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()
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前一阵子看了一下ffmpeg的源代码，并且做了一些注释，在此贴出来以作备忘。
本文分析一下ffmpeg注册复用器，编码器等的函数av_register_all()。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，才能使用复用器，编码器等。
可见解复用器注册都是用
REGISTER_DEMUXER (X,x)
例如：
REGISTER_DEMUXER (AAC, aac)
可见复用器注册都是用
REGISTER_MUXER (X,x))
例如：
REGISTER_MUXER (ADTS, adts)
既有解复用器又有复用器的话，可以用
REGISTER_MUXDEMUX (X,x));
例如：
REGISTER_MUXDEMUX (AC3, ac3);
我们来看一下宏的定义，这里以解复用器为例：
#define REGISTER_DEMUXER(X,x) {
extern AVInputFormat ff_##x##_demuxer;
if(CONFIG_##X##_DEMUXER) av_register_input_format(&ff_##x##_demuxer); }注意：define里面的##可能不太常见，它的含义就是拼接两个字符串，比如#define Conn(x,y) x##y
那么
int n = Conn(123,456); 结果就是n=123456;
我们以REGISTER_DEMUXER (AAC, aac)为例，则它等效于
extern AVInputFormat ff_aac_demuxer; if(CONFIG_AAC_DEMUXER) av_register_input_format(&ff_aac_demuxer);
从上面这段代码我们可以看出，真正注册的函数是av_register_input_format(&ff_aac_demuxer)，那我就看看这个和函数的作用，查看一下av_register_input_format()的代码：
void av_register_input_format(AVInputFormat *format)
{
AVInputFormat **p;
p = &first_iformat;
while (*p != NULL) p = &(*p)->next;
*p = format;
format->next = NULL;
}这段代码是比较容易理解的，首先先提一点，first_iformat是个什么东东呢？其实它是Input Format链表的头部地址，是一个全局静态变量，定义如下：
/** head of registered input format linked list */ static AVInputFormat *first_iformat = NULL;由此我们可以分析出av_register_input_format()的含义，一句话概括就是：遍历链表并把当前的Input Format加到链表的尾部。
至此REGISTER_DEMUXER (X, x)分析完毕。
同理，复用器道理是一样的，只是注册函数改为av_register_output_format()；
既有解复用器又有复用器的话，有一个宏定义：
#define REGISTER_MUXDEMUX(X,x) REGISTER_MUXER(X,x); REGISTER_DEMUXER(X,x)
可见是分别注册了复用器和解复用器。
此外还有网络协议的注册，注册函数为ffurl_register_protocol()，在此不再详述。
下面贴出它的源代码（allformats.c）
/*
*雷霄骅
*leixiaohua1020@126.com
*中国传媒大学/数字电视技术
*/
/*
* Register all the formats and protocols
* Copyright (c) 2000, 2001, 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
*/
#include "avformat.h"
#include "rtp.h"
#include "rdt.h"
#include "url.h"
//定义的宏？宏的速度会快一点？注册AVOutputFormat
//define中，#用来把参数转换成字符串，##则用来连接前后两个参数，把它们变成一个字符串。
//感觉有点像JAva中的EL，可以随意拼接字符串
#define REGISTER_MUXER(X,x) {
extern AVOutputFormat ff_##x##_muxer;
if(CONFIG_##X##_MUXER) av_register_output_format(&ff_##x##_muxer); }
