此前写了好几篇ffmpeg源代码分析文章，列表如下：
图解FFMPEG打开媒体的函数avformat_open_inputffmpeg 源代码简单分析 ： av_register_all()
ffmpeg 源代码简单分析 ： avcodec_register_all()
ffmpeg 源代码简单分析 ： av_read_frame()
ffmpeg 源代码简单分析 ： avcodec_decode_video2()
============================
本文分析一下ffmpeg注册编解码器等的函数avcodec_register_all()（注意不是av_register_all()，那是注册所有东西的）。该函数在所有基于ffmpeg的应用程序中几乎都是第一个被调用的。只有调用了该函数，才能使用编解码器等。
之前已经写过一篇文章了：
ffmpeg 源代码简单分析 ： av_register_all()
其实注册编解码器和注册复用器解复用器道理是差不多的，重复的内容不再多说。
编码器的注册是：
REGISTER_ENCODER (X,x);
例如：
REGISTER_ENCODER (LJPEG, ljpeg);
解码器的注册是：
REGISTER_DECODER (X,x);
例如：
REGISTER_DECODER (H264, h264);
既包含编码器有包含解码器的注册是：
REGISTER_ENCDEC (X,x);
例如：
REGISTER_ENCDEC (BMP, bmp);
此外还有几种注册：
Parser：
REGISTER_PARSER (X,x);
例如：
REGISTER_PARSER (H264, h264);
BSF（bitstream filters，比特流滤镜，有一个常用：h264_mp4toannexb）：
REGISTER_BSF (X,x);
例如：
REGISTER_BSF (H264_MP4TOANNEXB, h264_mp4toannexb);
HWACCEL（hardware accelerators，硬件加速器）：
REGISTER_HWACCEL (X,x);
例如：
REGISTER_HWACCEL (H264_DXVA2, h264_dxva2);
我们来看一下宏的定义，这里以编解码器为例：
#define REGISTER_ENCODER(X,x) {
extern AVCodec ff_##x##_encoder;
if(CONFIG_##X##_ENCODER) avcodec_register(&ff_##x##_encoder); }
#define REGISTER_DECODER(X,x) {
extern AVCodec ff_##x##_decoder;
if(CONFIG_##X##_DECODER) avcodec_register(&ff_##x##_decoder); }
#define REGISTER_ENCDEC(X,x) REGISTER_ENCODER(X,x); REGISTER_DECODER(X,x)在这里，我发现其实编码器和解码器用的注册函数都是一样的：avcodec_register()
以REGISTER_DECODER (H264, h264)为例，就是等效于
extern AVCodec ff_h264_decoder; if(CONFIG_H264_DECODER) avcodec_register(&ff_h264_decoder);
下面看一下avcodec_register()的源代码：
//注册所有的AVCodec
void avcodec_register(AVCodec *codec)
{
AVCodec **p;
//初始化
avcodec_init();
//从第一个开始
p = &first_avcodec;
while (*p != NULL) p = &(*p)->next;
*p = codec;
codec->next = NULL;
if (codec->init_static_data)
codec->init_static_data(codec);
}这段代码是比较容易理解的。首先先提一点，first_avcdec是就是AVCodec链表的头部地址，是一个全局静态变量，定义如下：
/* encoder management */ static AVCodec *first_avcodec = NULL;
由此我们可以分析出avcodec_register()的含义，一句话概括就是：遍历链表并把当前的AVCodec加到链表的尾部。
同理，Parser，BSF（bitstream filters，比特流滤镜），HWACCEL（hardware accelerators，硬件加速器）的注册方式都是类似的。不再详述。
下面贴出它的原代码：
/*
*雷霄骅
*leixiaohua1020@126.com
*中国传媒大学/数字电视技术
*/
/*
* Provide registration of all codecs, parsers and bitstream filters for libavcodec.
* Copyright (c) 2002 Fabrice Bellard
*
* This file is part of FFmpeg.
*
* FFmpeg is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 2.1 of the License, or (at your option) any later version.
