在 IOContext 中的 IStream 中寻找

设置自定义 IO 上下文的 API 调用 avio_alloc_context 接收指向 Seek 函数的指针。如果你正在阅读 IStream，则可以使用以下内容：

/**
 * Seeks to a given position on an IStream.
 * 
 * @param ptr     A pointer to the user-defined IO data structure.
 * @param pos     The position to seek to.
 * @param origin  The relative point (origin) from which the seek is performed.
 *
 * @return  The new position in the IStream.
 */
int64_t FileStreamSeek(void* ptr, int64_t pos, int origin){
    // Your custom IStream
    IStream* stream = reinterpret_cast<IStream*>(ptr);
 
    // Prevent overflows
    LARGE_INTEGER in = { pos };
    ULARGE_INTEGER out = { 0 };

    // Origin is an item from STREAM_SEEK enum.
    //   STREAM_SEEK_SET - relative to beginning of stream.
    //   STREAM_SEEK_CUR - relative to current position in stream.
    //   STREAM_SEEK_END - relative to end of stream.
    if(FAILED(stream->Seek(in, origin, &out)))
         return -1;
 
    // Return the new position
    return out.QuadPart;
}