//定义的宏？宏的速度会快一点？注册AVInputFormat
#define REGISTER_DEMUXER(X,x) {
extern AVInputFormat ff_##x##_demuxer;
if(CONFIG_##X##_DEMUXER) av_register_input_format(&ff_##x##_demuxer); }
//注册函数av_register_input_format
//定义的宏？宏的速度会快一点？两个一起注册！
#define REGISTER_MUXDEMUX(X,x) REGISTER_MUXER(X,x); REGISTER_DEMUXER(X,x)
//定义的宏？宏的速度会快一点？注册URLProtocol
//extern URLProtocol ff_##x##_protocol;
//在librtmp中，对应的就是ff_rtmp_protocol
//这样就把librtmp整合起来了
//由此可见URLProtocol的名字是固定的
#define REGISTER_PROTOCOL(X,x) {
extern URLProtocol ff_##x##_protocol;
if(CONFIG_##X##_PROTOCOL) ffurl_register_protocol(&ff_##x##_protocol, sizeof(ff_##x##_protocol)); }
//注册函数ffurl_register_protocol
void av_register_all(void)
{
static int initialized;
if (initialized)
return;
initialized = 1;
//注册所有的codec
avcodec_register_all();
//注册所有的MUXER（复用器和解复用器）
/* (de)muxers */
REGISTER_MUXER (A64, a64);
REGISTER_DEMUXER (AAC, aac);
REGISTER_MUXDEMUX (AC3, ac3);
REGISTER_DEMUXER (ACT, act);
REGISTER_DEMUXER (ADF, adf);
REGISTER_MUXER (ADTS, adts);
REGISTER_MUXDEMUX (ADX, adx);
REGISTER_DEMUXER (AEA, aea);
REGISTER_MUXDEMUX (AIFF, aiff);
REGISTER_MUXDEMUX (AMR, amr);
REGISTER_DEMUXER (ANM, anm);
REGISTER_DEMUXER (APC, apc);
REGISTER_DEMUXER (APE, ape);
REGISTER_DEMUXER (APPLEHTTP, applehttp);
REGISTER_MUXDEMUX (ASF, asf);
REGISTER_MUXDEMUX (ASS, ass);
REGISTER_MUXER (ASF_STREAM, asf_stream);
REGISTER_MUXDEMUX (AU, au);
REGISTER_MUXDEMUX (AVI, avi);
REGISTER_DEMUXER (AVISYNTH, avisynth);
REGISTER_MUXER (AVM2, avm2);
REGISTER_DEMUXER (AVS, avs);
REGISTER_DEMUXER (BETHSOFTVID, bethsoftvid);
REGISTER_DEMUXER (BFI, bfi);
REGISTER_DEMUXER (BINTEXT, bintext);
REGISTER_DEMUXER (BINK, bink);
REGISTER_MUXDEMUX (BIT, bit);
REGISTER_DEMUXER (BMV, bmv);
REGISTER_DEMUXER (C93, c93);
REGISTER_MUXDEMUX (CAF, caf);
REGISTER_MUXDEMUX (CAVSVIDEO, cavsvideo);
REGISTER_DEMUXER (CDG, cdg);
REGISTER_MUXER (CRC, crc);
REGISTER_MUXDEMUX (DAUD, daud);
REGISTER_DEMUXER (DFA, dfa);
REGISTER_MUXDEMUX (DIRAC, dirac);
REGISTER_MUXDEMUX (DNXHD, dnxhd);
REGISTER_DEMUXER (DSICIN, dsicin);
REGISTER_MUXDEMUX (DTS, dts);
REGISTER_MUXDEMUX (DV, dv);
REGISTER_DEMUXER (DXA, dxa);
REGISTER_DEMUXER (EA, ea);
REGISTER_DEMUXER (EA_CDATA, ea_cdata);
