AV系列结构体之AVCodec、AVCodecParameters、AVCodecParser、AVCodecParserContext、AVCodecDescriptor

AVCodec： 编解码器结构体
位于libavcodec/avcodec.h中

typedef struct AVCodec {
    const char *name; // codec的名字，保持全局唯一，标识名
    const char *long_name; // codec的名字，全名
    enum AVMediaType type; // Media类型，是视频，音频，还是字幕
    enum AVCodecID id;
    int capabilities; //  codec的容量，参考 AV_CODEC_CAP_*
    const AVRational *supported_framerates; //支持的帧率,如果是null，返回是{0，0}
    const enum AVPixelFormat *pix_fmts;  //支持的像素格式,如果是null或unknown，返回-1
    const int *supported_samplerates;     //支持的采样率，如果是null或unknown，返回0  
    const enum AVSampleFormat *sample_fmts;  //支持的采样率，如果是null，返回-1 
    const uint64_t *channel_layouts;     //支持的声道数， 如果是null，返回0 
    uint8_t max_lowres;                  //解码器支持的最大lowres  
    const AVClass *priv_class;    //定义AVClass 成员变量   
    const AVProfile *profiles;    //定义AVProfile 成员变量          
    /*****************************************************************
     * No fields below this line are part of the public API. They
     * may not be used outside of libavcodec and can be changed and
     * removed at will.
     * New public fields should be added right above.
     *****************************************************************
     */
    int priv_data_size;
    struct AVCodec *next;
    /**
     * @name Frame-level threading support functions
     * @{
     */
    /**
     * If defined, called on thread contexts when they are created.
     * If the codec allocates writable tables in init(), re-allocate them here.
     * priv_data will be set to a copy of the original.
     */
    int (*init_thread_copy)(AVCodecContext *);
    /**
     * Copy necessary context variables from a previous thread context to the current one.
     * If not defined, the next thread will start automatically; otherwise, the codec
     * must call ff_thread_finish_setup().
     *
     * dst and src will (rarely) point to the same context, in which case memcpy should be skipped.
     */
    int (*update_thread_context)(AVCodecContext *dst, const AVCodecContext *src);
    /** @} */
    /**
     * Private codec-specific defaults.
     */
    const AVCodecDefault *defaults;
    /**
     * Initialize codec static data, called from avcodec_register().
     */
    void (*init_static_data)(struct AVCodec *codec);
    int (*init)(AVCodecContext *);
    int (*encode_sub)(AVCodecContext *, uint8_t *buf, int buf_size,
                      const struct AVSubtitle *sub);
    /**
     * Encode data to an AVPacket.
     *
     * @param      avctx          codec context
     * @param      avpkt          output AVPacket (may contain a user-provided buffer)
     * @param[in]  frame          AVFrame containing the raw data to be encoded
     * @param[out] got_packet_ptr encoder sets to 0 or 1 to indicate that a
     *                            non-empty packet was returned in avpkt.
     * @return 0 on success, negative error code on failure
     */
    int (*encode2)(AVCodecContext *avctx, AVPacket *avpkt, const AVFrame *frame,
                   int *got_packet_ptr);
    int (*decode)(AVCodecContext *, void *outdata, int *outdata_size, AVPacket *avpkt);
    int (*close)(AVCodecContext *);
    /**
     * Decode/encode API with decoupled packet/frame dataflow. The API is the
     * same as the avcodec_ prefixed APIs (avcodec_send_frame() etc.), except
     * that:
     * - never called if the codec is closed or the wrong type,
     * - AVPacket parameter change side data is applied right before calling
     *   AVCodec->send_packet,
     * - if AV_CODEC_CAP_DELAY is not set, drain packets or frames are never sent,
     * - only one drain packet is ever passed down (until the next flush()),
     * - a drain AVPacket is always NULL (no need to check for avpkt->size).
     */
    int (*send_frame)(AVCodecContext *avctx, const AVFrame *frame);
    int (*send_packet)(AVCodecContext *avctx, const AVPacket *avpkt);
    int (*receive_frame)(AVCodecContext *avctx, AVFrame *frame);
    int (*receive_packet)(AVCodecContext *avctx, AVPacket *avpkt);
    /**
     * Flush buffers.
     * Will be called when seeking
     */
    void (*flush)(AVCodecContext *);
    /**
     * Internal codec capabilities.
     * See FF_CODEC_CAP_* in internal.h
     */
    int caps_internal;
} AVCodec;

AVCodecParameters:描述一个解码后的流的属性
sizeof(AVCodecParameters),并不是public的api, 这个结构必须通过avcodec_parameters_alloc()分配空间，通过avcodec_parameters_free()释放空间。

