How to properly pass an asset FileDescriptor to FFmpeg using JNI in Android

Question

I'm trying to retrieve metadata in Android using FFmpeg, JNI and a Java FileDescriptor and it isn't' working. I know FFmpeg supports the pipe protocol so I'm trying to emmulate: "cat test.mp3 | ffmpeg i pipe:0" programmatically. I use the following code to get a FileDescriptor from an asset bundled with the Android application:

FileDescriptor fd = getContext().getAssets().openFd("test.mp3").getFileDescriptor();
setDataSource(fd, 0, 0x7ffffffffffffffL); // native function, shown below

Then, in my native (In C++) code I get the FileDescriptor by calling:

static void wseemann_media_FFmpegMediaMetadataRetriever_setDataSource(JNIEnv *env, jobject thiz, jobject fileDescriptor, jlong offset, jlong length)
{
    //...

    int fd = jniGetFDFromFileDescriptor(env, fileDescriptor); // function contents show below

    //...
}

// function contents
static int jniGetFDFromFileDescriptor(JNIEnv * env, jobject fileDescriptor) {
    jint fd = -1;
    jclass fdClass = env->FindClass("java/io/FileDescriptor");

    if (fdClass != NULL) {
        jfieldID fdClassDescriptorFieldID = env->GetFieldID(fdClass, "descriptor", "I");
        if (fdClassDescriptorFieldID != NULL && fileDescriptor != NULL) {
            fd = env->GetIntField(fileDescriptor, fdClassDescriptorFieldID);
        }
    }

    return fd;
}

I then pass the file descriptor pipe # (In C) to FFmpeg:

char path[256] = "";

FILE *file = fdopen(fd, "rb");

if (file && (fseek(file, offset, SEEK_SET) == 0)) {
    char str[20];
    sprintf(str, "pipe:%d", fd);
    strcat(path, str);
}

State *state = av_mallocz(sizeof(State));
state->pFormatCtx = NULL;

if (avformat_open_input(&state->pFormatCtx, path, NULL, &options) != 0) { // Note: path is in the format "pipe:<the FD #>"
    printf("Metadata could not be retrieved\n");
    *ps = NULL;
    return FAILURE;
}

if (avformat_find_stream_info(state->pFormatCtx, NULL) < 0) {
    printf("Metadata could not be retrieved\n");
    avformat_close_input(&state->pFormatCtx);
    *ps = NULL;
    return FAILURE;
}

// Find the first audio and video stream
for (i = 0; i < state->pFormatCtx->nb_streams; i++) {
    if (state->pFormatCtx->streams[i]->codec->codec_type == AVMEDIA_TYPE_VIDEO && video_index < 0) {
        video_index = i;
    }

    if (state->pFormatCtx->streams[i]->codec->codec_type == AVMEDIA_TYPE_AUDIO && audio_index < 0) {
        audio_index = i;
    }

    set_codec(state->pFormatCtx, i);
}

if (audio_index >= 0) {
    stream_component_open(state, audio_index);
}

if (video_index >= 0) {
    stream_component_open(state, video_index);
}

printf("Found metadata\n");
AVDictionaryEntry *tag = NULL;
while ((tag = av_dict_get(state->pFormatCtx->metadata, "", tag, AV_DICT_IGNORE_SUFFIX))) {
    printf("Key %s: \n", tag->key);
    printf("Value %s: \n", tag->value);
}

*ps = state;
return SUCCESS;

My issue is avformat_open_input doesn't fail but it also doesn't let me retrieve any metadata or frames, The same code works if I use a regular file URI (e.g file://sdcard/test.mp3) as the path. What am I doing wrong? Thanks in advance.

Note: if you would like to look at all of the code I'm trying to solve the issue in order to provide this functionality for my library: FFmpegMediaMetadataRetriever.

Answer 1

Thks a lot for this post. That help me a lot to integrate Android 10 and scoped storage with FFmpeg using FileDescriptor.

Here the solution I'm using on Android 10:

Java

URI uri = ContentUris.withAppendedId(
   MediaStore.Audio.Media.EXTERNAL_CONTENT_URI,
   trackId // Coming from `MediaStore.Audio.Media._ID`
);
ParcelFileDescriptor parcelFileDescriptor = getContentResolver().openFileDescriptor(
   uri,
   "r"
);
int pid = android.os.Process.myPid();
String path = "/proc/" + pid + "/fd/" + parcelFileDescriptor.dup().getFd();
loadFFmpeg(path); // Call native code

CPP

// Native code, `path` coming from Java `loadFFmpeg(String)`
avformat_open_input(&format, path, nullptr, nullptr);

Answer 2

Java

AssetFileDescriptor afd = getContext().getAssets().openFd("test.mp3");
setDataSource(afd.getFileDescriptor(), afd.getStartOffset(), fd.getLength());

C

void ***_setDataSource(JNIEnv *env, jobject thiz, 
    jobject fileDescriptor, jlong offset, jlong length)
{
    int fd = jniGetFDFromFileDescriptor(env, fileDescriptor);

    char path[20];
    sprintf(path, "pipe:%d", fd);

    State *state = av_mallocz(sizeof(State));
    state->pFormatCtx =  avformat_alloc_context();
    state->pFormatCtx->skip_initial_bytes = offset;
    state->pFormatCtx->iformat = av_find_input_format("mp3");

and now we can continue as usual:

if (avformat_open_input(&state->pFormatCtx, path, NULL, &options) != 0) {
    printf("Metadata could not be retrieved\n");
    *ps = NULL;
    return FAILURE;
}
...

---

Even better, use <android/asset_manager.h>, like this:

Java

setDataSource(getContext().getAssets(), "test.mp3");

C

#include <android/asset_manager_jni.h>

void ***_setDataSource(JNIEnv *env, jobject thiz, 
    jobject assetManager, jstring assetName)
{
    AAssetManager* assetManager = AAssetManager_fromJava(env, assetManager);
    const char *szAssetName = (*env)->GetStringUTFChars(env, assetName, NULL);
    AAsset* asset = AAssetManager_open(assetManager, szAssetName, AASSET_MODE_RANDOM);
    (*env)->ReleaseStringUTFChars(env, assetName, szAssetName);
    off_t offset, length;
    int fd = AAsset_openFileDescriptor(asset, &offset, &length);
    AAsset_close(asset);

Disclaimer: error checking was omitted for brevity, but resources are released correctly, except for fd. You must close(fd) when finished.

Post Scriptum: note that some media formats, e.g. mp4 need seekable protocol, and pipe: cannot help. In such case, you may try sprintf(path, "/proc/self/fd/%d", fd);, or use the custom saf: protocol.

Answer 3

OK, I spent a lot of time trying to transfer media data to ffmpeg through Assetfiledescriptor. Finally, I found that there may be a bug in mov.c. When mov.c parsed the trak atom, the corresponding skip_initial_bytes was not set. I have tried to fix this problem.

Detail please refer to FFmpegForAndroidAssetFileDescriptor, demo refer to WhatTheCodec.