Android JNI C++读写本地文件

小结

进行Android JNI C++读写本地文件，取得了想要的效果。

Android JNI使用C++

对于Android的本地文件的操作，由于涉及到安全问题，并不是十分直接。
具体创建Andriod JNI应用，可以参考CSDN: android studio 3.2 使用jni 和 Add C and C++ code to your project

Android JNI读写本地文件

有关权限

要使用Android JNI读写本地文件，首先需要解决权限和授权的问题。否则会出现各种各样的错误。我碰到的有errno = 2 (No such file or directory) 和 errno = 13 (Permission denied)，具体错误码如下：

errno错误码：
errno0 :     Success

errno1 :     Operation not permitted

errno2 :     No such file or directory

errno3 :     No such process

errno4 :     Interrupted system call

errno5 :     Input/output error

errno6 :     No such device or address

errno7 :     Argument list too long

errno8 :     Exec format error

errno9 :     Bad file descriptor

errno10 :    No child processes

errno11 :    Resource temporarily unavailable

errno12 :    Cannot allocate memory

errno13 :    Permission denied

errno14 :    Bad address

errno15 :    Block device required

errno16 :    Device or resource busy

errno17 :    File exists

errno18 :    Invalid cross-device link

errno19 :    No such device

errno20 :    Not a directory

errno21 :    Is a directory

errno22 :    Invalid argument

errno23 :    Too many open files in system

errno24 :    Too many open files

errno25 :    Inappropriate ioctl for device

errno26 :    Text file busy

errno27 :    File too large

errno28 :    No space left on device

errno29 :    Illegal seek

errno30 :    Read-only file system

errno31 :    Too many links

errno32 :    Broken pipe

errno33 :    Numerical argument out of domain

errno34 :    Numerical result out of range

errno35 :    Resource deadlock avoided

errno36 :    File name too long

errno37 :    No locks available

errno38 :    Function not implemented

errno39 :    Directory not empty

errno40 :    Too many levels of symbolic links

errno41 :    Unknown error 41

errno42 :    No message of desired type

errno43 :    Identifier removed

errno44 :    Channel number out of range

errno45 :    Level 2 not synchronized

errno46 :    Level 3 halted

errno47 :    Level 3 reset

errno48 :    Link number out of range

errno49 :    Protocol driver not attached

errno50 :    No CSI structure available

errno51 :    Level 2 halted

errno52 :    Invalid exchange

errno53 :    Invalid request descriptor

errno54 :    Exchange full

errno55 :    No anode

errno56 :    Invalid request code

errno57 :    Invalid slot

errno58 :    Unknown error 58

errno59 :    Bad font file format

errno60 :    Device not a stream

errno61 :    No data available

errno62 :    Timer expired

errno63 :    Out of streams resources

errno64 :    Machine is not on the network

errno65 :    Package not installed

errno66 :    Object is remote

errno67 :    Link has been severed

errno68 :    Advertise error

errno69 :    Srmount error

errno70 :    Communication error on send

errno71 :    Protocol error

errno72 :    Multihop attempted

errno73 :    RFS specific error

errno74 :    Bad message

errno75 :    Value too large for defined datatype

errno76 :    Name not unique on network

errno77 :    File descriptor in bad state

errno78 :    Remote address changed

errno79 :    Can not access a needed sharedlibrary

errno80 :    Accessing a corrupted sharedlibrary

errno81 :    .lib section in a.out corrupted

errno82 :    Attempting to link in too manyshared libraries

errno83 :    Cannot exec a shared librarydirectly

errno84 :    Invalid or incomplete multibyte orwide character

errno85 :    Interrupted system call should berestarted

errno86 :    Streams pipe error

errno87 :    Too many users

errno88 :    Socket operation on non-socket

errno89 :    Destinationaddress required

errno90 :    Message too long

errno91 :    Protocol wrong type for socket

errno92 :    Protocol not available

errno93 :    Protocol not supported

errno94 :    Socket type not supported

errno95 :    Operation not supported

errno96 :    Protocol family not supported

errno97 :    Address family not supported byprotocol

errno98 :    Address already in use

errno99 :    Cannot assign requested address

errno100 :   Network is down

errno101 :   Network is unreachable

errno102 :   Network dropped connection onreset

errno103 :   Software caused connection abort

errno104 :   Connection reset by peer

errno105 :   No buffer space available

errno106 :   Transport endpoint is alreadyconnected

errno107 :   Transport endpoint is notconnected

errno108 :   Cannot send after transportendpoint shutdown

errno109 :   Too many references: cannot splice

errno110 :   Connection timed out

errno111 :   Connection refused

errno112 :   Host is down

errno113 :   No route to host

errno114 :   Operation already in progress

