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

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.

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.

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);