typedef struct AVCodecParameters {
    /**
     * General type of the encoded data.
     */
    enum AVMediaType codec_type;
    /**
     * Specific type of the encoded data (the codec used).
     */
    enum AVCodecID   codec_id;
    /**
     * Additional information about the codec (corresponds to the AVI FOURCC).
     */
    uint32_t         codec_tag;
    /**
     * Extra binary data needed for initializing the decoder, codec-dependent.
     *
     * Must be allocated with av_malloc() and will be freed by
     * avcodec_parameters_free(). The allocated size of extradata must be at
     * least extradata_size + AV_INPUT_BUFFER_PADDING_SIZE, with the padding
     * bytes zeroed.
     */
    uint8_t *extradata;
    /**
     * Size of the extradata content in bytes.
     */
    int      extradata_size;
    /**
     * - video: the pixel format, the value corresponds to enum AVPixelFormat.
     * - audio: the sample format, the value corresponds to enum AVSampleFormat.
     */
    int format;
    /**
     * The average bitrate of the encoded data (in bits per second).
     */
    int64_t bit_rate;
    /**
     * The number of bits per sample in the codedwords.
     *
     * This is basically the bitrate per sample. It is mandatory for a bunch of
     * formats to actually decode them. It's the number of bits for one sample in
     * the actual coded bitstream.
     *
     * This could be for example 4 for ADPCM
     * For PCM formats this matches bits_per_raw_sample
     * Can be 0
     */
    int bits_per_coded_sample;
    /**
     * This is the number of valid bits in each output sample. If the
     * sample format has more bits, the least significant bits are additional
     * padding bits, which are always 0. Use right shifts to reduce the sample
     * to its actual size. For example, audio formats with 24 bit samples will
     * have bits_per_raw_sample set to 24, and format set to AV_SAMPLE_FMT_S32.
     * To get the original sample use "(int32_t)sample >> 8"."
     *
     * For ADPCM this might be 12 or 16 or similar
     * Can be 0
     */
    int bits_per_raw_sample;
    /**
     * Codec-specific bitstream restrictions that the stream conforms to.
     */
    int profile;
    int level;
    /**
     * Video only. The dimensions of the video frame in pixels.
     */
    int width;
    int height;
    /**
     * Video only. The aspect ratio (width / height) which a single pixel
     * should have when displayed.
     *
     * When the aspect ratio is unknown / undefined, the numerator should be
     * set to 0 (the denominator may have any value).
     */
    AVRational sample_aspect_ratio;
    /**
     * Video only. The order of the fields in interlaced video.
     */
    enum AVFieldOrder                  field_order;
    /**
     * Video only. Additional colorspace characteristics.
     */
    enum AVColorRange                  color_range;
    enum AVColorPrimaries              color_primaries;
    enum AVColorTransferCharacteristic color_trc;
    enum AVColorSpace                  color_space;
    enum AVChromaLocation              chroma_location;
    /**
     * Video only. Number of delayed frames.
     */
    int video_delay;
    /**
     * Audio only. The channel layout bitmask. May be 0 if the channel layout is
     * unknown or unspecified, otherwise the number of bits set must be equal to
     * the channels field.
     */
    uint64_t channel_layout;
    /**
     * Audio only. The number of audio channels.
     */
    int      channels;
    /**
     * Audio only. The number of audio samples per second.
     */
    int      sample_rate;
    /**
     * Audio only. The number of bytes per coded audio frame, required by some
     * formats.
     *
     * Corresponds to nBlockAlign in WAVEFORMATEX.
     */
    int      block_align;
    /**
     * Audio only. Audio frame size, if known. Required by some formats to be static.
     */
    int      frame_size;
    /**
     * Audio only. The amount of padding (in samples) inserted by the encoder at
     * the beginning of the audio. I.e. this number of leading decoded samples
     * must be discarded by the caller to get the original audio without leading
     * padding.
     */
    int initial_padding;
    /**
     * Audio only. The amount of padding (in samples) appended by the encoder to
     * the end of the audio. I.e. this number of decoded samples must be
     * discarded by the caller from the end of the stream to get the original
     * audio without any trailing padding.
     */
    int trailing_padding;
    /**
     * Audio only. Number of samples to skip after a discontinuity.
     */
    int seek_preroll;
} AVCodecParameters;

AVCodecParser

typedef struct AVCodecParser {
    int codec_ids[5]; /* several codec IDs are permitted */
    int priv_data_size;
    int (*parser_init)(AVCodecParserContext *s);
    /* This callback never returns an error, a negative value means that
     * the frame start was in a previous packet. */
    int (*parser_parse)(AVCodecParserContext *s,
                        AVCodecContext *avctx,
                        const uint8_t **poutbuf, int *poutbuf_size,
                        const uint8_t *buf, int buf_size);
    void (*parser_close)(AVCodecParserContext *s);
    int (*split)(AVCodecContext *avctx, const uint8_t *buf, int buf_size);
    struct AVCodecParser *next;
} AVCodecParser;