REGISTER_MUXDEMUX (EAC3, eac3);
REGISTER_MUXDEMUX (FFM, ffm);
REGISTER_MUXDEMUX (FFMETADATA, ffmetadata);
REGISTER_MUXDEMUX (FILMSTRIP, filmstrip);
REGISTER_MUXDEMUX (FLAC, flac);
REGISTER_DEMUXER (FLIC, flic);
REGISTER_MUXDEMUX (FLV, flv);
REGISTER_DEMUXER (FOURXM, fourxm);
REGISTER_MUXER (FRAMECRC, framecrc);
REGISTER_MUXER (FRAMEMD5, framemd5);
REGISTER_MUXDEMUX (G722, g722);
REGISTER_MUXDEMUX (G723_1, g723_1);
REGISTER_DEMUXER (G729, g729);
REGISTER_MUXER (GIF, gif);
REGISTER_DEMUXER (GSM, gsm);
REGISTER_MUXDEMUX (GXF, gxf);
REGISTER_MUXDEMUX (H261, h261);
REGISTER_MUXDEMUX (H263, h263);
REGISTER_MUXDEMUX (H264, h264);
REGISTER_DEMUXER (ICO, ico);
REGISTER_DEMUXER (IDCIN, idcin);
REGISTER_DEMUXER (IDF, idf);
REGISTER_DEMUXER (IFF, iff);
REGISTER_MUXDEMUX (IMAGE2, image2);
REGISTER_MUXDEMUX (IMAGE2PIPE, image2pipe);
REGISTER_DEMUXER (INGENIENT, ingenient);
REGISTER_DEMUXER (IPMOVIE, ipmovie);
REGISTER_MUXER (IPOD, ipod);
REGISTER_MUXER (ISMV, ismv);
REGISTER_DEMUXER (ISS, iss);
REGISTER_DEMUXER (IV8, iv8);
REGISTER_MUXDEMUX (IVF, ivf);
REGISTER_DEMUXER (JV, jv);
REGISTER_MUXDEMUX (LATM, latm);
REGISTER_DEMUXER (LMLM4, lmlm4);
REGISTER_DEMUXER (LOAS, loas);
REGISTER_DEMUXER (LXF, lxf);
REGISTER_MUXDEMUX (M4V, m4v);
REGISTER_MUXER (MD5, md5);
REGISTER_MUXDEMUX (MATROSKA, matroska);
REGISTER_MUXER (MATROSKA_AUDIO, matroska_audio);
REGISTER_MUXDEMUX (MICRODVD, microdvd);
REGISTER_MUXDEMUX (MJPEG, mjpeg);
REGISTER_MUXDEMUX (MLP, mlp);
REGISTER_DEMUXER (MM, mm);
REGISTER_MUXDEMUX (MMF, mmf);
REGISTER_MUXDEMUX (MOV, mov);
REGISTER_MUXER (MP2, mp2);
REGISTER_MUXDEMUX (MP3, mp3);
REGISTER_MUXER (MP4, mp4);
REGISTER_DEMUXER (MPC, mpc);
REGISTER_DEMUXER (MPC8, mpc8);
REGISTER_MUXER (MPEG1SYSTEM, mpeg1system);
REGISTER_MUXER (MPEG1VCD, mpeg1vcd);
REGISTER_MUXER (MPEG1VIDEO, mpeg1video);
REGISTER_MUXER (MPEG2DVD, mpeg2dvd);
REGISTER_MUXER (MPEG2SVCD, mpeg2svcd);
REGISTER_MUXER (MPEG2VIDEO, mpeg2video);
REGISTER_MUXER (MPEG2VOB, mpeg2vob);
REGISTER_DEMUXER (MPEGPS, mpegps);
REGISTER_MUXDEMUX (MPEGTS, mpegts);
REGISTER_DEMUXER (MPEGTSRAW, mpegtsraw);
REGISTER_DEMUXER (MPEGVIDEO, mpegvideo);
REGISTER_MUXER (MPJPEG, mpjpeg);
