Android JNI C++读写本地文件
Posted scruffybear
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文章目录
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：
<?xml version="1.0" encoding="utf-8"?>
<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 <sys/stat.h>，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(¤tTime); /*获取time_t类型当前时间*/
LOG_D("Current time = %s", ctime(¤tTime));
putenv("TZ=Asia/Singapore");
//sCurrentTime = gmtime(¤tTime);
sCurrentTime = localtime(¤tTime);
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(¤tTime));
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(¤tTime);
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
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