AVCodecParserContext ：Frame parsing

typedef struct AVCodecParserContext {
    void *priv_data;
    struct AVCodecParser *parser;
    int64_t frame_offset; /* offset of the current frame */
    int64_t cur_offset; /* current offset
                           (incremented by each av_parser_parse()) */
    int64_t next_frame_offset; /* offset of the next frame */
    /* video info */
    int pict_type; /* XXX: Put it back in AVCodecContext. */
    /**
     * This field is used for proper frame duration computation in lavf.
     * It signals, how much longer the frame duration of the current frame
     * is compared to normal frame duration.
     *
     * frame_duration = (1 + repeat_pict) * time_base
     *
     * It is used by codecs like H.264 to display telecined material.
     */
    int repeat_pict; /* XXX: Put it back in AVCodecContext. */
    int64_t pts;     /* pts of the current frame */
    int64_t dts;     /* dts of the current frame */
    /* private data */
    int64_t last_pts;
    int64_t last_dts;
    int fetch_timestamp;
#define AV_PARSER_PTS_NB 4
    int cur_frame_start_index;
    int64_t cur_frame_offset[AV_PARSER_PTS_NB];
    int64_t cur_frame_pts[AV_PARSER_PTS_NB];
    int64_t cur_frame_dts[AV_PARSER_PTS_NB];
    int flags;
#define PARSER_FLAG_COMPLETE_FRAMES           0x0001
#define PARSER_FLAG_ONCE                      0x0002
/// Set if the parser has a valid file offset
#define PARSER_FLAG_FETCHED_OFFSET            0x0004
#define PARSER_FLAG_USE_CODEC_TS              0x1000
    int64_t offset;      ///< byte offset from starting packet start
    int64_t cur_frame_end[AV_PARSER_PTS_NB];
    /**
     * Set by parser to 1 for key frames and 0 for non-key frames.
     * It is initialized to -1, so if the parser doesn't set this flag,
     * old-style fallback using AV_PICTURE_TYPE_I picture type as key frames
     * will be used.
     */
    int key_frame;
#if FF_API_CONVERGENCE_DURATION
    /**
     * @deprecated unused
     */
    attribute_deprecated
    int64_t convergence_duration;
#endif
    // Timestamp generation support:
    /**
     * Synchronization point for start of timestamp generation.
     *
     * Set to >0 for sync point, 0 for no sync point and <0 for undefined
     * (default).
     *
     * For example, this corresponds to presence of H.264 buffering period
     * SEI message.
     */
    int dts_sync_point;
    /**
     * Offset of the current timestamp against last timestamp sync point in
     * units of AVCodecContext.time_base.
     *
     * Set to INT_MIN when dts_sync_point unused. Otherwise, it must
     * contain a valid timestamp offset.
     *
     * Note that the timestamp of sync point has usually a nonzero
     * dts_ref_dts_delta, which refers to the previous sync point. Offset of
     * the next frame after timestamp sync point will be usually 1.
     *
     * For example, this corresponds to H.264 cpb_removal_delay.
     */
    int dts_ref_dts_delta;
    /**
     * Presentation delay of current frame in units of AVCodecContext.time_base.
     *
     * Set to INT_MIN when dts_sync_point unused. Otherwise, it must
     * contain valid non-negative timestamp delta (presentation time of a frame
     * must not lie in the past).
     *
     * This delay represents the difference between decoding and presentation
     * time of the frame.
     *
     * For example, this corresponds to H.264 dpb_output_delay.
     */
    int pts_dts_delta;
    /**
     * Position of the packet in file.
     *
     * Analogous to cur_frame_pts/dts
     */
    int64_t cur_frame_pos[AV_PARSER_PTS_NB];
    /**
     * Byte position of currently parsed frame in stream.
     */
    int64_t pos;
    /**
     * Previous frame byte position.
     */
    int64_t last_pos;
    /**
     * Duration of the current frame.
     * For audio, this is in units of 1 / AVCodecContext.sample_rate.
     * For all other types, this is in units of AVCodecContext.time_base.
     */
    int duration;
    enum AVFieldOrder field_order;
    /**
     * Indicate whether a picture is coded as a frame, top field or bottom field.
     *
     * For example, H.264 field_pic_flag equal to 0 corresponds to
     * AV_PICTURE_STRUCTURE_FRAME. An H.264 picture with field_pic_flag
     * equal to 1 and bottom_field_flag equal to 0 corresponds to
     * AV_PICTURE_STRUCTURE_TOP_FIELD.
     */
    enum AVPictureStructure picture_structure;
    /**
     * Picture number incremented in presentation or output order.
     * This field may be reinitialized at the first picture of a new sequence.
     *
     * For example, this corresponds to H.264 PicOrderCnt.
     */
    int output_picture_number;
    /**
     * Dimensions of the decoded video intended for presentation.
     */
    int width;
    int height;
    /**
     * Dimensions of the coded video.
     */
    int coded_width;
    int coded_height;
    /**
     * The format of the coded data, corresponds to enum AVPixelFormat for video
     * and for enum AVSampleFormat for audio.
     *
     * Note that a decoder can have considerable freedom in how exactly it
     * decodes the data, so the format reported here might be different from the
     * one returned by a decoder.
     */
    int format;
} AVCodecParserContext;

AVCodecDescriptor:描述单个解码器的属性，单个解码器被描述是通过AVCodecID

typedef struct AVCodecDescriptor {
    enum AVCodecID     id;
    enum AVMediaType type;
    const char      *name;
    const char *long_name;
    /**
     * Codec properties, a combination of AV_CODEC_PROP_* flags.
     */
    int             props;
    /**
     * MIME type(s) associated with the codec.
     * May be NULL; if not, a NULL-terminated array of MIME types.
     * The first item is always non-NULL and is the preferred MIME type.
     */
    const char *const *mime_types;
    /**
     * If non-NULL, an array of profiles recognized for this codec.
     * Terminated with FF_PROFILE_UNKNOWN.
     */
    const struct AVProfile *profiles;
} AVCodecDescriptor;

