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You should carefully read the documentation of avcodec_receive_frame:

Return decoded output data from a decoder.
Parameters
avctx codec context
frame This will be set to a reference-counted video or audio frame (depending on the decoder type) allocated by the decoder. Note that the function will always call av_frame_unref(frame) before doing anything else.

Returns 0: success, a frame was returned
AVERROR(EAGAIN): output is not available in this state - user must try to send new input
AVERROR_EOF: the decoder has been fully flushed, and there will be no more output frames AVERROR(EINVAL): codec not opened, or it is an encoder other negative values: legitimate decoding errors

I have highlighted two key pieces of information:

• First, the call to avcodec_receive_frame will invalidate vFrameOne. If your other thread is in the middle of decoding, your program will crash. You will need to establish a synchronization mechanism between receiving and displaying threads to make sure the receiver is always receiving into a frame only it has access to, and that it only passes full frames to the displaying side. (see next point)
• Second, you should check the return value of avcodec_receive_frame. If you see an AVERROR(EAGAIN) you have not received enough packets to produce a full frame. Only if this function produces 0 can you take the full frame and hand it over to the displaying thread.