本机代码中外部存储的应用路径

Question

我正在尝试为我的应用程序编写一个本机库，以便我可以在本机代码中执行所有文件操作。我读到getExternalStorageDirectory()给出目录外部存储的路径。

我的问题是如何在不将位置硬编码为某个字符串的情况下访问相同的位置？Android ndk 中是否有任何函数可以在C++代码中提供与 java getExternalStorageDirectory()相同的功能？

Answer 1

JNI是你的朋友，这并不太复杂，因为getExternalStorageDirectory是一个静态方法。此函数获取值，并将工作目录更改为该值，以便进行良好的度量。

#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

我不确定该功能是否存在，但我认为您可以通过读取/proc/mounts 然后获取外部存储的信息来实现它，例如 JellyBean 上的/storage/sdcardx，旧版本上的 mnt/sdcardx。您可以签入 *.rc 文件，也许可以为其定义一个向后兼容的符号链接。存在另一个环境变量，用于定义外部存储，EXTERNAL_STORAGE因此您可以尝试getenv(EXTERNAL_STORAGE)获取挂载点。希望它能有所帮助。