AVCodecID:枚举出FFmpeg所有解码器id

enum AVCodecID {
    AV_CODEC_ID_NONE,
    /* video codecs */
    AV_CODEC_ID_MPEG1VIDEO,
    AV_CODEC_ID_MPEG2VIDEO, ///< preferred ID for MPEG-1/2 video decoding
#if FF_API_XVMC
    AV_CODEC_ID_MPEG2VIDEO_XVMC,
#endif /* FF_API_XVMC */
    AV_CODEC_ID_H261,
    AV_CODEC_ID_H263,
    AV_CODEC_ID_RV10,
    AV_CODEC_ID_RV20,
    AV_CODEC_ID_MJPEG,
    AV_CODEC_ID_MJPEGB,
    AV_CODEC_ID_LJPEG,
    AV_CODEC_ID_SP5X,
    AV_CODEC_ID_JPEGLS,
    AV_CODEC_ID_MPEG4,
    AV_CODEC_ID_RAWVIDEO,
    AV_CODEC_ID_MSMPEG4V1,
    AV_CODEC_ID_MSMPEG4V2,
    AV_CODEC_ID_MSMPEG4V3,
    AV_CODEC_ID_WMV1,
    AV_CODEC_ID_WMV2,
    AV_CODEC_ID_H263P,
    AV_CODEC_ID_H263I,
    AV_CODEC_ID_FLV1,
    AV_CODEC_ID_SVQ1,
    AV_CODEC_ID_SVQ3,
    AV_CODEC_ID_DVVIDEO,
    AV_CODEC_ID_HUFFYUV,
    AV_CODEC_ID_CYUV,
    AV_CODEC_ID_H264,
    AV_CODEC_ID_INDEO3,
    AV_CODEC_ID_VP3,
    AV_CODEC_ID_THEORA,
    AV_CODEC_ID_ASV1,
    AV_CODEC_ID_ASV2,
    AV_CODEC_ID_FFV1,
    AV_CODEC_ID_4XM,
    AV_CODEC_ID_VCR1,
    AV_CODEC_ID_CLJR,
    AV_CODEC_ID_MDEC,
    AV_CODEC_ID_ROQ,
    AV_CODEC_ID_INTERPLAY_VIDEO,
    AV_CODEC_ID_XAN_WC3,
    AV_CODEC_ID_XAN_WC4,
    AV_CODEC_ID_RPZA,
    AV_CODEC_ID_CINEPAK,
    AV_CODEC_ID_WS_VQA,
    AV_CODEC_ID_MSRLE,
    AV_CODEC_ID_MSVIDEO1,
    AV_CODEC_ID_IDCIN,
    AV_CODEC_ID_8BPS,
    AV_CODEC_ID_SMC,
    AV_CODEC_ID_FLIC,
    AV_CODEC_ID_TRUEMOTION1,
    AV_CODEC_ID_VMDVIDEO,
    AV_CODEC_ID_MSZH,
    AV_CODEC_ID_ZLIB,
    AV_CODEC_ID_QTRLE,
    AV_CODEC_ID_TSCC,
    AV_CODEC_ID_ULTI,
    AV_CODEC_ID_QDRAW,
    AV_CODEC_ID_VIXL,
    AV_CODEC_ID_QPEG,
    AV_CODEC_ID_PNG,
    AV_CODEC_ID_PPM,
    AV_CODEC_ID_PBM,
    AV_CODEC_ID_PGM,
    AV_CODEC_ID_PGMYUV,
    AV_CODEC_ID_PAM,
    AV_CODEC_ID_FFVHUFF,
    AV_CODEC_ID_RV30,
    AV_CODEC_ID_RV40,
    AV_CODEC_ID_VC1,
    AV_CODEC_ID_WMV3,
    AV_CODEC_ID_LOCO,
    AV_CODEC_ID_WNV1,
    AV_CODEC_ID_AASC,
    AV_CODEC_ID_INDEO2,
    AV_CODEC_ID_FRAPS,
    AV_CODEC_ID_TRUEMOTION2,
    AV_CODEC_ID_BMP,
    AV_CODEC_ID_CSCD,
    AV_CODEC_ID_MMVIDEO,
    AV_CODEC_ID_ZMBV,
    AV_CODEC_ID_AVS,
    AV_CODEC_ID_SMACKVIDEO,
    AV_CODEC_ID_NUV,
    AV_CODEC_ID_KMVC,
    AV_CODEC_ID_FLASHSV,
    AV_CODEC_ID_CAVS,
    AV_CODEC_ID_JPEG2000,
    AV_CODEC_ID_VMNC,
    AV_CODEC_ID_VP5,
    AV_CODEC_ID_VP6,
    AV_CODEC_ID_VP6F,
    AV_CODEC_ID_TARGA,
    AV_CODEC_ID_DSICINVIDEO,
    AV_CODEC_ID_TIERTEXSEQVIDEO,
    AV_CODEC_ID_TIFF,
    AV_CODEC_ID_GIF,
    AV_CODEC_ID_DXA,
    AV_CODEC_ID_DNXHD,
    AV_CODEC_ID_THP,
    AV_CODEC_ID_SGI,
    AV_CODEC_ID_C93,
    AV_CODEC_ID_BETHSOFTVID,
    AV_CODEC_ID_PTX,
    AV_CODEC_ID_TXD,
    AV_CODEC_ID_VP6A,
    AV_CODEC_ID_AMV,
    AV_CODEC_ID_VB,
    AV_CODEC_ID_PCX,
    AV_CODEC_ID_SUNRAST,
    AV_CODEC_ID_INDEO4,
    AV_CODEC_ID_INDEO5,
    AV_CODEC_ID_MIMIC,
    AV_CODEC_ID_RL2,
    AV_CODEC_ID_ESCAPE124,
    AV_CODEC_ID_DIRAC,
    AV_CODEC_ID_BFI,
    AV_CODEC_ID_CMV,
    AV_CODEC_ID_MOTIONPIXELS,
    AV_CODEC_ID_TGV,
    AV_CODEC_ID_TGQ,
    AV_CODEC_ID_TQI,
