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

作者：小蓝xlanll | 2024-04-18 14:00:22

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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：

  1. AVCodec *avcodec_find_decoder(enum AVCodecID id)
  2. {
  3.     return find_codec(id, av_codec_is_decoder);
  4. }

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：

  1. static AVCodec *find_codec(enum AVCodecID id, int (*x)(const AVCodec *))
  2. {
  3.     const AVCodec *p, *experimental = NULL;
  4.     void *i = 0;
  5.  
  6.     id = remap_deprecated_codec_id(id);
  7.  
  8.     while ((p = av_codec_iterate(&i))) {
  9.         if (!x(p))
  10.             continue;
  11.         if (p->id == id) {
  12.             if (p->capabilities & AV_CODEC_CAP_EXPERIMENTAL && !experimental) {
  13.                 experimental = p;
  14.             } else
  15.                 return (AVCodec*)p;
  16.         }
  17.     }
  18.  
  19.     return (AVCodec*)experimental;
  20. }

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：

  1. static void av_codec_init_static(void)
  2. {
  3.     for (int i = 0; codec_list[i]; i++) {
  4.         if (codec_list[i]->init_static_data)
  5.             codec_list[i]->init_static_data((AVCodec*)codec_list[i]);
  6.     }
  7. }
  8.  
  9. const AVCodec *av_codec_iterate(void **opaque)
  10. {
  11.     uintptr_t i = (uintptr_t)*opaque;
  12.     const AVCodec *c = codec_list[i];
  13.  
  14.     ff_thread_once(&av_codec_static_init, av_codec_init_static);
  15.  
  16.     if (c)
  17.         *opaque = (void*)(i + 1);
  18.  
  19.     return c;
  20. }

2、avcodec_find_decoder_by_name

通过名字查找解码器的方法如下：

  1. AVCodec *avcodec_find_decoder_by_name(const char *name)
  2. {
  3.     return find_codec_by_name(name, av_codec_is_decoder);
  4. }

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：

  1. static AVCodec *find_codec_by_name(const char *name, int (*x)(const AVCodec *))
  2. {
  3.     void *i = 0;
  4.     const AVCodec *p;
  5.  
  6.     if (!name)
  7.         return NULL;
  8.  
  9.     while ((p = av_codec_iterate(&i))) {
  10.         if (!x(p))
  11.             continue;
  12.         if (strcmp(name, p->name) == 0)
  13.             return (AVCodec*)p;
  14.     }
  15.  
  16.     return NULL;
  17. }

 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：

  1. AVCodec *avcodec_find_encoder(enum AVCodecID id)
  2. {
  3.     return find_codec(id, av_codec_is_encoder);
  4. }

2、avcodec_find_encoder_by_name

通过名字查找编码器的方法如下：

  1. AVCodec *avcodec_find_encoder_by_name(const char *name)
  2. {
  3.     return find_codec_by_name(name, av_codec_is_encoder);
  4. }

