FFmpeg源码分析：avcodec_find_decoder()查找解码器

FFmpeg提供两种方式查找解码器，通过codecId查找avcodec_find_decoder()与通过名字查找avcodec_find_decoder_by_name()。同样地，也提供两种方式查找编码器，通过codecId查找avcodec_find_encoder()与通过名字查找avcodec_find_encoder_by_name()。源码位于libavcodec/allcodecs.c中。

查找编解码器过程如下图所示：

一、查找解码器

1、avcodec_find_decoder
通codecId来查找解码器的方法如下：

AVCodec *avcodec_find_decoder(enum AVCodecID id)
{return find_codec(id, av_codec_is_decoder);
}

函数体只有一行代码，就是调用find_codec()来查找，第一个参数为codecId，第二个参数为属于解码器类型，具体方法如下：

static AVCodec *find_codec(enum AVCodecID id, int (*x)(const AVCodec *))
{const AVCodec *p, *experimental = NULL;void *i = 0;id = remap_deprecated_codec_id(id);while ((p = av_codec_iterate(&i))) {if (!x(p))continue;if (p->id == id) {if (p->capabilities & AV_CODEC_CAP_EXPERIMENTAL && !experimental) {experimental = p;} elsereturn (AVCodec*)p;}}return (AVCodec*)experimental;
}

可以看到，是通过while循环调用av_codec_iterate()来迭代遍历，获取AVCodec，然后用id判断是否相等。如果id相等，就返回对应的AVCodec。我们来看下av_codec_iterate()的实现：

static void av_codec_init_static(void)
{for (int i = 0; codec_list[i]; i++) {if (codec_list[i]->init_static_data)codec_list[i]->init_static_data((AVCodec*)codec_list[i]);}
}const AVCodec *av_codec_iterate(void **opaque)
{uintptr_t i = (uintptr_t)*opaque;const AVCodec *c = codec_list[i];ff_thread_once(&av_codec_static_init, av_codec_init_static);if (c)*opaque = (void*)(i + 1);return c;
}

2、avcodec_find_decoder_by_name
通过名字查找解码器的方法如下：

AVCodec *avcodec_find_decoder_by_name(const char *name)
{return find_codec_by_name(name, av_codec_is_decoder);
}

主要调用find_codec_by_name()，第一个参数为解码器名字，第二个参数为属于解码器类型，具体方法如下：

static AVCodec *find_codec_by_name(const char *name, int (*x)(const AVCodec *))
{void *i = 0;const AVCodec *p;if (!name)return NULL;while ((p = av_codec_iterate(&i))) {if (!x(p))continue;if (strcmp(name, p->name) == 0)return (AVCodec*)p;}return NULL;
}

可以看到，是通过while循环调用av_codec_iterate()来迭代遍历，获取AVCodec，然后用名字进行比较。如果匹配成功，就返回对应的AVCodec。

二、查找编码器

1、avcodec_find_encoder
通过codecId查找编码器的方法如下：

AVCodec *avcodec_find_encoder(enum AVCodecID id)
{return find_codec(id, av_codec_is_encoder);
}

2、avcodec_find_encoder_by_name
通过名字查找编码器的方法如下：

AVCodec *avcodec_find_encoder_by_name(const char *name)
{return find_codec_by_name(name, av_codec_is_encoder);
}