    AV_CODEC_ID_AURA,
    AV_CODEC_ID_AURA2,
    AV_CODEC_ID_V210X,
    AV_CODEC_ID_TMV,
    AV_CODEC_ID_V210,
    AV_CODEC_ID_DPX,
    AV_CODEC_ID_MAD,
    AV_CODEC_ID_FRWU,
    AV_CODEC_ID_FLASHSV2,
    AV_CODEC_ID_CDGRAPHICS,
    AV_CODEC_ID_R210,
    AV_CODEC_ID_ANM,
    AV_CODEC_ID_BINKVIDEO,
    AV_CODEC_ID_IFF_ILBM,
#define AV_CODEC_ID_IFF_BYTERUN1 AV_CODEC_ID_IFF_ILBM
    AV_CODEC_ID_KGV1,
    AV_CODEC_ID_YOP,
    AV_CODEC_ID_VP8,
    AV_CODEC_ID_PICTOR,
    AV_CODEC_ID_ANSI,
    AV_CODEC_ID_A64_MULTI,
    AV_CODEC_ID_A64_MULTI5,
    AV_CODEC_ID_R10K,
    AV_CODEC_ID_MXPEG,
    AV_CODEC_ID_LAGARITH,
    AV_CODEC_ID_PRORES,
    AV_CODEC_ID_JV,
    AV_CODEC_ID_DFA,
    AV_CODEC_ID_WMV3IMAGE,
    AV_CODEC_ID_VC1IMAGE,
    AV_CODEC_ID_UTVIDEO,
    AV_CODEC_ID_BMV_VIDEO,
    AV_CODEC_ID_VBLE,
    AV_CODEC_ID_DXTORY,
    AV_CODEC_ID_V410,
    AV_CODEC_ID_XWD,
    AV_CODEC_ID_CDXL,
    AV_CODEC_ID_XBM,
    AV_CODEC_ID_ZEROCODEC,
    AV_CODEC_ID_MSS1,
    AV_CODEC_ID_MSA1,
    AV_CODEC_ID_TSCC2,
    AV_CODEC_ID_MTS2,
    AV_CODEC_ID_CLLC,
    AV_CODEC_ID_MSS2,
    AV_CODEC_ID_VP9,
    AV_CODEC_ID_AIC,
    AV_CODEC_ID_ESCAPE130,
    AV_CODEC_ID_G2M,
    AV_CODEC_ID_WEBP,
    AV_CODEC_ID_HNM4_VIDEO,
    AV_CODEC_ID_HEVC,
#define AV_CODEC_ID_H265 AV_CODEC_ID_HEVC
    AV_CODEC_ID_FIC,
    AV_CODEC_ID_ALIAS_PIX,
    AV_CODEC_ID_BRENDER_PIX,
    AV_CODEC_ID_PAF_VIDEO,
    AV_CODEC_ID_EXR,
    AV_CODEC_ID_VP7,
    AV_CODEC_ID_SANM,
    AV_CODEC_ID_SGIRLE,
    AV_CODEC_ID_MVC1,
    AV_CODEC_ID_MVC2,
    AV_CODEC_ID_HQX,
    AV_CODEC_ID_TDSC,
    AV_CODEC_ID_HQ_HQA,
    AV_CODEC_ID_HAP,
    AV_CODEC_ID_DDS,
    AV_CODEC_ID_DXV,
    AV_CODEC_ID_SCREENPRESSO,
    AV_CODEC_ID_RSCC,
    AV_CODEC_ID_Y41P = 0x8000,
    AV_CODEC_ID_AVRP,
    AV_CODEC_ID_012V,
    AV_CODEC_ID_AVUI,
    AV_CODEC_ID_AYUV,
    AV_CODEC_ID_TARGA_Y216,
    AV_CODEC_ID_V308,
    AV_CODEC_ID_V408,
    AV_CODEC_ID_YUV4,
    AV_CODEC_ID_AVRN,
    AV_CODEC_ID_CPIA,
    AV_CODEC_ID_XFACE,
    AV_CODEC_ID_SNOW,
    AV_CODEC_ID_SMVJPEG,
    AV_CODEC_ID_APNG,
    AV_CODEC_ID_DAALA,
    AV_CODEC_ID_CFHD,
    AV_CODEC_ID_TRUEMOTION2RT,
    AV_CODEC_ID_M101,
    AV_CODEC_ID_MAGICYUV,
    AV_CODEC_ID_SHEERVIDEO,
    AV_CODEC_ID_YLC,
    /* various PCM "codecs" */
    AV_CODEC_ID_FIRST_AUDIO = 0x10000,     ///< A dummy id pointing at the start of audio codecs
    AV_CODEC_ID_PCM_S16LE = 0x10000,
    AV_CODEC_ID_PCM_S16BE,
    AV_CODEC_ID_PCM_U16LE,
    AV_CODEC_ID_PCM_U16BE,
    AV_CODEC_ID_PCM_S8,
    AV_CODEC_ID_PCM_U8,
    AV_CODEC_ID_PCM_MULAW,
    AV_CODEC_ID_PCM_ALAW,
    AV_CODEC_ID_PCM_S32LE,
    AV_CODEC_ID_PCM_S32BE,
    AV_CODEC_ID_PCM_U32LE,
    AV_CODEC_ID_PCM_U32BE,
    AV_CODEC_ID_PCM_S24LE,
    AV_CODEC_ID_PCM_S24BE,
    AV_CODEC_ID_PCM_U24LE,
    AV_CODEC_ID_PCM_U24BE,
    AV_CODEC_ID_PCM_S24DAUD,
    AV_CODEC_ID_PCM_ZORK,
    AV_CODEC_ID_PCM_S16LE_PLANAR,
    AV_CODEC_ID_PCM_DVD,
    AV_CODEC_ID_PCM_F32BE,
    AV_CODEC_ID_PCM_F32LE,
    AV_CODEC_ID_PCM_F64BE,
    AV_CODEC_ID_PCM_F64LE,