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)：

  1. extern AVCodec ff_flv_encoder;
  2. extern AVCodec ff_flv_decoder;
  3. extern AVCodec ff_h264_decoder;
  4. extern AVCodec ff_hevc_decoder;
  5. extern AVCodec ff_mpeg4_encoder;
  6. extern AVCodec ff_mpeg4_decoder;
  7. extern AVCodec ff_msmpeg4v1_decoder;
  8. extern AVCodec ff_msmpeg4v2_encoder;
  9. extern AVCodec ff_msmpeg4v2_decoder;
  10. extern AVCodec ff_msmpeg4v3_encoder;
  11. extern AVCodec ff_msmpeg4v3_decoder;
  12. extern AVCodec ff_vp8_decoder;
  13. extern AVCodec ff_vp9_decoder;
  14. extern AVCodec ff_wmv1_encoder;
  15. extern AVCodec ff_wmv1_decoder;
  16. extern AVCodec ff_wmv2_encoder;
  17. extern AVCodec ff_wmv2_decoder;
  18. extern AVCodec ff_wmv3_decoder;
  19.  
  20. /* audio codecs */
  21. extern AVCodec ff_aac_encoder;
  22. extern AVCodec ff_aac_decoder;
  23. extern AVCodec ff_ac3_encoder;
  24. extern AVCodec ff_ac3_decoder;
  25. extern AVCodec ff_amrnb_decoder;
  26. extern AVCodec ff_amrwb_decoder;
  27. extern AVCodec ff_ape_decoder;
  28. extern AVCodec ff_eac3_encoder;
  29. extern AVCodec ff_eac3_decoder;
  30. extern AVCodec ff_flac_encoder;
  31. extern AVCodec ff_flac_decoder;
  32. extern AVCodec ff_mp3_decoder;
  33. extern AVCodec ff_opus_encoder;
  34. extern AVCodec ff_opus_decoder;
  35. extern AVCodec ff_truehd_encoder;
  36. extern AVCodec ff_truehd_decoder;
  37. extern AVCodec ff_vorbis_encoder;
  38. extern AVCodec ff_vorbis_decoder;
  39. extern AVCodec ff_wmav1_encoder;
  40. extern AVCodec ff_wmav1_decoder;
  41. extern AVCodec ff_wmav2_encoder;
  42. extern AVCodec ff_wmav2_decoder;
  43.  
  44. /* PCM codecs */
  45. extern AVCodec ff_pcm_alaw_encoder;
  46. extern AVCodec ff_pcm_alaw_decoder;
  47. extern AVCodec ff_pcm_mulaw_encoder;
  48. extern AVCodec ff_pcm_mulaw_decoder;
  49. extern AVCodec ff_pcm_s8_encoder;
  50. extern AVCodec ff_pcm_s8_decoder;
  51. extern AVCodec ff_pcm_s16le_encoder;
  52. extern AVCodec ff_pcm_s16le_decoder;
  53. extern AVCodec ff_pcm_s16le_planar_encoder;
  54. extern AVCodec ff_pcm_s16le_planar_decoder;
  55. extern AVCodec ff_pcm_s24le_encoder;
  56. extern AVCodec ff_pcm_s24le_decoder;
  57. extern AVCodec ff_pcm_s32le_encoder;
  58. extern AVCodec ff_pcm_s32le_decoder;
  59. extern AVCodec ff_pcm_s64le_encoder;
  60. extern AVCodec ff_pcm_s64le_decoder;
  61. extern AVCodec ff_pcm_u8_encoder;
  62. extern AVCodec ff_pcm_u8_decoder;
  63.  
  64. /* ADPCM codecs */
  65. extern AVCodec ff_adpcm_ima_wav_encoder;
  66. extern AVCodec ff_adpcm_ima_wav_decoder;
  67. extern AVCodec ff_adpcm_ms_encoder;
  68. extern AVCodec ff_adpcm_ms_decoder;
  69.  
  70. /* subtitles */
  71. extern AVCodec ff_ssa_encoder;
  72. extern AVCodec ff_ssa_decoder;
  73. extern AVCodec ff_ass_encoder;
  74. extern AVCodec ff_ass_decoder;
  75. extern AVCodec ff_movtext_encoder;
  76. extern AVCodec ff_movtext_decoder;