errno115 :   Operation now in progress

errno116 :   Stale NFS file handle

errno117 :   Structure needs cleaning

errno118 :   Not a XENIX named type file

errno119 :   No XENIX semaphores available

errno120 :   Is a named type file

errno121 :   Remote I/O error

errno122 :   Disk quota exceeded

errno123 :   No medium found

errno124 :   Wrong medium type

errno125 :   Operation canceled

errno126 :   Required key not available

errno127 :   Key has expired

errno128 :   Key has been revoked

errno129 :   Key was rejected by service

errno130 :   Owner died

errno131 :   State not recoverable

errno132 :   Operation not possible due toRF-kill

errno133 :   Unknown error 133

errno134 :   Unknown error 134

errno135 :   Unknown error 135

errno136 :   Unknown error 136

errno137 :   Unknown error 137

errno138 :   Unknown error 138

errno139 :   Unknown error 139
 

有关权限的问题的具体解决办法可能参考此篇文章：CSDN: Android文件读写权限 fopen errno=13

默认设置是App可以访问本应用所在的目录, 例如/data/data/example.jniwritefile/，这里example.jniwritefile是应用名字。但是要访问其它存储，需要考虑到权限问题。

权限的问题的解决办法是在AndroidManifest.xml加了几个文件操作权限，并在application中加入了android:requestLegacyExternalStorage="true"，文件具体如下：

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools">

    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
    <uses-permission android:name="android.permission.MOUNT_UNMOUNT_FILESYSTEMS" tools:ignore="ProtectedPermissions" />
    <uses-permission android:name="android.permission.MANAGE_EXTERNAL_STORAGE"
        tools:ignore="ScopedStorage" />
        
    <application
        android:allowBackup="true"
        android:dataExtractionRules="@xml/data_extraction_rules"
        android:fullBackupContent="@xml/backup_rules"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/Theme.JNIWriteFile"
        android:requestLegacyExternalStorage="true"
        tools:targetApi="31">


    <activity
            android:name=".MainActivity"
            android:exported="true">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

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

            <meta-data
                android:name="android.app.lib_name"
                android:value="" />
        activity>
    application>

manifest>

以下Java代码是在应用启动时开启弹窗让用户确认开启权限。

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        verifyStoragePermission(this);

 ...
 此处省略
 ...

    }

    private static final int REQUEST_EXTERNAL_STORAGE = 1;
    private static String[] PERMISSIONS_STORAGE = {
            "android.permission.READ_EXTERNAL_STORAGE",
            "android.permission.WRITE_EXTERNAL_STORAGE"
    };

    public void verifyStoragePermission(Activity activity){
        try{
            int permission = ActivityCompat.checkSelfPermission(activity,"android.permission.WRITE_EXTERNAL_STORAGE");
            if(permission!= PackageManager.PERMISSION_GRANTED){
                ActivityCompat.requestPermissions(activity,PERMISSIONS_STORAGE, REQUEST_EXTERNAL_STORAGE);
            }
        }catch (Exception e){
            e.printStackTrace();
            e.printStackTrace();
        }
    }

创建文件夹

mkdir函数需要包括头文件：#include ，参考CSDN: 文件编程：创建目录mkdir()函数，具体C代码如下：

//方便用日志查看

#define LOG_D(...)  __android_log_print(ANDROID_LOG_DEBUG, "jni", __VA_ARGS__)

...
省略
...
    int errNum = 0;
    if(0 == access(WriteFileFolder,0)) {//目录存在

    } else{
        if(0 == mkdir(WriteFileFolder,777)) {

        }
        else {
            LOG_D("open fail errno = %d, reason = %s", errNum, strerror(errNum));
        }
    }
...

访问 /storage/emulated/0/

/storage/emulated/0/是可以通过Android手机文件应用/文件浏览器进行访问的。
首先，需要获取获取手机内部存储卡的根目录，Java代码获取比较方便，这里使用Android ndk来获取。
参考CSDN: Android ndk获取手机内部存储卡的根目录 和 CSDN: Android Native APP开发笔记：文件存储与访问 ，代码如下：

    //get the external file directory

    jclass envcls = env->FindClass("android/os/Environment"); //获得类引用
    if (envcls == nullptr) return 0;

    //找到对应的类，该类是静态的返回值是File
    jmethodID id = env->GetStaticMethodID(envcls, "getExternalStorageDirectory", "()Ljava/io/File;");

    //调用上述id获得的方法，返回对象即File file=Enviroment.getExternalStorageDirectory()
    //其实就是通过Enviroment调用 getExternalStorageDirectory()
    jobject fileObj = env->CallStaticObjectMethod(envcls,id);

    //通过上述方法返回的对象创建一个引用即File对象
    jclass flieClass = env->GetObjectClass(fileObj); //或得类引用
    //在调用File对象的getPath()方法获取该方法的ID，返回值为String 参数为空
    jmethodID getpathId = env->GetMethodID(flieClass, "getPath", "()Ljava/lang/String;");
    //调用该方法及最终获得存储卡的根目录
    jstring pathStr = (jstring)env->CallObjectMethod(fileObj,getpathId);

    const char* pathStrC = env->GetStringUTFChars(pathStr,NULL);

    char WriteFileFolder[100];

    sprintf(WriteFileFolder, "%s/DocumentTest", pathStrC);

