App's path for external storage in native code

Question

I am trying to write a native library for my application so that i can do all file operation in the native code. I read that getExternalStorageDirectory() give the path of the external storage of directory.

My question is how can i access the same without hard-coding the location to some string? Is there any function in android ndk that can give the same function as getExternalStorageDirectory() of java in C++ code?

Answer 1

JNI is your friend, and this isn't too complicated, as getExternalStorageDirectory is a static method. This function gets the value, and changes the working directory to it, for good measure.

#include <jni.h>
#include <unistd.h> // chdir()
#include <sys/param.h> // MAXPATHLEN

// To call Java methods when running native code inside an Android activity,
// a reference is needed to the JavaVM.
static JavaVM *gJavaVM;

JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM *vm, void *reserved)
{
    gJavaVM = vm;
    return JNI_VERSION_1_6;
}

int cdToExtStorage(void) {

    // Make JNI calls to get the external storage directory, and cd to it.
    
    // To begin, get a reference to the env and attach to it.
    JNIEnv *env;
    int isAttached = 0;
    int ret = 0;
    jthrowable exception;
    if (((*gJavaVM)->GetEnv(gJavaVM, (void**)&env, JNI_VERSION_1_6)) < 0) {
        // Couldn't get JNI environment, so this thread is native.
        if (((*gJavaVM)->AttachCurrentThread(gJavaVM, &env, NULL)) < 0) {
            fprintf(stderr, "Error: Couldn't attach to Java VM.\n");
            return (-1);
        }
        isAttached = 1;
    }
    
    // Get File object for the external storage directory.
    jclass classEnvironment = (*env)->FindClass(env, "android/os/Environment");
    if (!classEnvironment) goto bailAndroid;
    jmethodID methodIDgetExternalStorageDirectory = (*env)->GetStaticMethodID(env, classEnvironment, "getExternalStorageDirectory", "()Ljava/io/File;"); // public static File getExternalStorageDirectory ()
    if (!methodIDgetExternalStorageDirectory) goto bailAndroid;
    jobject objectFile = (*env)->CallStaticObjectMethod(env, classEnvironment, methodIDgetExternalStorageDirectory);
    exception = (*env)->ExceptionOccurred(env);
    if (exception) {
        (*env)->ExceptionDescribe(env);
        (*env)->ExceptionClear(env);
    }
    
    // Call method on File object to retrieve String object.
    jclass classFile = (*env)->GetObjectClass(env, objectFile);
    if (!classFile) goto bailAndroid;
    jmethodID methodIDgetAbsolutePath = (*env)->GetMethodID(env, classFile, "getAbsolutePath", "()Ljava/lang/String;");
    if (!methodIDgetAbsolutePath) goto bailAndroid;
    jstring stringPath = (*env)->CallObjectMethod(env, objectFile, methodIDgetAbsolutePath);
    exception = (*env)->ExceptionOccurred(env);
    if (exception) {
        (*env)->ExceptionDescribe(env);
        (*env)->ExceptionClear(env);
    }
    // Extract a C string from the String object, and chdir() to it.
    const char *wpath3 = (*env)->GetStringUTFChars(env, stringPath, NULL);
    if (chdir(wpath3) != 0) {
        fprintf(stderr, "Error: Unable to change working directory to %s.\n", wpath3);
        perror(NULL);
    } else if (path) {
        if (chdir(path) != 0) {
            fprintf(stderr, "Error: Unable to change working directory to %s.\n", path);
            perror(NULL);
        }
    }
    
    (*env)->ReleaseStringUTFChars(env, stringPath, wpath3);
    
    goto retAndroid;
    
bailAndroid:
    fprintf(stderr, "Error: JNI call failure.\n");
    ret = -1;
retAndroid:
    if (isAttached) (*gJavaVM)->DetachCurrentThread(gJavaVM); // Clean up.
    return (ret);
}

Answer 2

I'm not sure the existence of that function, but I think you can achieve it by reading /proc/mounts then get info of external storage,e.g /storage/sdcardx on JellyBean, mnt/sdcardx on older versions. You can check in *.rc file, maybe it can be defined a symlink for backward compatiblility. There exist another environment variable which is used to define external storage, EXTERNAL_STORAGE so you can try getenv(EXTERNAL_STORAGE) to get mount point. Hope it can help some ways.