  77. extern AVCodec ff_pgssub_decoder;
  78. extern AVCodec ff_sami_decoder;
  79. extern AVCodec ff_srt_encoder;
  80. extern AVCodec ff_srt_decoder;
  81. extern AVCodec ff_subrip_encoder;
  82. extern AVCodec ff_subrip_decoder;
  83. extern AVCodec ff_ttml_encoder;
  84. extern AVCodec ff_webvtt_encoder;
  85. extern AVCodec ff_webvtt_decoder;
  86.  
  87. /* external libraries */
  88. extern AVCodec ff_aac_at_encoder;
  89. extern AVCodec ff_aac_at_decoder;
  90. extern AVCodec ff_ac3_at_decoder;
  91. extern AVCodec ff_amr_nb_at_decoder;
  92. extern AVCodec ff_eac3_at_decoder;
  93. extern AVCodec ff_mp3_at_decoder;
  94. extern AVCodec ff_libaom_av1_encoder;
  95. extern AVCodec ff_libdav1d_decoder;
  96. extern AVCodec ff_libfdk_aac_encoder;
  97. extern AVCodec ff_libfdk_aac_decoder;
  98. extern AVCodec ff_libmp3lame_encoder;
  99. extern AVCodec ff_libopencore_amrnb_decoder;
  100. extern AVCodec ff_libopencore_amrwb_decoder;
  101. extern AVCodec ff_libopus_encoder;
  102. extern AVCodec ff_libopus_decoder;
  103. extern AVCodec ff_libshine_encoder;
  104. extern AVCodec ff_libspeex_encoder;
  105. extern AVCodec ff_libspeex_decoder;
  106. extern AVCodec ff_libvorbis_encoder;
  107. extern AVCodec ff_libvorbis_decoder;
  108. extern AVCodec ff_libvpx_vp8_encoder;
  109. extern AVCodec ff_libvpx_vp8_decoder;
  110. extern AVCodec ff_libvpx_vp9_encoder;
  111. extern AVCodec ff_libvpx_vp9_decoder;
  112. extern AVCodec ff_libwebp_encoder;
  113. extern AVCodec ff_libx264_encoder;
  114. extern AVCodec ff_libx265_encoder;
  115.  
  116. /* hwaccel hooks only, so prefer external decoders */
  117. extern AVCodec ff_av1_decoder;
  118. extern AVCodec ff_av1_cuvid_decoder;
  119. extern AVCodec ff_av1_qsv_decoder;
  120. extern AVCodec ff_libopenh264_encoder;
  121. extern AVCodec ff_libopenh264_decoder;
  122. extern AVCodec ff_h264_cuvid_decoder;
  123. extern AVCodec ff_h264_nvenc_encoder;
  124. extern AVCodec ff_h264_omx_encoder;
  125. extern AVCodec ff_h264_qsv_encoder;
  126. extern AVCodec ff_h264_v4l2m2m_encoder;
  127. extern AVCodec ff_h264_vaapi_encoder;
  128. extern AVCodec ff_h264_videotoolbox_encoder;
  129. extern AVCodec ff_hevc_mediacodec_decoder;
  130. extern AVCodec ff_hevc_nvenc_encoder;
  131. extern AVCodec ff_hevc_qsv_encoder;
  132. extern AVCodec ff_hevc_v4l2m2m_encoder;
  133. extern AVCodec ff_hevc_vaapi_encoder;
  134. extern AVCodec ff_hevc_videotoolbox_encoder;
  135. extern AVCodec ff_mp3_mf_encoder;
  136. extern AVCodec ff_mpeg4_cuvid_decoder;
  137. extern AVCodec ff_mpeg4_mediacodec_decoder;
  138. extern AVCodec ff_mpeg4_omx_encoder;
  139. extern AVCodec ff_mpeg4_v4l2m2m_encoder;
  140. extern AVCodec ff_vp9_cuvid_decoder;
  141. extern AVCodec ff_vp9_mediacodec_decoder;
  142. extern AVCodec ff_vp9_qsv_decoder;
  143. extern AVCodec ff_vp9_vaapi_encoder;
  144. 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)：