*
* FFmpeg is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with FFmpeg; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
*/
/**
* @file
* Provide registration of all codecs, parsers and bitstream filters for libavcodec.
*/
#include "avcodec.h"
//硬件加速
#define REGISTER_HWACCEL(X,x) {
extern AVHWAccel ff_##x##_hwaccel;
if(CONFIG_##X##_HWACCEL) av_register_hwaccel(&ff_##x##_hwaccel); }
#define REGISTER_ENCODER(X,x) {
extern AVCodec ff_##x##_encoder;
if(CONFIG_##X##_ENCODER) avcodec_register(&ff_##x##_encoder); }
//定义的宏？宏的速度会快一点？注册AVCodec
//extern AVCodec ff_##x##_decoder;
//注意：extern表明全局唯一
//在h264中，对应的就是ff_h264_decoder
//由此可见AVCodecParser的名字是固定的
#define REGISTER_DECODER(X,x) {
extern AVCodec ff_##x##_decoder;
if(CONFIG_##X##_DECODER) avcodec_register(&ff_##x##_decoder); }
#define REGISTER_ENCDEC(X,x) REGISTER_ENCODER(X,x); REGISTER_DECODER(X,x)
//定义的宏？宏的速度会快一点？注册AVCodecParser
//extern AVCodecParser ff_##x##_parser;
//在h264中，对应的就是ff_h264_parser
//由此可见AVCodecParser的名字是固定的
#define REGISTER_PARSER(X,x) {
extern AVCodecParser ff_##x##_parser;
if(CONFIG_##X##_PARSER) av_register_codec_parser(&ff_##x##_parser); }
#define REGISTER_BSF(X,x) {
extern AVBitStreamFilter ff_##x##_bsf;
if(CONFIG_##X##_BSF) av_register_bitstream_filter(&ff_##x##_bsf); }
void avcodec_register_all(void)
{
static int initialized;
if (initialized)
return;
initialized = 1;
/* hardware accelerators */
REGISTER_HWACCEL (H263_VAAPI, h263_vaapi);
REGISTER_HWACCEL (H264_DXVA2, h264_dxva2);
REGISTER_HWACCEL (H264_VAAPI, h264_vaapi);
REGISTER_HWACCEL (H264_VDA, h264_vda);
REGISTER_HWACCEL (MPEG1_VDPAU, mpeg1_vdpau);
REGISTER_HWACCEL (MPEG2_DXVA2, mpeg2_dxva2);
REGISTER_HWACCEL (MPEG2_VAAPI, mpeg2_vaapi);
REGISTER_HWACCEL (MPEG2_VDPAU, mpeg2_vdpau);
REGISTER_HWACCEL (MPEG4_VAAPI, mpeg4_vaapi);
REGISTER_HWACCEL (VC1_DXVA2, vc1_dxva2);
REGISTER_HWACCEL (VC1_VAAPI, vc1_vaapi);
REGISTER_HWACCEL (WMV3_DXVA2, wmv3_dxva2);
REGISTER_HWACCEL (WMV3_VAAPI, wmv3_vaapi);
/* video codecs */
REGISTER_ENCODER (A64MULTI, a64multi);
REGISTER_ENCODER (A64MULTI5, a64multi5);
REGISTER_DECODER (AASC, aasc);
REGISTER_ENCDEC (AMV, amv);
REGISTER_DECODER (ANM, anm);
REGISTER_DECODER (ANSI, ansi);
REGISTER_ENCDEC (ASV1, asv1);
REGISTER_ENCDEC (ASV2, asv2);
REGISTER_DECODER (AURA, aura);
REGISTER_DECODER (AURA2, aura2);