    AV_CODEC_ID_PCM_BLURAY,
    AV_CODEC_ID_PCM_LXF,
    AV_CODEC_ID_S302M,
    AV_CODEC_ID_PCM_S8_PLANAR,
    AV_CODEC_ID_PCM_S24LE_PLANAR,
    AV_CODEC_ID_PCM_S32LE_PLANAR,
    AV_CODEC_ID_PCM_S16BE_PLANAR,
    /* new PCM "codecs" should be added right below this line starting with
     * an explicit value of for example 0x10800
     */
    /* various ADPCM codecs */
    AV_CODEC_ID_ADPCM_IMA_QT = 0x11000,
    AV_CODEC_ID_ADPCM_IMA_WAV,
    AV_CODEC_ID_ADPCM_IMA_DK3,
    AV_CODEC_ID_ADPCM_IMA_DK4,
    AV_CODEC_ID_ADPCM_IMA_WS,
    AV_CODEC_ID_ADPCM_IMA_SMJPEG,
    AV_CODEC_ID_ADPCM_MS,
    AV_CODEC_ID_ADPCM_4XM,
    AV_CODEC_ID_ADPCM_XA,
    AV_CODEC_ID_ADPCM_ADX,
    AV_CODEC_ID_ADPCM_EA,
    AV_CODEC_ID_ADPCM_G726,
    AV_CODEC_ID_ADPCM_CT,
    AV_CODEC_ID_ADPCM_SWF,
    AV_CODEC_ID_ADPCM_YAMAHA,
    AV_CODEC_ID_ADPCM_SBPRO_4,
    AV_CODEC_ID_ADPCM_SBPRO_3,
    AV_CODEC_ID_ADPCM_SBPRO_2,
    AV_CODEC_ID_ADPCM_THP,
    AV_CODEC_ID_ADPCM_IMA_AMV,
    AV_CODEC_ID_ADPCM_EA_R1,
    AV_CODEC_ID_ADPCM_EA_R3,
    AV_CODEC_ID_ADPCM_EA_R2,
    AV_CODEC_ID_ADPCM_IMA_EA_SEAD,
    AV_CODEC_ID_ADPCM_IMA_EA_EACS,
    AV_CODEC_ID_ADPCM_EA_XAS,
    AV_CODEC_ID_ADPCM_EA_MAXIS_XA,
    AV_CODEC_ID_ADPCM_IMA_ISS,
    AV_CODEC_ID_ADPCM_G722,
    AV_CODEC_ID_ADPCM_IMA_APC,
    AV_CODEC_ID_ADPCM_VIMA,
#if FF_API_VIMA_DECODER
    AV_CODEC_ID_VIMA = AV_CODEC_ID_ADPCM_VIMA,
#endif
    AV_CODEC_ID_ADPCM_AFC = 0x11800,
    AV_CODEC_ID_ADPCM_IMA_OKI,
    AV_CODEC_ID_ADPCM_DTK,
    AV_CODEC_ID_ADPCM_IMA_RAD,
    AV_CODEC_ID_ADPCM_G726LE,
    AV_CODEC_ID_ADPCM_THP_LE,
    AV_CODEC_ID_ADPCM_PSX,
    AV_CODEC_ID_ADPCM_AICA,
    AV_CODEC_ID_ADPCM_IMA_DAT4,
    AV_CODEC_ID_ADPCM_MTAF,
    /* AMR */
    AV_CODEC_ID_AMR_NB = 0x12000,
    AV_CODEC_ID_AMR_WB,
    /* RealAudio codecs*/
    AV_CODEC_ID_RA_144 = 0x13000,
    AV_CODEC_ID_RA_288,
    /* various DPCM codecs */
    AV_CODEC_ID_ROQ_DPCM = 0x14000,
    AV_CODEC_ID_INTERPLAY_DPCM,
    AV_CODEC_ID_XAN_DPCM,
    AV_CODEC_ID_SOL_DPCM,
    AV_CODEC_ID_SDX2_DPCM = 0x14800,
    /* audio codecs */
    AV_CODEC_ID_MP2 = 0x15000,
    AV_CODEC_ID_MP3, ///< preferred ID for decoding MPEG audio layer 1, 2 or 3
    AV_CODEC_ID_AAC,
    AV_CODEC_ID_AC3,
    AV_CODEC_ID_DTS,
    AV_CODEC_ID_VORBIS,
    AV_CODEC_ID_DVAUDIO,
    AV_CODEC_ID_WMAV1,
    AV_CODEC_ID_WMAV2,
    AV_CODEC_ID_MACE3,
    AV_CODEC_ID_MACE6,
    AV_CODEC_ID_VMDAUDIO,
    AV_CODEC_ID_FLAC,
    AV_CODEC_ID_MP3ADU,
    AV_CODEC_ID_MP3ON4,
    AV_CODEC_ID_SHORTEN,
    AV_CODEC_ID_ALAC,
    AV_CODEC_ID_WESTWOOD_SND1,
    AV_CODEC_ID_GSM, ///< as in Berlin toast format
    AV_CODEC_ID_QDM2,
    AV_CODEC_ID_COOK,
    AV_CODEC_ID_TRUESPEECH,
    AV_CODEC_ID_TTA,
    AV_CODEC_ID_SMACKAUDIO,
    AV_CODEC_ID_QCELP,
    AV_CODEC_ID_WAVPACK,
    AV_CODEC_ID_DSICINAUDIO,
    AV_CODEC_ID_IMC,
    AV_CODEC_ID_MUSEPACK7,
    AV_CODEC_ID_MLP,
    AV_CODEC_ID_GSM_MS, /* as found in WAV */
    AV_CODEC_ID_ATRAC3,
#if FF_API_VOXWARE
    AV_CODEC_ID_VOXWARE,
#endif
    AV_CODEC_ID_APE,
    AV_CODEC_ID_NELLYMOSER,