  1. enum AVCodecID {
  2.     AV_CODEC_ID_NONE,
  3.  
  4.     /* video codecs */
  5.     AV_CODEC_ID_MPEG1VIDEO,
  6.     AV_CODEC_ID_MPEG2VIDEO, ///< preferred ID for MPEG-1/2 video decoding
  7.     AV_CODEC_ID_MPEG4,
  8.     AV_CODEC_ID_MSMPEG4V1,
  9.     AV_CODEC_ID_MSMPEG4V2,
  10.     AV_CODEC_ID_MSMPEG4V3,
  11.     AV_CODEC_ID_WMV1,
  12.     AV_CODEC_ID_WMV2,
  13.     AV_CODEC_ID_FLV1,
  14.     AV_CODEC_ID_H264,
  15.     AV_CODEC_ID_PNG,
  16.     AV_CODEC_ID_VC1,
  17.     AV_CODEC_ID_WMV3,
  18.     AV_CODEC_ID_AVS,
  19.     AV_CODEC_ID_JPEG2000,
  20.     AV_CODEC_ID_GIF,
  21.     AV_CODEC_ID_VP8,
  22.     AV_CODEC_ID_VP9,
  23.     AV_CODEC_ID_HEVC,
  24. #define AV_CODEC_ID_H265 AV_CODEC_ID_HEVC
  25.     AV_CODEC_ID_VVC,
  26. #define AV_CODEC_ID_H266 AV_CODEC_ID_VVC
  27.     AV_CODEC_ID_AV1,
  28.  
  29.     /* various PCM "codecs" */
  30.     AV_CODEC_ID_PCM_S16LE = 0x10000,
  31.     AV_CODEC_ID_PCM_S16BE,
  32.     AV_CODEC_ID_PCM_S8,
  33.     AV_CODEC_ID_PCM_U8,
  34.     AV_CODEC_ID_PCM_MULAW,
  35.     AV_CODEC_ID_PCM_ALAW,
  36.     AV_CODEC_ID_PCM_S32LE,
  37.     AV_CODEC_ID_PCM_S24LE,
  38.     AV_CODEC_ID_PCM_S24DAUD,
  39.     AV_CODEC_ID_PCM_S16LE_PLANAR,
  40.     AV_CODEC_ID_PCM_F32LE,
  41.     AV_CODEC_ID_PCM_F64LE,
  42.     AV_CODEC_ID_PCM_S8_PLANAR,
  43.     AV_CODEC_ID_PCM_S24LE_PLANAR,
  44.     AV_CODEC_ID_PCM_S32LE_PLANAR,
  45.     AV_CODEC_ID_PCM_S64LE = 0x10800,
  46.     AV_CODEC_ID_PCM_F16LE,
  47.     AV_CODEC_ID_PCM_F24LE,
  48.  
  49.     /* various ADPCM codecs */
  50.     AV_CODEC_ID_ADPCM_IMA_QT = 0x11000,
  51.     AV_CODEC_ID_ADPCM_IMA_WAV,
  52.     AV_CODEC_ID_ADPCM_MS,
  53.  
  54.     AV_CODEC_ID_ADPCM_AFC = 0x11800,
  55.     AV_CODEC_ID_ADPCM_IMA_OKI,
  56.  
  57.     /* AMR */
  58.     AV_CODEC_ID_AMR_NB = 0x12000,
  59.     AV_CODEC_ID_AMR_WB,
  60.  
  61.     /* audio codecs */
  62.     AV_CODEC_ID_MP2 = 0x15000,
  63.     AV_CODEC_ID_MP3, ///< preferred ID for decoding MPEG audio layer 1, 2 or 3
  64.     AV_CODEC_ID_AAC,
  65.     AV_CODEC_ID_AC3,
  66.     AV_CODEC_ID_VORBIS,
  67.     AV_CODEC_ID_WMAV1,
  68.     AV_CODEC_ID_WMAV2,
  69.     AV_CODEC_ID_FLAC,
  70.     AV_CODEC_ID_APE,
  71.     AV_CODEC_ID_EAC3,
  72.     AV_CODEC_ID_TRUEHD,
  73.     AV_CODEC_ID_OPUS,
  74.  
  75.     /* subtitle codecs */
  76.     AV_CODEC_ID_DVD_SUBTITLE = 0x17000,
  77.     AV_CODEC_ID_DVB_SUBTITLE,
  78.     AV_CODEC_ID_SSA,
  79.     AV_CODEC_ID_MOV_TEXT,
  80.     AV_CODEC_ID_SRT,
  81.  
  82.     AV_CODEC_ID_MICRODVD   = 0x17800,
  83.     AV_CODEC_ID_SAMI,
  84.     AV_CODEC_ID_SUBRIP,
  85.     AV_CODEC_ID_WEBVTT,
  86.     AV_CODEC_ID_ASS,
  87.     AV_CODEC_ID_TTML,
  88.  
  89.     AV_CODEC_ID_TTF = 0x18000,
  90. };

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