REGISTER_ENCDEC (AVRP, avrp);
REGISTER_DECODER (AVS, avs);
REGISTER_DECODER (BETHSOFTVID, bethsoftvid);
REGISTER_DECODER (BFI, bfi);
REGISTER_DECODER (BINK, bink);
REGISTER_ENCDEC (BMP, bmp);
REGISTER_DECODER (BMV_VIDEO, bmv_video);
REGISTER_DECODER (C93, c93);
REGISTER_DECODER (CAVS, cavs);
REGISTER_DECODER (CDGRAPHICS, cdgraphics);
REGISTER_DECODER (CINEPAK, cinepak);
REGISTER_ENCDEC (CLJR, cljr);
REGISTER_DECODER (CSCD, cscd);
REGISTER_DECODER (CYUV, cyuv);
REGISTER_DECODER (DFA, dfa);
REGISTER_DECODER (DIRAC, dirac);
REGISTER_ENCDEC (DNXHD, dnxhd);
REGISTER_ENCDEC (DPX, dpx);
REGISTER_DECODER (DSICINVIDEO, dsicinvideo);
REGISTER_ENCDEC (DVVIDEO, dvvideo);
REGISTER_DECODER (DXA, dxa);
REGISTER_DECODER (DXTORY, dxtory);
REGISTER_DECODER (EACMV, eacmv);
REGISTER_DECODER (EAMAD, eamad);
REGISTER_DECODER (EATGQ, eatgq);
REGISTER_DECODER (EATGV, eatgv);
REGISTER_DECODER (EATQI, eatqi);
REGISTER_DECODER (EIGHTBPS, eightbps);
REGISTER_DECODER (EIGHTSVX_EXP, eightsvx_exp);
REGISTER_DECODER (EIGHTSVX_FIB, eightsvx_fib);
REGISTER_DECODER (ESCAPE124, escape124);
REGISTER_DECODER (ESCAPE130, escape130);
REGISTER_ENCDEC (FFV1, ffv1);
REGISTER_ENCDEC (FFVHUFF, ffvhuff);
REGISTER_ENCDEC (FLASHSV, flashsv);
REGISTER_ENCDEC (FLASHSV2, flashsv2);
REGISTER_DECODER (FLIC, flic);
REGISTER_ENCDEC (FLV, flv);
REGISTER_DECODER (FOURXM, fourxm);
REGISTER_DECODER (FRAPS, fraps);
REGISTER_DECODER (FRWU, frwu);
REGISTER_ENCDEC (GIF, gif);
REGISTER_ENCDEC (H261, h261);
REGISTER_ENCDEC (H263, h263);
REGISTER_DECODER (H263I, h263i);
REGISTER_ENCODER (H263P, h263p);
REGISTER_DECODER (H264, h264);
REGISTER_DECODER (H264_CRYSTALHD, h264_crystalhd);
REGISTER_DECODER (H264_VDPAU, h264_vdpau);
REGISTER_ENCDEC (HUFFYUV, huffyuv);
REGISTER_DECODER (IDCIN, idcin);
REGISTER_DECODER (IFF_BYTERUN1, iff_byterun1);
REGISTER_DECODER (IFF_ILBM, iff_ilbm);
REGISTER_DECODER (INDEO2, indeo2);
REGISTER_DECODER (INDEO3, indeo3);
REGISTER_DECODER (INDEO4, indeo4);
REGISTER_DECODER (INDEO5, indeo5);
REGISTER_DECODER (INTERPLAY_VIDEO, interplay_video);
REGISTER_ENCDEC (JPEG2000, jpeg2000);
REGISTER_ENCDEC (JPEGLS, jpegls);
REGISTER_DECODER (JV, jv);
REGISTER_DECODER (KGV1, kgv1);
REGISTER_DECODER (KMVC, kmvc);
REGISTER_DECODER (LAGARITH, lagarith);
REGISTER_ENCODER (LJPEG, ljpeg);