以下代码实现了在前面创建的 /storage/emulated/0/DocumentTest目录下创建5个文件，目录是放在以上所获取的变量WriteFileFolder里的，文件名以JohnTest开头，以时间戳来命名的TXT文件，创建文件后写入This is test to write to file! Timestamp:并加上时间戳。

FILE *dumpFile = NULL;

    for (int j = 0; j < 5; j++) {

        time_t currentTime;
        struct tm *sCurrentTime;

        time(&currentTime); /*获取time_t类型当前时间*/

        LOG_D("Current time = %s", ctime(&currentTime));

        putenv("TZ=Asia/Singapore");
        //sCurrentTime = gmtime(¤tTime);
        sCurrentTime = localtime(&currentTime);


        char dumpfileName[100];
        
        sprintf(dumpfileName, "%s/JohnTest%04d%02d%02d%02d%02d%02d.txt",
                WriteFileFolder,
                sCurrentTime->tm_year + 1900,
                sCurrentTime->tm_mon + 1,
                sCurrentTime->tm_mday,
                sCurrentTime->tm_hour,
                sCurrentTime->tm_min,
                sCurrentTime->tm_sec);
        dumpFile = fopen(dumpfileName, "w+");

        char rpucData[100];
        
        sprintf(rpucData, "This is test to write to file! Timestamp: %s", ctime(&currentTime));

        if (dumpFile == NULL) {
            int errNum = 0;
            errNum = errno;
            LOG_D("open fail errno = %d, reason = %s", errNum, strerror(errNum));
        } else {
            fwrite(rpucData, sizeof(char), (unsigned) strlen(rpucData), dumpFile);
            fclose(dumpFile);
        }
    }

程序运行后会在相应的目录里写入5个TXT文件，并写入相应的内容。

访问/data/data/example.jniwritefile/

访问/data/data/example.jniwritefile/ 并不需要申请权限，类似以上程序，只需要进行以下修改：

...
省略
...
        sprintf(dumpfileName, "/data/data/example.jniwritefile/JohnTest%04d%02d%02d%02d%02d%02d.txt",
                sCurrentTime->tm_year + 1900,
                sCurrentTime->tm_mon + 1,
                sCurrentTime->tm_mday,
                sCurrentTime->tm_hour,
                sCurrentTime->tm_min,
                sCurrentTime->tm_sec);
        dumpFile = fopen(dumpfileName, "w+");

时间戳

有关时间戳的问题可以参考CSDN: C语言应用（1）——Unix时间戳和北京时间的相互转换 ， cppreference.com: gmtime, gmtime_r, gmtime_s 和 CSDN: c++ 时间类型详解 time_t

这里使用了localtime函数，注意localtime与gmtime的时差，例如新加坡/北京时间与GMT时间隔了8个小时。

        putenv("TZ=Asia/Singapore");
        //sCurrentTime = gmtime(¤tTime);
        sCurrentTime = localtime(&currentTime);

Can’t determine type for tag

参考Can’t determine type for tag macro name=“m3_comp_assist_chip_container_shape”>?attr/shapeAppearanceCornerSmall
这个问题在build.gradle(:app)里通过修改几个版本号解决，我使用了以下版本：

dependencies {

    implementation 'androidx.appcompat:appcompat:1.4.0'
    implementation 'com.google.android.material:material:1.6.0'
    implementation 'androidx.constraintlayout:constraintlayout:2.1.4'
    testImplementation 'junit:junit:4.13.2'
    androidTestImplementation 'androidx.test.ext:junit:1.1.3'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.4.0'
}

参考

C library function - fopen()
CSDN: android通过JNI用C/C++创建本地文件
CSDN: android studio 3.2 使用jni
CSDN Android JNI读取本地文件和读取文件并且写入其他文件
Stackoverflow: Android NDK fopen returns error 2 “No such file or directory” on a file I know exits
Stackoverlfow: Write file to location other than SDcard using Android NDK?
Stackoverlfow: File Operations in Android NDK
CSDN: Android Native APP开发笔记：文件存储与访问
Add C and C++ code to your project
Can’t determine type for tag macro name=“m3_comp_assist_chip_container_shape”>?attr/shapeAppearanceCornerSmall
CSDN: c++ 时间类型详解 time_t
cppreference.com: gmtime, gmtime_r, gmtime_s
CSDN: Android文件读写权限 fopen errno=13
CSDN: C语言应用（1）——Unix时间戳和北京时间的相互转换
CSDN: Android ndk获取手机内部存储卡的根目录
CSDN: 文件编程：创建目录mkdir()函数