ffmpeg enumeration decoders, encoders analysis

ffmpeg 枚举decoders, encoders 分析
$ffmpeg -decoders
    调用 print_codecs(0);

$ffmpeg -encoders
    调用 print_codecs(1);

$ffmpeg -codecs

     调用 show_codecs() Document a list of similarities.

下面对 print_codecs ()进行代码分析.
第一步, 获取所有codecs 描述符
    const AVCodecDescriptor **codecs;
    unsigned i, nb_codecs = get_codecs_sorted(&codecs);
    怎样获取codecs Pointer to the descriptor pointer？
    原来codec_descriptors[] is a large array.codec A descriptor is a structure.
static const AVCodecDescriptor codec_descriptors[] = {
    /* video codecs */
    {
        .id        = AV_CODEC_ID_MPEG1VIDEO,
        .type      = AVMEDIA_TYPE_VIDEO,
        .name      = "mpeg1video",
        .long_name = NULL_IF_CONFIG_SMALL("MPEG-1 video"),
        .props     = AV_CODEC_PROP_LOSSY | AV_CODEC_PROP_REORDER,
    },
    {
        .id        = AV_CODEC_ID_MPEG2VIDEO,
        .type      = AVMEDIA_TYPE_VIDEO,
        .name      = "mpeg2video",
        .long_name = NULL_IF_CONFIG_SMALL("MPEG-2 video"),
        .props     = AV_CODEC_PROP_LOSSY | AV_CODEC_PROP_REORDER,
        .profiles  = NULL_IF_CONFIG_SMALL(ff_mpeg2_video_profiles),
    },
    {
        .id        = AV_CODEC_ID_H261,
        .type      = AVMEDIA_TYPE_VIDEO,
        .name      = "h261",
        .long_name = NULL_IF_CONFIG_SMALL("H.261"),
        .props     = AV_CODEC_PROP_LOSSY,
    },
... //忽略, 很多
};

typedef struct AVCodecDescriptor {
    enum AVCodecID     id;
    enum AVMediaType type;
    const char      *name;
    const char *long_name;
    int             props;
    const char *const *mime_types;
    const struct AVProfile *profiles;
} AVCodecDescriptor;

    All this function does is organize the descriptor pointers into an array. 采用了2Sub-scanning principle,
    Got the number the first time, Then pointer memory is allocated,The second pass fills in the addresses.

第二步,由id可以获得对应的codec指针.

codec Lists also define a large array.
static const AVCodec * const codec_list[] = {
    &ff_a64multi_encoder,
    &ff_a64multi5_encoder,
    &ff_alias_pix_encoder,
    &ff_amv_encoder,
    &ff_apng_encoder,
    &ff_asv1_encoder,
    &ff_asv2_encoder,
    &ff_avrp_encoder,
    ....  // 很多,180个encoder, 480个decoder
    }
    每种codec 是一种对象,其对应的类为：

  type = const struct AVCodec {
      const char *name;
      const char *long_name;
      enum AVMediaType type;
      enum AVCodecID id;
      int capabilities;
      const AVRational *supported_framerates;
      const enum AVPixelFormat *pix_fmts;
      const int *supported_samplerates;
      const enum AVSampleFormat *sample_fmts;
      const uint64_t *channel_layouts;
      uint8_t max_lowres;
      const AVClass *priv_class;
      const AVProfile *profiles;
      const char *wrapper_name;
      int priv_data_size;
      struct AVCodec *next;
      int (*update_thread_context)(struct AVCodecContext *, const struct AVCodecContext *);
      const AVCodecDefault *defaults;
      void (*init_static_data)(struct AVCodec *);
      int (*init)(struct AVCodecContext *);
      int (*encode_sub)(struct AVCodecContext *, uint8_t *, int, const struct AVSubtitle *);
      int (*encode2)(struct AVCodecContext *, struct AVPacket *, const struct AVFrame *, int *);
      int (*decode)(struct AVCodecContext *, void *, int *, struct AVPacket *);
      int (*close)(struct AVCodecContext *);
      int (*receive_packet)(struct AVCodecContext *, struct AVPacket *);
      int (*receive_frame)(struct AVCodecContext *, struct AVFrame *);
      void (*flush)(struct AVCodecContext *);
      int caps_internal;
      const char *bsfs;
      const struct AVCodecHWConfigInternal * const *hw_configs;
      const uint32_t *codec_tags;
  }
  when you implement onecodec 时,It may be enough to implement the required few functional interfaces.
  列出codec 列表,Also just enumerate eachcodec,Its capabilities are printed,名称,long name only.
  found after analysis,概念比较简单,It's just a data organization.