REGISTER_DEMUXER (MSNWC_TCP, msnwc_tcp);
REGISTER_DEMUXER (MTV, mtv);
REGISTER_DEMUXER (MVI, mvi);
REGISTER_MUXDEMUX (MXF, mxf);
REGISTER_MUXER (MXF_D10, mxf_d10);
REGISTER_DEMUXER (MXG, mxg);
REGISTER_DEMUXER (NC, nc);
REGISTER_DEMUXER (NSV, nsv);
REGISTER_MUXER (NULL, null);
REGISTER_MUXDEMUX (NUT, nut);
REGISTER_DEMUXER (NUV, nuv);
REGISTER_MUXDEMUX (OGG, ogg);
REGISTER_MUXDEMUX (OMA, oma);
REGISTER_MUXDEMUX (PCM_ALAW, pcm_alaw);
REGISTER_MUXDEMUX (PCM_MULAW, pcm_mulaw);
REGISTER_MUXDEMUX (PCM_F64BE, pcm_f64be);
REGISTER_MUXDEMUX (PCM_F64LE, pcm_f64le);
REGISTER_MUXDEMUX (PCM_F32BE, pcm_f32be);
REGISTER_MUXDEMUX (PCM_F32LE, pcm_f32le);
REGISTER_MUXDEMUX (PCM_S32BE, pcm_s32be);
REGISTER_MUXDEMUX (PCM_S32LE, pcm_s32le);
REGISTER_MUXDEMUX (PCM_S24BE, pcm_s24be);
REGISTER_MUXDEMUX (PCM_S24LE, pcm_s24le);
REGISTER_MUXDEMUX (PCM_S16BE, pcm_s16be);
REGISTER_MUXDEMUX (PCM_S16LE, pcm_s16le);
REGISTER_MUXDEMUX (PCM_S8, pcm_s8);
REGISTER_MUXDEMUX (PCM_U32BE, pcm_u32be);
REGISTER_MUXDEMUX (PCM_U32LE, pcm_u32le);
REGISTER_MUXDEMUX (PCM_U24BE, pcm_u24be);
REGISTER_MUXDEMUX (PCM_U24LE, pcm_u24le);
REGISTER_MUXDEMUX (PCM_U16BE, pcm_u16be);
REGISTER_MUXDEMUX (PCM_U16LE, pcm_u16le);
REGISTER_MUXDEMUX (PCM_U8, pcm_u8);
REGISTER_DEMUXER (PMP, pmp);
REGISTER_MUXER (PSP, psp);
REGISTER_DEMUXER (PVA, pva);
REGISTER_DEMUXER (QCP, qcp);
REGISTER_DEMUXER (R3D, r3d);
REGISTER_MUXDEMUX (RAWVIDEO, rawvideo);
REGISTER_DEMUXER (RL2, rl2);
REGISTER_MUXDEMUX (RM, rm);
REGISTER_MUXDEMUX (ROQ, roq);
REGISTER_DEMUXER (RPL, rpl);
REGISTER_MUXDEMUX (RSO, rso);
REGISTER_MUXDEMUX (RTP, rtp);
REGISTER_MUXDEMUX (RTSP, rtsp);
REGISTER_MUXDEMUX (SAP, sap);
REGISTER_DEMUXER (SBG, sbg);
REGISTER_DEMUXER (SDP, sdp);
#if CONFIG_RTPDEC
av_register_rtp_dynamic_payload_handlers();
av_register_rdt_dynamic_payload_handlers();
#endif
REGISTER_DEMUXER (SEGAFILM, segafilm);
REGISTER_MUXER (SEGMENT, segment);
REGISTER_DEMUXER (SHORTEN, shorten);
REGISTER_DEMUXER (SIFF, siff);
REGISTER_DEMUXER (SMACKER, smacker);
REGISTER_MUXDEMUX (SMJPEG, smjpeg);
REGISTER_DEMUXER (SOL, sol);
REGISTER_MUXDEMUX (SOX, sox);
REGISTER_MUXDEMUX (SPDIF, spdif);
REGISTER_MUXDEMUX (SRT, srt);