REGISTER_DECODER (LOCO, loco);
REGISTER_DECODER (MDEC, mdec);
REGISTER_DECODER (MIMIC, mimic);
REGISTER_ENCDEC (MJPEG, mjpeg);
REGISTER_DECODER (MJPEGB, mjpegb);
REGISTER_DECODER (MMVIDEO, mmvideo);
REGISTER_DECODER (MOTIONPIXELS, motionpixels);
REGISTER_DECODER (MPEG_XVMC, mpeg_xvmc);
REGISTER_ENCDEC (MPEG1VIDEO, mpeg1video);
REGISTER_ENCDEC (MPEG2VIDEO, mpeg2video);
REGISTER_ENCDEC (MPEG4, mpeg4);
REGISTER_DECODER (MPEG4_CRYSTALHD, mpeg4_crystalhd);
REGISTER_DECODER (MPEG4_VDPAU, mpeg4_vdpau);
REGISTER_DECODER (MPEGVIDEO, mpegvideo);
REGISTER_DECODER (MPEG_VDPAU, mpeg_vdpau);
REGISTER_DECODER (MPEG1_VDPAU, mpeg1_vdpau);
REGISTER_DECODER (MPEG2_CRYSTALHD, mpeg2_crystalhd);
REGISTER_DECODER (MSMPEG4_CRYSTALHD, msmpeg4_crystalhd);
REGISTER_DECODER (MSMPEG4V1, msmpeg4v1);
REGISTER_ENCDEC (MSMPEG4V2, msmpeg4v2);
REGISTER_ENCDEC (MSMPEG4V3, msmpeg4v3);
REGISTER_DECODER (MSRLE, msrle);
REGISTER_ENCDEC (MSVIDEO1, msvideo1);
REGISTER_DECODER (MSZH, mszh);
REGISTER_DECODER (MXPEG, mxpeg);
REGISTER_DECODER (NUV, nuv);
REGISTER_ENCDEC (PAM, pam);
REGISTER_ENCDEC (PBM, pbm);
REGISTER_ENCDEC (PCX, pcx);
REGISTER_ENCDEC (PGM, pgm);
REGISTER_ENCDEC (PGMYUV, pgmyuv);
REGISTER_DECODER (PICTOR, pictor);
REGISTER_ENCDEC (PNG, png);
REGISTER_ENCDEC (PPM, ppm);
REGISTER_ENCDEC (PRORES, prores);
REGISTER_DECODER (PRORES_LGPL, prores_lgpl);
REGISTER_DECODER (PTX, ptx);
REGISTER_DECODER (QDRAW, qdraw);
REGISTER_DECODER (QPEG, qpeg);
REGISTER_ENCDEC (QTRLE, qtrle);
REGISTER_ENCDEC (R10K, r10k);
REGISTER_ENCDEC (R210, r210);
REGISTER_ENCDEC (RAWVIDEO, rawvideo);
REGISTER_DECODER (RL2, rl2);
REGISTER_ENCDEC (ROQ, roq);
REGISTER_DECODER (RPZA, rpza);
REGISTER_ENCDEC (RV10, rv10);
REGISTER_ENCDEC (RV20, rv20);
REGISTER_DECODER (RV30, rv30);
REGISTER_DECODER (RV40, rv40);
REGISTER_DECODER (S302M, s302m);
REGISTER_ENCDEC (SGI, sgi);
REGISTER_DECODER (SMACKER, smacker);
REGISTER_DECODER (SMC, smc);
REGISTER_ENCDEC (SNOW, snow);
REGISTER_DECODER (SP5X, sp5x);
REGISTER_DECODER (SUNRAST, sunrast);
REGISTER_ENCDEC (SVQ1, svq1);
REGISTER_DECODER (SVQ3, svq3);
REGISTER_ENCDEC (TARGA, targa);
REGISTER_DECODER (THEORA, theora);
REGISTER_DECODER (THP, thp);
REGISTER_DECODER (TIERTEXSEQVIDEO, tiertexseqvideo);
REGISTER_ENCDEC (TIFF, tiff);
REGISTER_DECODER (TMV, tmv);
REGISTER_DECODER (TRUEMOTION1, truemotion1);
