port");
            /* TODO: throw an exception */
            return NULL;
        }
    }

    /* Configure device */
    {
        struct termios cfg;
        LOGD("Configuring serial port");
        if (tcgetattr(fd, &cfg)) {
            LOGE("tcgetattr() failed");
            close(fd);
            /* TODO: throw an exception */
            return NULL;
        }

        cfmakeraw(&cfg);
        cfsetispeed(&cfg, speed);
        cfsetospeed(&cfg, speed);
		ioctl(fd, 0x544d);
		
        cfg.c_cflag &= ~CSIZE;
        switch (dataBits) {
            case 5:
                cfg.c_cflag |= CS5;    //使用5位数据位
                break;
            case 6:
                cfg.c_cflag |= CS6;    //使用6位数据位
                break;
            case 7:
                cfg.c_cflag |= CS7;    //使用7位数据位
                break;
            case 8:
                cfg.c_cflag |= CS8;    //使用8位数据位
                break;
            default:
                cfg.c_cflag |= CS8;
                break;
        }

        switch (parity) {
            case 0:
                cfg.c_cflag &= ~PARENB;    //无奇偶校验
                break;
            case 1:
                cfg.c_cflag |= (PARODD | PARENB);   //奇校验
                break;
            case 2:
                cfg.c_iflag &= ~(IGNPAR | PARMRK); // 偶校验
                cfg.c_iflag |= INPCK;
                cfg.c_cflag |= PARENB;
                cfg.c_cflag &= ~PARODD;
                break;
            default:
                cfg.c_cflag &= ~PARENB;
                break;
        }

        switch (stopBits) {
            case 1:
                cfg.c_cflag &= ~CSTOPB;    //1位停止位
                break;
            case 2:
                cfg.c_cflag |= CSTOPB;    //2位停止位
                break;
            default:
                break;
        }

        // hardware flow control
        switch (flowCon) {
            case 0:
                cfg.c_cflag &= ~CRTSCTS;    //不使用流控
                break;
            case 1:
                cfg.c_cflag |= CRTSCTS;    //硬件流控
                break;
            case 2:
                cfg.c_cflag |= IXON | IXOFF | IXANY;    //软件流控
                break;
            default:
                cfg.c_cflag &= ~CRTSCTS;
                break;
        }


        if (tcsetattr(fd, TCSANOW, &cfg)) {
            LOGE("tcsetattr() failed");
            close(fd);
            /* TODO: throw an exception */
            return NULL;
        }
    }

    /* Create a corresponding file descriptor */
    {
        jclass cFileDescriptor = (*env)->FindClass(env, "java/io/FileDescriptor");
        jmethodID iFileDescriptor = (*env)->GetMethodID(env, cFileDescriptor, "<init>", "()V");
        jfieldID descriptorID = (*env)->GetFieldID(env, cFileDescriptor, "descriptor", "I");
        mFileDescriptor = (*env)->NewObject(env, cFileDescriptor, iFileDescriptor);
        (*env)->SetIntField(env, mFileDescriptor, descriptorID, (jint) fd);
    }

    return mFileDescriptor;
}

/*
 * Class:     cedric_serial_SerialPort
 * Method:    close
 * Signature: ()V
 */
JNIEXPORT void JNICALL Java_android_1serialport_1api_SerialPort_close
        (JNIEnv *env, jobject thiz) {
    jclass SerialPortClass = (*env)->GetObjectClass(env, thiz);
    jclass FileDescriptorClass = (*env)->FindClass(env, "java/io/FileDescriptor");

    jfieldID mFdID = (*env)->GetFieldID(env, SerialPortClass, "mFd", "Ljava/io/FileDescriptor;");
    jfieldID descriptorID = (*env)->GetFieldID(env, FileDescriptorClass, "descriptor", "I");

    jobject mFd = (*env)->GetObjectField(env, thiz, mFdID);
    jint descriptor = (*env)->GetIntField(env, mFd, descriptorID);
	ioctl(descriptor, 0x544e);
    LOGD("close(fd = %d)", descriptor);
    close(descriptor);
}

3.通过Android studio的run apk按钮可以安装成功apk,但是生成的串口工具给别人安装，却闪退。

原因是安装完串口app没有赋予系统权限。此时需要系统签名。

（1）修改AndroidManifest.xml文件，加入android.uid.system权限。注意此代码加载的位置

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.ysn.serialport"
    android:sharedUserId="android.uid.system">
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme"
        android:requestLegacyExternalStorage="true">
        <activity android:name=".MainActivity" android:screenOrientation="portrait">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

    </application>

</manifest>

 （2）使用Android studio自带工具进行签名。Build--Generate Signed Bundle /apk--选择APK--点击NEXT--如果没有keystore,点击Create new,填写其中的内容，如果已有keystore,点击choose existing,然后填写key store password、key alias、key password，然后点击next，选择debug，选择V1和V2,然后点击finish。等待编译完成，最后生成的签名好的apk，放在app--debug目录下，名为app-debug.apk（此apk完成了系统签名）。将此apk拖到应用界面即可完成安装，也可以将此apk分享给其他人进行安装。

注意，此时如果用Android studio 的run app按钮，往往无法安装成功。这给调试带来麻烦。因此需要加上步骤（3），方便调试。

（3）在build.gradle(:app)文件中配置签名。

apply plugin: 'com.android.application'

android {
    compileSdkVersion 30
    defaultConfig {
        applicationId "com.ysn.serialport"
        minSdkVersion 26
        targetSdkVersion 30
        versionCode 1
        versionName "1.0"

    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
    buildToolsVersion '30.0.2'
    signingConfigs {
        debug {
            storeFile file('release.keystore')
            storePassword '123qwe'
            keyAlias 'androidreleasekey'
            keyPassword '123qwe'
        }
        release {
            storeFile file('release.keystore')
            storePassword '123qwe'
            keyAlias 'androidreleasekey'
            keyPassword '123qwe'
        }
    }
}

dependencies {
    implementation fileTree(include: ['*.jar'], dir: 'libs')
    implementation 'com.android.support:appcompat-v7:28.0.0'
    implementation 'com.android.support.constraint:constraint-layout:1.1.3'
    implementation 'com.android.support:recyclerview-v7:28.0.0'
    implementation 'com.github.CymChad:BaseRecyclerViewAdapterHelper:2.9.42'
    implementation project(':serialport')
    implementation 'tp.xmaihh:serialport:2.0'
    implementation 'pub.devrel:easypermissions:1.2.0'
}

注意，完成（1）（2）（3）步骤后，可以使用Android studio 的run app按钮可以安装成功，方便调试，但是无法在app下生成debug文件夹，在build--output--apk--debug中生成的apk也无法给其他人进行安装。