ffmpeg学习六：avcodec_open2函数源码分析

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上一节我们尝试分析了avformat_open_input函数的源码，这个函数的虽然比较复杂，但是它基本是围绕着创建和初始化一些数据结构来展开的，比如，avformat_open_input函数会创建和初始化AVFormatContext，AVClass ，AVOption,URLContext,URLProtocol ,AVInputFormat ,AVStream等数据结构，这些数据结构的关系如下：
这里写图片描述
（这里的箭头是包含关系，不是继承关系）

那么，我们可以推测，同样作为Open系列的函数，avcodec_open2的使命也必然是构建和初始化一系列的数据结构，那么是不是这样呢？
avcodec_open2函数定义在libavcodec/aviocodec.h中:

/*** 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.** @warning This function is not thread safe!** @note Always call this function before using decoding routines (such as* @ref avcodec_receive_frame()).** @code* avcodec_register_all();* 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);

从它的注释中我们可以得到如下信息：
1.使用所给的AVCodec结构体构造AVCodecContext结构体。
2.avcodec_find_decoder_by_name(), avcodec_find_encoder_by_name(),
* avcodec_find_decoder() and avcodec_find_encoder()，这几个函数提供了方便获取AVCodecContext实例的途径。
3.这个函数是线程不安全函数
4.在使用解码程序之前，必须调用此函数。

综上，avcodec_open2函数的核心人物是构造AVCodecContext结构体。
avcodec_open2函数在libavcodec/utils.c中实现：

int attribute_align_arg avcodec_open2(AVCodecContext *avctx, const AVCodec *codec, AVDictionary **options)
{int ret = 0;AVDictionary *tmp = NULL;const AVPixFmtDescriptor *pixdesc;if (avcodec_is_open(avctx))return 0;if ((!codec && !avctx->codec)) {av_log(avctx, AV_LOG_ERROR, "No codec provided to avcodec_open2()\n");return AVERROR(EINVAL);}if ((codec && avctx->codec && codec != avctx->codec)) {av_log(avctx, AV_LOG_ERROR, "This AVCodecContext was allocated for %s, ""but %s passed to avcodec_open2()\n", avctx->codec->name, codec->name);return AVERROR(EINVAL);}if (!codec)codec = avctx->codec;if (avctx->extradata_size < 0 || avctx->extradata_size >= FF_MAX_EXTRADATA_SIZE)return AVERROR(EINVAL);if (options)av_dict_copy(&tmp, *options, 0);ret = ff_lock_avcodec(avctx, codec);if (ret < 0)return ret;avctx->internal = av_mallocz(sizeof(AVCodecInternal));if (!avctx->internal) {ret = AVERROR(ENOMEM);goto end;}avctx->internal->pool = av_mallocz(sizeof(*avctx->internal->pool));if (!avctx->internal->pool) {ret = AVERROR(ENOMEM);goto free_and_end;}avctx->internal->to_free = av_frame_alloc();if (!avctx->internal->to_free) {ret = AVERROR(ENOMEM);goto free_and_end;}avctx->internal->buffer_frame = av_frame_alloc();if (!avctx->internal->buffer_frame) {ret = AVERROR(ENOMEM);goto free_and_end;}avctx->internal->buffer_pkt = av_packet_alloc();if (!avctx->internal->buffer_pkt) {ret = AVERROR(ENOMEM);goto free_and_end;}if (codec->priv_data_size > 0) {if (!avctx->priv_data) {avctx->priv_data = av_mallocz(codec->priv_data_size);if (!avctx->priv_data) {ret = AVERROR(ENOMEM);goto end;}if (codec->priv_class) {*(const AVClass **)avctx->priv_data = codec->priv_class;av_opt_set_defaults(avctx->priv_data);}}if (codec->priv_class && (ret = av_opt_set_dict(avctx->priv_data, &tmp)) < 0)goto free_and_end;} else {avctx->priv_data = NULL;}if ((ret = av_opt_set_dict(avctx, &tmp)) < 0)goto free_and_end;if (avctx->codec_whitelist && av_match_list(codec->name, avctx->codec_whitelist, ',') <= 0) {av_log(avctx, AV_LOG_ERROR, "Codec (%s) not on whitelist \'%s\'\n", codec->name, avctx->codec_whitelist);ret = AVERROR(EINVAL);goto free_and_end;}// only call ff_set_dimensions() for non H.264/VP6F/DXV codecs so as not to overwrite previously setup dimensionsif (!(avctx->coded_width && avctx->coded_height && avctx->width && avctx->height &&(avctx->codec_id == AV_CODEC_ID_H264 || avctx->codec_id == AV_CODEC_ID_VP6F || avctx->codec_id == AV_CODEC_ID_DXV))) {if (avctx->coded_width && avctx->coded_height)ret = ff_set_dimensions(avctx, avctx->coded_width, avctx->coded_height);else if (avctx->width && avctx->height)ret = ff_set_dimensions(avctx, avctx->width, avctx->height);if (ret < 0)goto free_and_end;}if ((avctx->coded_width || avctx->coded_height || avctx->width || avctx->height)&& (  av_image_check_size(avctx->coded_width, avctx->coded_height, 0, avctx) < 0|| av_image_check_size(avctx->width,       avctx->height,       0, avctx) < 0)) {av_log(avctx, AV_LOG_WARNING, "Ignoring invalid width/height values\n");ff_set_dimensions(avctx, 0, 0);}if (avctx->width > 0 && avctx->height > 0) {if (av_image_check_sar(avctx->width, avctx->