REGISTER_DECODER (TRUEMOTION2, truemotion2);
REGISTER_DECODER (TSCC, tscc);
REGISTER_DECODER (TXD, txd);
REGISTER_DECODER (ULTI, ulti);
REGISTER_DECODER (UTVIDEO, utvideo);
REGISTER_ENCDEC (V210, v210);
REGISTER_DECODER (V210X, v210x);
REGISTER_ENCDEC (V308, v308);
REGISTER_ENCDEC (V410, v410);
REGISTER_DECODER (VB, vb);
REGISTER_DECODER (VBLE, vble);
REGISTER_DECODER (VC1, vc1);
REGISTER_DECODER (VC1_CRYSTALHD, vc1_crystalhd);
REGISTER_DECODER (VC1_VDPAU, vc1_vdpau);
REGISTER_DECODER (VC1IMAGE, vc1image);
REGISTER_DECODER (VCR1, vcr1);
REGISTER_DECODER (VMDVIDEO, vmdvideo);
REGISTER_DECODER (VMNC, vmnc);
REGISTER_DECODER (VP3, vp3);
REGISTER_DECODER (VP5, vp5);
REGISTER_DECODER (VP6, vp6);
REGISTER_DECODER (VP6A, vp6a);
REGISTER_DECODER (VP6F, vp6f);
REGISTER_DECODER (VP8, vp8);
REGISTER_DECODER (VQA, vqa);
REGISTER_ENCDEC (WMV1, wmv1);
REGISTER_ENCDEC (WMV2, wmv2);
REGISTER_DECODER (WMV3, wmv3);
REGISTER_DECODER (WMV3_CRYSTALHD, wmv3_crystalhd);
REGISTER_DECODER (WMV3_VDPAU, wmv3_vdpau);
REGISTER_DECODER (WMV3IMAGE, wmv3image);
REGISTER_DECODER (WNV1, wnv1);
REGISTER_DECODER (XAN_WC3, xan_wc3);
REGISTER_DECODER (XAN_WC4, xan_wc4);
REGISTER_DECODER (XL, xl);
REGISTER_ENCDEC (XWD, xwd);
REGISTER_ENCDEC (Y41P, y41p);
REGISTER_DECODER (YOP, yop);
REGISTER_ENCDEC (YUV4, yuv4);
REGISTER_ENCDEC (ZLIB, zlib);
REGISTER_ENCDEC (ZMBV, zmbv);
/* audio codecs */
REGISTER_ENCDEC (AAC, aac);
REGISTER_DECODER (AAC_LATM, aac_latm);
REGISTER_ENCDEC (AC3, ac3);
REGISTER_ENCODER (AC3_FIXED, ac3_fixed);
REGISTER_ENCDEC (ALAC, alac);
REGISTER_DECODER (ALS, als);
REGISTER_DECODER (AMRNB, amrnb);
REGISTER_DECODER (AMRWB, amrwb);
REGISTER_DECODER (APE, ape);
REGISTER_DECODER (ATRAC1, atrac1);
REGISTER_DECODER (ATRAC3, atrac3);
REGISTER_DECODER (BINKAUDIO_DCT, binkaudio_dct);
REGISTER_DECODER (BINKAUDIO_RDFT, binkaudio_rdft);
REGISTER_DECODER (BMV_AUDIO, bmv_audio);
REGISTER_DECODER (COOK, cook);
REGISTER_ENCDEC (DCA, dca);
REGISTER_DECODER (DSICINAUDIO, dsicinaudio);
REGISTER_ENCDEC (EAC3, eac3);
REGISTER_DECODER (FFWAVESYNTH, ffwavesynth);
REGISTER_ENCDEC (FLAC, flac);
REGISTER_ENCDEC (G723_1, g723_1);
REGISTER_DECODER (G729, g729);
REGISTER_DECODER (GSM, gsm);
REGISTER_DECODER (GSM_MS, gsm_ms);
REGISTER_DECODER (IMC, imc);
REGISTER_DECODER (MACE3, mace3);