    AV_CODEC_ID_MUSEPACK8,
    AV_CODEC_ID_SPEEX,
    AV_CODEC_ID_WMAVOICE,
    AV_CODEC_ID_WMAPRO,
    AV_CODEC_ID_WMALOSSLESS,
    AV_CODEC_ID_ATRAC3P,
    AV_CODEC_ID_EAC3,
    AV_CODEC_ID_SIPR,
    AV_CODEC_ID_MP1,
    AV_CODEC_ID_TWINVQ,
    AV_CODEC_ID_TRUEHD,
    AV_CODEC_ID_MP4ALS,
    AV_CODEC_ID_ATRAC1,
    AV_CODEC_ID_BINKAUDIO_RDFT,
    AV_CODEC_ID_BINKAUDIO_DCT,
    AV_CODEC_ID_AAC_LATM,
    AV_CODEC_ID_QDMC,
    AV_CODEC_ID_CELT,
    AV_CODEC_ID_G723_1,
    AV_CODEC_ID_G729,
    AV_CODEC_ID_8SVX_EXP,
    AV_CODEC_ID_8SVX_FIB,
    AV_CODEC_ID_BMV_AUDIO,
    AV_CODEC_ID_RALF,
    AV_CODEC_ID_IAC,
    AV_CODEC_ID_ILBC,
    AV_CODEC_ID_OPUS,
    AV_CODEC_ID_COMFORT_NOISE,
    AV_CODEC_ID_TAK,
    AV_CODEC_ID_METASOUND,
    AV_CODEC_ID_PAF_AUDIO,
    AV_CODEC_ID_ON2AVC,
    AV_CODEC_ID_DSS_SP,
    AV_CODEC_ID_FFWAVESYNTH = 0x15800,
    AV_CODEC_ID_SONIC,
    AV_CODEC_ID_SONIC_LS,
    AV_CODEC_ID_EVRC,
    AV_CODEC_ID_SMV,
    AV_CODEC_ID_DSD_LSBF,
    AV_CODEC_ID_DSD_MSBF,
    AV_CODEC_ID_DSD_LSBF_PLANAR,
    AV_CODEC_ID_DSD_MSBF_PLANAR,
    AV_CODEC_ID_4GV,
    AV_CODEC_ID_INTERPLAY_ACM,
    AV_CODEC_ID_XMA1,
    AV_CODEC_ID_XMA2,
    AV_CODEC_ID_DST,
    /* subtitle codecs */
    AV_CODEC_ID_FIRST_SUBTITLE = 0x17000,          ///< A dummy ID pointing at the start of subtitle codecs.
    AV_CODEC_ID_DVD_SUBTITLE = 0x17000,
    AV_CODEC_ID_DVB_SUBTITLE,
    AV_CODEC_ID_TEXT,  ///< raw UTF-8 text
    AV_CODEC_ID_XSUB,
    AV_CODEC_ID_SSA,
    AV_CODEC_ID_MOV_TEXT,
    AV_CODEC_ID_HDMV_PGS_SUBTITLE,
    AV_CODEC_ID_DVB_TELETEXT,
    AV_CODEC_ID_SRT,
    AV_CODEC_ID_MICRODVD   = 0x17800,
    AV_CODEC_ID_EIA_608,
    AV_CODEC_ID_JACOSUB,
    AV_CODEC_ID_SAMI,
    AV_CODEC_ID_REALTEXT,
    AV_CODEC_ID_STL,
    AV_CODEC_ID_SUBVIEWER1,
    AV_CODEC_ID_SUBVIEWER,
    AV_CODEC_ID_SUBRIP,
    AV_CODEC_ID_WEBVTT,
    AV_CODEC_ID_MPL2,
    AV_CODEC_ID_VPLAYER,
    AV_CODEC_ID_PJS,
    AV_CODEC_ID_ASS,
    AV_CODEC_ID_HDMV_TEXT_SUBTITLE,
    /* other specific kind of codecs (generally used for attachments) */
    AV_CODEC_ID_FIRST_UNKNOWN = 0x18000,           ///< A dummy ID pointing at the start of various fake codecs.
    AV_CODEC_ID_TTF = 0x18000,
    AV_CODEC_ID_BINTEXT    = 0x18800,
    AV_CODEC_ID_XBIN,
    AV_CODEC_ID_IDF,
    AV_CODEC_ID_OTF,
    AV_CODEC_ID_SMPTE_KLV,
    AV_CODEC_ID_DVD_NAV,
    AV_CODEC_ID_TIMED_ID3,
    AV_CODEC_ID_BIN_DATA,
    AV_CODEC_ID_PROBE = 0x19000, ///< codec_id is not known (like AV_CODEC_ID_NONE) but lavf should attempt to identify it
    AV_CODEC_ID_MPEG2TS = 0x20000, /**< _FAKE_ codec to indicate a raw MPEG-2 TS
                                * stream (only used by libavformat) */
    AV_CODEC_ID_MPEG4SYSTEMS = 0x20001, /**< _FAKE_ codec to indicate a MPEG-4 Systems
                                * stream (only used by libavformat) */
    AV_CODEC_ID_FFMETADATA = 0x21000,   ///< Dummy codec for streams containing only metadata information.
    AV_CODEC_ID_WRAPPED_AVFRAME = 0x21001, ///< Passthrough codec, AVFrames wrapped in AVPacket
};