三、编解码器列表与id

1、编解码器列表
编解码器列表来自libavcodec/allcodecs.c，有声明全局变量(详情可查看：avcodec注册与介绍)：

extern AVCodec ff_flv_encoder;
extern AVCodec ff_flv_decoder;
extern AVCodec ff_h264_decoder;
extern AVCodec ff_hevc_decoder;
extern AVCodec ff_mpeg4_encoder;
extern AVCodec ff_mpeg4_decoder;
extern AVCodec ff_msmpeg4v1_decoder;
extern AVCodec ff_msmpeg4v2_encoder;
extern AVCodec ff_msmpeg4v2_decoder;
extern AVCodec ff_msmpeg4v3_encoder;
extern AVCodec ff_msmpeg4v3_decoder;
extern AVCodec ff_vp8_decoder;
extern AVCodec ff_vp9_decoder;
extern AVCodec ff_wmv1_encoder;
extern AVCodec ff_wmv1_decoder;
extern AVCodec ff_wmv2_encoder;
extern AVCodec ff_wmv2_decoder;
extern AVCodec ff_wmv3_decoder;/* audio codecs */
extern AVCodec ff_aac_encoder;
extern AVCodec ff_aac_decoder;
extern AVCodec ff_ac3_encoder;
extern AVCodec ff_ac3_decoder;
extern AVCodec ff_amrnb_decoder;
extern AVCodec ff_amrwb_decoder;
extern AVCodec ff_ape_decoder;
extern AVCodec ff_eac3_encoder;
extern AVCodec ff_eac3_decoder;
extern AVCodec ff_flac_encoder;
extern AVCodec ff_flac_decoder;
extern AVCodec ff_mp3_decoder;
extern AVCodec ff_opus_encoder;
extern AVCodec ff_opus_decoder;
extern AVCodec ff_truehd_encoder;
extern AVCodec ff_truehd_decoder;
extern AVCodec ff_vorbis_encoder;
extern AVCodec ff_vorbis_decoder;
extern AVCodec ff_wmav1_encoder;
extern AVCodec ff_wmav1_decoder;
extern AVCodec ff_wmav2_encoder;
extern AVCodec ff_wmav2_decoder;/* PCM codecs */
extern AVCodec ff_pcm_alaw_encoder;
extern AVCodec ff_pcm_alaw_decoder;
extern AVCodec ff_pcm_mulaw_encoder;
extern AVCodec ff_pcm_mulaw_decoder;
extern AVCodec ff_pcm_s8_encoder;
extern AVCodec ff_pcm_s8_decoder;
extern AVCodec ff_pcm_s16le_encoder;
extern AVCodec ff_pcm_s16le_decoder;
extern AVCodec ff_pcm_s16le_planar_encoder;
extern AVCodec ff_pcm_s16le_planar_decoder;
extern AVCodec ff_pcm_s24le_encoder;
extern AVCodec ff_pcm_s24le_decoder;
extern AVCodec ff_pcm_s32le_encoder;
extern AVCodec ff_pcm_s32le_decoder;
extern AVCodec ff_pcm_s64le_encoder;
extern AVCodec ff_pcm_s64le_decoder;
extern AVCodec ff_pcm_u8_encoder;
extern AVCodec ff_pcm_u8_decoder;/* ADPCM codecs */
extern AVCodec ff_adpcm_ima_wav_encoder;
extern AVCodec ff_adpcm_ima_wav_decoder;
extern AVCodec ff_adpcm_ms_encoder;
extern AVCodec ff_adpcm_ms_decoder;/* subtitles */
extern AVCodec ff_ssa_encoder;
extern AVCodec ff_ssa_decoder;
extern AVCodec ff_ass_encoder;
extern AVCodec ff_ass_decoder;
extern AVCodec ff_movtext_encoder;
extern AVCodec ff_movtext_decoder;
extern AVCodec ff_pgssub_decoder;
extern AVCodec ff_sami_decoder;
extern AVCodec ff_srt_encoder;
extern AVCodec ff_srt_decoder;
extern AVCodec ff_subrip_encoder;
extern AVCodec ff_subrip_decoder;
extern AVCodec ff_ttml_encoder;
extern AVCodec ff_webvtt_encoder;
extern AVCodec ff_webvtt_decoder;/* external libraries */
extern AVCodec ff_aac_at_encoder;
extern AVCodec ff_aac_at_decoder;
extern AVCodec ff_ac3_at_decoder;
extern AVCodec ff_amr_nb_at_decoder;
extern AVCodec ff_eac3_at_decoder;
extern AVCodec ff_mp3_at_decoder;
extern AVCodec ff_libaom_av1_encoder;
extern AVCodec ff_libdav1d_decoder;
extern AVCodec ff_libfdk_aac_encoder;
extern AVCodec ff_libfdk_aac_decoder;
extern AVCodec ff_libmp3lame_encoder;
extern AVCodec ff_libopencore_amrnb_decoder;
extern AVCodec ff_libopencore_amrwb_decoder;
extern AVCodec ff_libopus_encoder;
extern AVCodec ff_libopus_decoder;
extern AVCodec ff_libshine_encoder;
extern AVCodec ff_libspeex_encoder;
extern AVCodec ff_libspeex_decoder;
extern AVCodec ff_libvorbis_encoder;
extern AVCodec ff_libvorbis_decoder;
extern AVCodec ff_libvpx_vp8_encoder;
extern AVCodec ff_libvpx_vp8_decoder;
extern AVCodec ff_libvpx_vp9_encoder;
extern AVCodec ff_libvpx_vp9_decoder;
extern AVCodec ff_libwebp_encoder;
extern AVCodec ff_libx264_encoder;
extern AVCodec ff_libx265_encoder;/* hwaccel hooks only, so prefer external decoders */
extern AVCodec ff_av1_decoder;
extern AVCodec ff_av1_cuvid_decoder;
extern AVCodec ff_av1_qsv_decoder;
extern AVCodec ff_libopenh264_encoder;
extern AVCodec ff_libopenh264_decoder;
extern AVCodec ff_h264_cuvid_decoder;
extern AVCodec ff_h264_nvenc_encoder;
extern AVCodec ff_h264_omx_encoder;
extern AVCodec ff_h264_qsv_encoder;
extern AVCodec ff_h264_v4l2m2m_encoder;
extern AVCodec ff_h264_vaapi_encoder;
extern AVCodec ff_h264_videotoolbox_encoder;
extern AVCodec ff_hevc_mediacodec_decoder;
extern AVCodec ff_hevc_nvenc_encoder;
extern AVCodec ff_hevc_qsv_encoder;
extern AVCodec ff_hevc_v4l2m2m_encoder;
extern AVCodec ff_hevc_vaapi_encoder;
extern AVCodec ff_hevc_videotoolbox_encoder;
extern AVCodec ff_mp3_mf_encoder;
extern AVCodec ff_mpeg4_cuvid_decoder;
extern AVCodec ff_mpeg4_mediacodec_decoder;
extern AVCodec ff_mpeg4_omx_encoder;
extern AVCodec ff_mpeg4_v4l2m2m_encoder;
extern AVCodec ff_vp9_cuvid_decoder;
extern AVCodec ff_vp9_mediacodec_decoder;
extern AVCodec ff_vp9_qsv_decoder;
extern AVCodec ff_vp9_vaapi_encoder;
extern AVCodec ff_vp9_qsv_encoder;