REGISTER_DECODER (MACE6, mace6);
REGISTER_DECODER (MLP, mlp);
REGISTER_DECODER (MP1, mp1);
REGISTER_DECODER (MP1FLOAT, mp1float);
REGISTER_ENCDEC (MP2, mp2);
REGISTER_DECODER (MP2FLOAT, mp2float);
REGISTER_DECODER (MP3, mp3);
REGISTER_DECODER (MP3FLOAT, mp3float);
REGISTER_DECODER (MP3ADU, mp3adu);
REGISTER_DECODER (MP3ADUFLOAT, mp3adufloat);
REGISTER_DECODER (MP3ON4, mp3on4);
REGISTER_DECODER (MP3ON4FLOAT, mp3on4float);
REGISTER_DECODER (MPC7, mpc7);
REGISTER_DECODER (MPC8, mpc8);
REGISTER_ENCDEC (NELLYMOSER, nellymoser);
REGISTER_DECODER (QCELP, qcelp);
REGISTER_DECODER (QDM2, qdm2);
REGISTER_ENCDEC (RA_144, ra_144);
REGISTER_DECODER (RA_288, ra_288);
REGISTER_DECODER (SHORTEN, shorten);
REGISTER_DECODER (SIPR, sipr);
REGISTER_DECODER (SMACKAUD, smackaud);
REGISTER_ENCDEC (SONIC, sonic);
REGISTER_ENCODER (SONIC_LS, sonic_ls);
REGISTER_DECODER (TRUEHD, truehd);
REGISTER_DECODER (TRUESPEECH, truespeech);
REGISTER_DECODER (TTA, tta);
REGISTER_DECODER (TWINVQ, twinvq);
REGISTER_DECODER (VMDAUDIO, vmdaudio);
REGISTER_ENCDEC (VORBIS, vorbis);
REGISTER_DECODER (WAVPACK, wavpack);
REGISTER_DECODER (WMALOSSLESS, wmalossless);
REGISTER_DECODER (WMAPRO, wmapro);
REGISTER_ENCDEC (WMAV1, wmav1);
REGISTER_ENCDEC (WMAV2, wmav2);
REGISTER_DECODER (WMAVOICE, wmavoice);
REGISTER_DECODER (WS_SND1, ws_snd1);
/* PCM codecs */
REGISTER_ENCDEC (PCM_ALAW, pcm_alaw);
REGISTER_DECODER (PCM_BLURAY, pcm_bluray);
REGISTER_DECODER (PCM_DVD, pcm_dvd);
REGISTER_ENCDEC (PCM_F32BE, pcm_f32be);
REGISTER_ENCDEC (PCM_F32LE, pcm_f32le);
REGISTER_ENCDEC (PCM_F64BE, pcm_f64be);
REGISTER_ENCDEC (PCM_F64LE, pcm_f64le);
REGISTER_DECODER (PCM_LXF, pcm_lxf);
REGISTER_ENCDEC (PCM_MULAW, pcm_mulaw);
REGISTER_ENCDEC (PCM_S8, pcm_s8);
REGISTER_DECODER (PCM_S8_PLANAR, pcm_s8_planar);
REGISTER_ENCDEC (PCM_S16BE, pcm_s16be);
REGISTER_ENCDEC (PCM_S16LE, pcm_s16le);
REGISTER_DECODER (PCM_S16LE_PLANAR, pcm_s16le_planar);
REGISTER_ENCDEC (PCM_S24BE, pcm_s24be);
REGISTER_ENCDEC (PCM_S24DAUD, pcm_s24daud);
REGISTER_ENCDEC (PCM_S24LE, pcm_s24le);
REGISTER_ENCDEC (PCM_S32BE, pcm_s32be);
REGISTER_ENCDEC (PCM_S32LE, pcm_s32le);
REGISTER_ENCDEC (PCM_U8, pcm_u8);
REGISTER_ENCDEC (PCM_U16BE, pcm_u16be);
REGISTER_ENCDEC (PCM_U16LE, pcm_u16le);
REGISTER_ENCDEC (PCM_U24BE, pcm_u24be);
REGISTER_ENCDEC (PCM_U24LE, pcm_u24le);