REGISTER_DEMUXER (STR, str);
REGISTER_MUXDEMUX (SWF, swf);
REGISTER_MUXER (TG2, tg2);
REGISTER_MUXER (TGP, tgp);
REGISTER_DEMUXER (THP, thp);
REGISTER_DEMUXER (TIERTEXSEQ, tiertexseq);
REGISTER_MUXER (MKVTIMESTAMP_V2, mkvtimestamp_v2);
REGISTER_DEMUXER (TMV, tmv);
REGISTER_MUXDEMUX (TRUEHD, truehd);
REGISTER_DEMUXER (TTA, tta);
REGISTER_DEMUXER (TXD, txd);
REGISTER_DEMUXER (TTY, tty);
REGISTER_DEMUXER (VC1, vc1);
REGISTER_MUXDEMUX (VC1T, vc1t);
REGISTER_DEMUXER (VMD, vmd);
REGISTER_MUXDEMUX (VOC, voc);
REGISTER_DEMUXER (VQF, vqf);
REGISTER_DEMUXER (W64, w64);
REGISTER_MUXDEMUX (WAV, wav);
REGISTER_DEMUXER (WC3, wc3);
REGISTER_MUXER (WEBM, webm);
REGISTER_DEMUXER (WSAUD, wsaud);
REGISTER_DEMUXER (WSVQA, wsvqa);
REGISTER_MUXDEMUX (WTV, wtv);
REGISTER_DEMUXER (WV, wv);
REGISTER_DEMUXER (XA, xa);
REGISTER_DEMUXER (XBIN, xbin);
REGISTER_DEMUXER (XMV, xmv);
REGISTER_DEMUXER (XWMA, xwma);
REGISTER_DEMUXER (YOP, yop);
REGISTER_MUXDEMUX (YUV4MPEGPIPE, yuv4mpegpipe);
/* external libraries */
#if CONFIG_LIBMODPLUG
REGISTER_DEMUXER (LIBMODPLUG, libmodplug);
#endif
REGISTER_MUXDEMUX (LIBNUT, libnut);
//注册所有的Protocol（位于DEMUXER之前（我的理解~~））
//文件也是一种Protocol
/* protocols */
REGISTER_PROTOCOL (APPLEHTTP, applehttp);
REGISTER_PROTOCOL (CACHE, cache);
REGISTER_PROTOCOL (CONCAT, concat);
REGISTER_PROTOCOL (CRYPTO, crypto);
REGISTER_PROTOCOL (FILE, file);
REGISTER_PROTOCOL (GOPHER, gopher);
REGISTER_PROTOCOL (HTTP, http);
REGISTER_PROTOCOL (HTTPPROXY, httpproxy);
REGISTER_PROTOCOL (HTTPS, https);
REGISTER_PROTOCOL (MMSH, mmsh);
REGISTER_PROTOCOL (MMST, mmst);
REGISTER_PROTOCOL (MD5, md5);
REGISTER_PROTOCOL (PIPE, pipe);
REGISTER_PROTOCOL (RTMP, rtmp);
//如果包含了LibRTMP
#if CONFIG_LIBRTMP
REGISTER_PROTOCOL (RTMP, rtmpt);
REGISTER_PROTOCOL (RTMP, rtmpe);
REGISTER_PROTOCOL (RTMP, rtmpte);
REGISTER_PROTOCOL (RTMP, rtmps);
#endif
REGISTER_PROTOCOL (RTP, rtp);
REGISTER_PROTOCOL (TCP, tcp);
REGISTER_PROTOCOL (TLS, tls);
REGISTER_PROTOCOL (UDP, udp);
}整个代码没太多可说的，首先确定是不是已经初始化过了（initialized），如果没有，就调用avcodec_register_all()注册编解码器（这个先不分析），然后就是注册，注册，注册...直到完成所有注册。
PS：曾经研究过一阵子RTMP协议，以及对应的开源工程librtmp。在这里发现有一点值得注意，ffmpeg自带了RTMP协议的支持，只有使用 rtmpt://, rtmpe://, rtmpte://等的时候才会使用librtmp库。