2、编解码器id
编解码器id是个枚举类型AVCodecId，定义位于libavcodec/codec_id.h(有删减)：

enum AVCodecID {AV_CODEC_ID_NONE,/* video codecs */AV_CODEC_ID_MPEG1VIDEO,AV_CODEC_ID_MPEG2VIDEO, ///< preferred ID for MPEG-1/2 video decodingAV_CODEC_ID_MPEG4,AV_CODEC_ID_MSMPEG4V1,AV_CODEC_ID_MSMPEG4V2,AV_CODEC_ID_MSMPEG4V3,AV_CODEC_ID_WMV1,AV_CODEC_ID_WMV2,AV_CODEC_ID_FLV1,AV_CODEC_ID_H264,AV_CODEC_ID_PNG,AV_CODEC_ID_VC1,AV_CODEC_ID_WMV3,AV_CODEC_ID_AVS,AV_CODEC_ID_JPEG2000,AV_CODEC_ID_GIF,AV_CODEC_ID_VP8,AV_CODEC_ID_VP9,AV_CODEC_ID_HEVC,
#define AV_CODEC_ID_H265 AV_CODEC_ID_HEVCAV_CODEC_ID_VVC,
#define AV_CODEC_ID_H266 AV_CODEC_ID_VVCAV_CODEC_ID_AV1,/* various PCM "codecs" */AV_CODEC_ID_PCM_S16LE = 0x10000,AV_CODEC_ID_PCM_S16BE,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_S24LE,AV_CODEC_ID_PCM_S24DAUD,AV_CODEC_ID_PCM_S16LE_PLANAR,AV_CODEC_ID_PCM_F32LE,AV_CODEC_ID_PCM_F64LE,AV_CODEC_ID_PCM_S8_PLANAR,AV_CODEC_ID_PCM_S24LE_PLANAR,AV_CODEC_ID_PCM_S32LE_PLANAR,AV_CODEC_ID_PCM_S64LE = 0x10800,AV_CODEC_ID_PCM_F16LE,AV_CODEC_ID_PCM_F24LE,/* various ADPCM codecs */AV_CODEC_ID_ADPCM_IMA_QT = 0x11000,AV_CODEC_ID_ADPCM_IMA_WAV,AV_CODEC_ID_ADPCM_MS,AV_CODEC_ID_ADPCM_AFC = 0x11800,AV_CODEC_ID_ADPCM_IMA_OKI,/* AMR */AV_CODEC_ID_AMR_NB = 0x12000,AV_CODEC_ID_AMR_WB,/* audio codecs */AV_CODEC_ID_MP2 = 0x15000,AV_CODEC_ID_MP3, ///< preferred ID for decoding MPEG audio layer 1, 2 or 3AV_CODEC_ID_AAC,AV_CODEC_ID_AC3,AV_CODEC_ID_VORBIS,AV_CODEC_ID_WMAV1,AV_CODEC_ID_WMAV2,AV_CODEC_ID_FLAC,AV_CODEC_ID_APE,AV_CODEC_ID_EAC3,AV_CODEC_ID_TRUEHD,AV_CODEC_ID_OPUS,/* subtitle codecs */AV_CODEC_ID_DVD_SUBTITLE = 0x17000,AV_CODEC_ID_DVB_SUBTITLE,AV_CODEC_ID_SSA,AV_CODEC_ID_MOV_TEXT,AV_CODEC_ID_SRT,AV_CODEC_ID_MICRODVD   = 0x17800,AV_CODEC_ID_SAMI,AV_CODEC_ID_SUBRIP,AV_CODEC_ID_WEBVTT,AV_CODEC_ID_ASS,AV_CODEC_ID_TTML,AV_CODEC_ID_TTF = 0x18000,
};