REGISTER_ENCDEC (PCM_U32BE, pcm_u32be);
REGISTER_ENCDEC (PCM_U32LE, pcm_u32le);
REGISTER_DECODER (PCM_ZORK , pcm_zork);
/* DPCM codecs */
REGISTER_DECODER (INTERPLAY_DPCM, interplay_dpcm);
REGISTER_ENCDEC (ROQ_DPCM, roq_dpcm);
REGISTER_DECODER (SOL_DPCM, sol_dpcm);
REGISTER_DECODER (XAN_DPCM, xan_dpcm);
/* ADPCM codecs */
REGISTER_DECODER (ADPCM_4XM, adpcm_4xm);
REGISTER_ENCDEC (ADPCM_ADX, adpcm_adx);
REGISTER_DECODER (ADPCM_CT, adpcm_ct);
REGISTER_DECODER (ADPCM_EA, adpcm_ea);
REGISTER_DECODER (ADPCM_EA_MAXIS_XA, adpcm_ea_maxis_xa);
REGISTER_DECODER (ADPCM_EA_R1, adpcm_ea_r1);
REGISTER_DECODER (ADPCM_EA_R2, adpcm_ea_r2);
REGISTER_DECODER (ADPCM_EA_R3, adpcm_ea_r3);
REGISTER_DECODER (ADPCM_EA_XAS, adpcm_ea_xas);
REGISTER_ENCDEC (ADPCM_G722, adpcm_g722);
REGISTER_ENCDEC (ADPCM_G726, adpcm_g726);
REGISTER_DECODER (ADPCM_IMA_AMV, adpcm_ima_amv);
REGISTER_DECODER (ADPCM_IMA_APC, adpcm_ima_apc);
REGISTER_DECODER (ADPCM_IMA_DK3, adpcm_ima_dk3);
REGISTER_DECODER (ADPCM_IMA_DK4, adpcm_ima_dk4);
REGISTER_DECODER (ADPCM_IMA_EA_EACS, adpcm_ima_ea_eacs);
REGISTER_DECODER (ADPCM_IMA_EA_SEAD, adpcm_ima_ea_sead);
REGISTER_DECODER (ADPCM_IMA_ISS, adpcm_ima_iss);
REGISTER_ENCDEC (ADPCM_IMA_QT, adpcm_ima_qt);
REGISTER_DECODER (ADPCM_IMA_SMJPEG, adpcm_ima_smjpeg);
REGISTER_ENCDEC (ADPCM_IMA_WAV, adpcm_ima_wav);
REGISTER_DECODER (ADPCM_IMA_WS, adpcm_ima_ws);
REGISTER_ENCDEC (ADPCM_MS, adpcm_ms);
REGISTER_DECODER (ADPCM_SBPRO_2, adpcm_sbpro_2);
REGISTER_DECODER (ADPCM_SBPRO_3, adpcm_sbpro_3);
REGISTER_DECODER (ADPCM_SBPRO_4, adpcm_sbpro_4);
REGISTER_ENCDEC (ADPCM_SWF, adpcm_swf);
REGISTER_DECODER (ADPCM_THP, adpcm_thp);
REGISTER_DECODER (ADPCM_XA, adpcm_xa);
REGISTER_ENCDEC (ADPCM_YAMAHA, adpcm_yamaha);
/* subtitles */
REGISTER_ENCDEC (ASS, ass);
REGISTER_ENCDEC (DVBSUB, dvbsub);
REGISTER_ENCDEC (DVDSUB, dvdsub);
REGISTER_DECODER (PGSSUB, pgssub);
REGISTER_ENCDEC (SRT, srt);
REGISTER_ENCDEC (XSUB, xsub);
/* external libraries */
REGISTER_ENCODER (LIBAACPLUS, libaacplus);
REGISTER_DECODER (LIBCELT, libcelt);
REGISTER_ENCDEC (LIBDIRAC, libdirac);
REGISTER_ENCODER (LIBFAAC, libfaac);
REGISTER_ENCDEC (LIBGSM, libgsm);
REGISTER_ENCDEC (LIBGSM_MS, libgsm_ms);
REGISTER_ENCODER (LIBMP3LAME, libmp3lame);
REGISTER_ENCDEC (LIBOPENCORE_AMRNB, libopencore_amrnb);