如我们找一个标准的h.264+aac的解码，h.264解码器在h264.c中的声明如下，注意id(AV_CODEC_ID_H264)对应我们上面AVCodcID中的AV_CODEC_ID_H264。

AVCodec ff_h264_decoder = {
    .name                  = "h264",
    .long_name             = NULL_IF_CONFIG_SMALL("H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10"),
    .type                  = AVMEDIA_TYPE_VIDEO,
    .id                    = AV_CODEC_ID_H264,
    .priv_data_size        = sizeof(H264Context),
    .init                  = ff_h264_decode_init,
    .close                 = h264_decode_end,
    .decode                = h264_decode_frame,
    .capabilities          = /*AV_CODEC_CAP_DRAW_HORIZ_BAND |*/ AV_CODEC_CAP_DR1 |
                             AV_CODEC_CAP_DELAY | AV_CODEC_CAP_SLICE_THREADS |
                             AV_CODEC_CAP_FRAME_THREADS,
    .caps_internal         = FF_CODEC_CAP_INIT_THREADSAFE,
    .flush                 = flush_dpb,
    .init_thread_copy      = ONLY_IF_THREADS_ENABLED(decode_init_thread_copy),
    .update_thread_context = ONLY_IF_THREADS_ENABLED(ff_h264_update_thread_context),
    .profiles              = NULL_IF_CONFIG_SMALL(ff_h264_profiles),
    .priv_class            = &h264_class,
};

aac中解码器声明如下，位于aacdec.c文件中,注意id(AV_CODEC_ID_AAC)与上面上面的AVCodcID中的AV_CODEC_ID_AAC。

AVCodec ff_aac_decoder = {
    .name            = "aac",
    .long_name       = NULL_IF_CONFIG_SMALL("AAC (Advanced Audio Coding)"),
    .type            = AVMEDIA_TYPE_AUDIO,
    .id              = AV_CODEC_ID_AAC,
    .priv_data_size  = sizeof(AACContext),
    .init            = aac_decode_init,
    .close           = aac_decode_close,
    .decode          = aac_decode_frame,
    .sample_fmts     = (const enum AVSampleFormat[]) {
        AV_SAMPLE_FMT_FLTP, AV_SAMPLE_FMT_NONE
    },
    .capabilities    = AV_CODEC_CAP_CHANNEL_CONF | AV_CODEC_CAP_DR1,
    .caps_internal   = FF_CODEC_CAP_INIT_THREADSAFE,
    .channel_layouts = aac_channel_layout,
    .flush = flush,
    .priv_class      = &aac_decoder_class,
    .profiles        = NULL_IF_CONFIG_SMALL(ff_aac_profiles),
};

每一个编解码器对应一个结构体AVCodec