FFmpeg：在 Android Q 上无法使用文件描述符进行查找

鉴于公共文件路径通常在具有范围存储的 Android Q 中不可用，我试图弄清楚如何使我的 FFmpeg 音频解码器使用文件描述符，而不将文件复制到我的应用程序的私有目录。

我们可以使用中描述的方法轻松获取文件描述符Android Q 隐私更改，并且可以使用管道协议打开文件描述符，如中所述从可打开的 URI 将本机 fd int 传递到 FFMPEG。但是，使用以下方法无法查找结果av_seek_frame并且使用持续时间成员也无法获得持续时间AVFormatContext.

有没有办法用 FFmpeg 寻找文件描述符并检索持续时间？

可以使用管道协议打开文件描述符，如下所述

我很好奇为什么需要通过管道协议打开文件描述符？sView播放器通过自定义打开文件描述符AVIO上下文，这是可搜索的，至少在较旧的经过测试的 Android 版本上是这样。这是使用自定义 AVIOContext 打开 AVFormatContext 的伪代码。

    int aFileDescriptor = myResMgr->openFileDescriptor(theFileToLoad);
    AVFormatContext* aFormatCtx = avformat_alloc_context();
    StAVIOContext myAvioContext;
    if(!myAvioContext.openFromDescriptor(aFileDescriptor, "rb")) {
       // error
    }

    aFormatCtx->pb = myAvioContext.getAvioContext();
    int avErrCode = avformat_open_input(&aFormatCtx, theFileToLoad, NULL, NULL);

下面尝试提取简化的 StAVIOFileContext 类定义。

//! Wrapper over AVIOContext for passing the custom I/O.
class StAVIOContext {
public:
  //! Main constructor.
  StAVIOContext() {
    const int aBufferSize = 32768;
    unsigned char* aBufferIO = (unsigned char* )av_malloc(aBufferSize + AV_INPUT_BUFFER_PADDING_SIZE);
    AVIOContext* myAvioCtx = avio_alloc_context (aBufferIO, aBufferSize, 0, this, readCallback, writeCallback, seekCallback);
  }

  //! Destructor.
  virtual ~StAVIOContext() {
    close();
    if (myAvioCtx != NULL) { av_free (myAvioCtx); }
  }

  //! Close the file.
  void close() {
    if(myFile != NULL) {
        fclose(myFile);
        myFile = NULL;
    }
  }

  //! Associate a stream with a file that was previously opened for low-level I/O.
  //! The associated file will be automatically closed on destruction.
  bool openFromDescriptor(int theFD, const char* theMode) {
    close();
  #ifdef _WIN32
    myFile = ::_fdopen(theFD, theMode);
  #else
    myFile =  ::fdopen(theFD, theMode);
  #endif
    return myFile != NULL;
  }

  //! Access AVIO context.
  AVIOContext* getAvioContext() const { return myAvioCtx; }

public:

  //! Virtual method for reading the data.
  virtual int read (uint8_t* theBuf,
                    int theBufSize) {
    if(myFile == NULL) { return -1; }

    int aNbRead = (int )::fread(theBuf, 1, theBufSize, myFile);
    if(aNbRead == 0 && feof(myFile) != 0) { return AVERROR_EOF; }
    return aNbRead;
  }

  //! Virtual method for writing the data.
  virtual int write (uint8_t* theBuf,
                     int theBufSize) {
    if(myFile == NULL) { return -1; }
    return (int )::fwrite(theBuf, 1, theBufSize, myFile);
  }

  //! Virtual method for seeking to new position.
  virtual int64_t seek (int64_t theOffset,
                        int theWhence) {
    if(theWhence == AVSEEK_SIZE || myFile == NULL) { return -1; }
  #ifdef _WIN32
    bool isOk = ::_fseeki64(myFile, theOffset, theWhence) == 0;
  #else
    bool isOk =    ::fseeko(myFile, theOffset, theWhence) == 0;
  #endif
    if(!isOk) { return -1; }
  #ifdef _WIN32
    return ::_ftelli64(myFile);
  #else
    return ::ftello(myFile);
  #endif
  }

private:
  //! Callback for reading the data.
  static int readCallback(void* theOpaque,
                          uint8_t* theBuf,
                          int  theBufSize) {
    return theOpaque != NULL
         ? ((StAVIOContext* )theOpaque)->read(theBuf, theBufSize)
         : 0;
  }

  //! Callback for writing the data.
  static int writeCallback(void* theOpaque,
                           uint8_t* theBuf,
                           int theBufSize) {
    return theOpaque != NULL
         ? ((StAVIOContext* )theOpaque)->write(theBuf, theBufSize)
         : 0;
  }

  //! Callback for seeking to new position.
  static int64_t seekCallback(void*   theOpaque,
                              int64_t theOffset,
                              int     theWhence) {
    return theOpaque != NULL
        ? ((StAVIOContext* )theOpaque)->seek(theOffset, theWhence)
        : -1;
  }

protected:
  AVIOContext* myAvioCtx;
  FILE* myFile;
};