REGISTER_DECODER (LIBOPENCORE_AMRWB, libopencore_amrwb);
REGISTER_ENCDEC (LIBOPENJPEG, libopenjpeg);
REGISTER_ENCDEC (LIBSCHROEDINGER, libschroedinger);
REGISTER_ENCDEC (LIBSPEEX, libspeex);
REGISTER_DECODER (LIBSTAGEFRIGHT_H264, libstagefright_h264);
REGISTER_ENCODER (LIBTHEORA, libtheora);
REGISTER_DECODER (LIBUTVIDEO, libutvideo);
REGISTER_ENCODER (LIBVO_AACENC, libvo_aacenc);
REGISTER_ENCODER (LIBVO_AMRWBENC, libvo_amrwbenc);
REGISTER_ENCODER (LIBVORBIS, libvorbis);
REGISTER_ENCDEC (LIBVPX, libvpx);
REGISTER_ENCODER (LIBX264, libx264);
REGISTER_ENCODER (LIBX264RGB, libx264rgb);
REGISTER_ENCODER (LIBXAVS, libxavs);
REGISTER_ENCODER (LIBXVID, libxvid);
/* text */
REGISTER_DECODER (BINTEXT, bintext);
REGISTER_DECODER (XBIN, xbin);
REGISTER_DECODER (IDF, idf);
/* parsers */
REGISTER_PARSER (AAC, aac);
REGISTER_PARSER (AAC_LATM, aac_latm);
REGISTER_PARSER (AC3, ac3);
REGISTER_PARSER (ADX, adx);
REGISTER_PARSER (CAVSVIDEO, cavsvideo);
REGISTER_PARSER (DCA, dca);
REGISTER_PARSER (DIRAC, dirac);
REGISTER_PARSER (DNXHD, dnxhd);
REGISTER_PARSER (DVBSUB, dvbsub);
REGISTER_PARSER (DVDSUB, dvdsub);
REGISTER_PARSER (FLAC, flac);
REGISTER_PARSER (GSM, gsm);
REGISTER_PARSER (H261, h261);
REGISTER_PARSER (H263, h263);
REGISTER_PARSER (H264, h264);
REGISTER_PARSER (MJPEG, mjpeg);
REGISTER_PARSER (MLP, mlp);
REGISTER_PARSER (MPEG4VIDEO, mpeg4video);
REGISTER_PARSER (MPEGAUDIO, mpegaudio);
REGISTER_PARSER (MPEGVIDEO, mpegvideo);
REGISTER_PARSER (PNM, pnm);
REGISTER_PARSER (RV30, rv30);
REGISTER_PARSER (RV40, rv40);
REGISTER_PARSER (VC1, vc1);
REGISTER_PARSER (VP3, vp3);
REGISTER_PARSER (VP8, vp8);
/* bitstream filters */
REGISTER_BSF (AAC_ADTSTOASC, aac_adtstoasc);
REGISTER_BSF (CHOMP, chomp);
REGISTER_BSF (DUMP_EXTRADATA, dump_extradata);
REGISTER_BSF (H264_MP4TOANNEXB, h264_mp4toannexb);
REGISTER_BSF (IMX_DUMP_HEADER, imx_dump_header);
REGISTER_BSF (MJPEG2JPEG, mjpeg2jpeg);
REGISTER_BSF (MJPEGA_DUMP_HEADER, mjpega_dump_header);
REGISTER_BSF (MP3_HEADER_COMPRESS, mp3_header_compress);
REGISTER_BSF (MP3_HEADER_DECOMPRESS, mp3_header_decompress);
REGISTER_BSF (MOV2TEXTSUB, mov2textsub);
REGISTER_BSF (NOISE, noise);
REGISTER_BSF (REMOVE_EXTRADATA, remove_extradata);
REGISTER_BSF (TEXT2MOVSUB, text2movsub);
}整个代码的过程就是首先确定是不是已经初始化过了（initialized），如果没有，就注册，注册，注册...直到完成所有注册。