avcodec_alloc_context3 是创建编解码器上下文，需要使用 avcodec_free_context释放
需要使用avcodec_free_context 释放
/**
* Allocate an AVCodecContext and set its fields to default values. The
* resulting struct should be freed with avcodec_free_context().
*
* @param codec if non-NULL, allocate private data and initialize defaults
* for the given codec. It is illegal to then call avcodec_open2()
* with a different codec.
* If NULL, then the codec-specific defaults won't be initialized,
* which may result in suboptimal default settings (this is
* important mainly for encoders, e.g. libx264).
*
* @return An AVCodecContext filled with default values or NULL on failure.
*/
AVCodecContext *avcodec_alloc_context3(const AVCodec *codec);
/**
* Free the codec context and everything associated with it and write NULL to
* the provided pointer.
*/
void avcodec_free_context(AVCodecContext **avctx);
avcodec_open2 打开编解码器上下文，需要使用 avcodec_free_context 释放
实际上更加合理是说：avcodec_open2 需要使用avcodec_close释放。avcodec_close函数的说明是：只会释放 avcodecContext 内部的元素，但是不会释放 avcodecContext自己。因此也需要使用 avcodec_free_context释放，avcodec_free_context的内部有用的第一句代码就是调用 avcodec_close函数。
/**
* Initialize the AVCodecContext to use the given AVCodec. Prior to using this
* function the context has to be allocated with avcodec_alloc_context3().
*
* The functions avcodec_find_decoder_by_name(), avcodec_find_encoder_by_name(),
* avcodec_find_decoder() and avcodec_find_encoder() provide an easy way for
* retrieving a codec.
*
* @note Always call this function before using decoding routines (such as
* @ref avcodec_receive_frame()).
*
* @code
* av_dict_set(&opts, "b", "2.5M", 0);
* codec = avcodec_find_decoder(AV_CODEC_ID_H264);
* if (!codec)
* exit(1);
*
* context = avcodec_alloc_context3(codec);
*
* if (avcodec_open2(context, codec, opts) < 0)
* exit(1);
* @endcode
*
* @param avctx The context to initialize.
* @param codec The codec to open this context for. If a non-NULL codec has been
* previously passed to avcodec_alloc_context3() or
* for this context, then this parameter MUST be either NULL or
* equal to the previously passed codec.
* @param options A dictionary filled with AVCodecContext and codec-private options.
* On return this object will be filled with options that were not found.
*
* @return zero on success, a negative value on error
* @see avcodec_alloc_context3(), avcodec_find_decoder(), avcodec_find_encoder(),
* av_dict_set(), av_opt_find().
*/
int avcodec_open2(AVCodecContext *avctx, const AVCodec *codec, AVDictionary **options);
/**
* Close a given AVCodecContext and free all the data associated with it
* (but not the AVCodecContext itself).
*
* Calling this function on an AVCodecContext that hasn't been opened will free
* the codec-specific data allocated in avcodec_alloc_context3() with a non-NULL
* codec. Subsequent calls will do nothing.
*
* @note Do not use this function. Use avcodec_free_context() to destroy a
* codec context (either open or closed). Opening and closing a codec context
* multiple times is not supported anymore -- use multiple codec contexts
* instead.
*/
int avcodec_close(AVCodecContext *avctx);
void avcodec_free_context(AVCodecContext **pavctx)
{
AVCodecContext *avctx = *pavctx;
if (!avctx)
return;
avcodec_close(avctx);
av_freep(&avctx->extradata);
av_freep(&avctx->subtitle_header);
av_freep(&avctx->intra_matrix);
av_freep(&avctx->inter_matrix);
av_freep(&avctx->rc_override);
av_channel_layout_uninit(&avctx->ch_layout);
av_freep(pavctx);
}