一、av_register_all()

av_register_all()的申明在文件libavformat\avformat.h中，如下所示

/*** Initialize libavformat and register all the muxers, demuxers and* protocols. If you do not call this function, then you can select* exactly which formats you want to support.** @see av_register_input_format()* @see av_register_output_format()*/
attribute_deprecated
void av_register_all(void);

从定义中可以看出av_register_all()过时了，取而代之的是av_register_input_format()、av_register_output_format()这两个函数，不过查看申明也可发现，这两个函数也过时了，我们先看依稀。av_register_all()注册所有的demuxer，muxer,outdev,indev设备。其定义在文件libavformat\allformats.c中，如下所示：

void av_register_all(void)
{ff_thread_once(&av_format_next_init, av_format_init_next);
}void av_register_input_format(AVInputFormat *format)
{ff_thread_once(&av_format_next_init, av_format_init_next);
}void av_register_output_format(AVOutputFormat *format)
{ff_thread_once(&av_format_next_init, av_format_init_next);
}

av_register_all()、av_register_input_format()、av_register_output_format()这三个函数调用了一个只运行一次的线程，线程的执行函数是av_format_init_next()，其定义在文件libavcodec\allcodecs.c中，如下所示：

static void av_format_init_next(void)
{AVOutputFormat *prevout = NULL, *out;AVInputFormat *previn = NULL, *in;ff_mutex_lock(&avpriv_register_devices_mutex);for (int i = 0; (out = (AVOutputFormat*)muxer_list[i]); i++) {if (prevout)prevout->next = out;prevout = out;}if (outdev_list) {for (int i = 0; (out = (AVOutputFormat*)outdev_list[i]); i++) {if (prevout)prevout->next = out;prevout = out;}}for (int i = 0; (in = (AVInputFormat*)demuxer_list[i]); i++) {if (previn)previn->next = in;previn = in;}if (indev_list) {for (int i = 0; (in = (AVInputFormat*)indev_list[i]); i++) {if (previn)previn->next = in;previn = in;}}ff_mutex_unlock(&avpriv_register_devices_mutex);
}

在av_format_init_next()函数中，他涉及到四个变量muxer_list、outdev_list、demuxer_list和indev_list，这四个变量是makefile文件自动生成的，正常是没有的，需要我们编译一下ffmpeg才会出现。其定义如下所示：

libavformat\muxer_list.c
static const AVOutputFormat * const muxer_list[] = {......&ff_flv_muxer,&ff_framecrc_muxer,&ff_framehash_muxer,&ff_framemd5_muxer,&ff_g722_muxer,&ff_g723_1_muxer,&ff_g726_muxer,&ff_g726le_muxer,&ff_gif_muxer,&ff_gsm_muxer,&ff_gxf_muxer,&ff_h261_muxer,&ff_h263_muxer,&ff_h264_muxer,.....NULL };libavformat\demuxer_list.c
static const AVInputFormat * const demuxer_list[] = {&ff_aa_demuxer,&ff_aac_demuxer,&ff_ac3_demuxer,.......NULL };libavdevice\indev_list.c
static const AVInputFormat * const indev_list[] = {&ff_fbdev_demuxer,&ff_lavfi_demuxer,&ff_oss_demuxer,&ff_v4l2_demuxer,NULL };libavdevice\outdev_list.c
static const AVOutputFormat * const outdev_list[] = {&ff_fbdev_muxer,&ff_oss_muxer,&ff_v4l2_muxer,NULL };

这四个变量都是已经定义了的全局变量，不需要在用其他函数进行注册，所以有关这些变量的注册也就不需要了。

二、avcodec_register_all()

avcodec_register_all()功能和av_register_all函数相似，注册编码器，其申明在文件libavcodec\avcodec.h中，如下所示。

/*** Register all the codecs, parsers and bitstream filters which were enabled at* configuration time. If you do not call this function you can select exactly* which formats you want to support, by using the individual registration* functions.** @see avcodec_register* @see av_register_codec_parser* @see av_register_bitstream_filter*/
attribute_deprecated
void avcodec_register_all(void);

看申明也是过时了，猜测和上面的情况差不多。

void avcodec_register_all(void)
{ff_thread_once(&av_codec_next_init, av_codec_init_next);
}static void av_codec_init_next(void)
{AVCodec *prev = NULL, *p;void *i = 0;while ((p = (AVCodec*)av_codec_iterate(&i))) {if (prev)prev->next = p;prev = p;}
}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;
}

codec_list全局变量也是make自动生成的，生成文件libavcodec\codec_list.c。也就不需要进行codec注册了。

static const AVCodec * const codec_list[] = {.....&ff_mpeg4_encoder,......&ff_mpeg4_decoder,.....NULL };

ffmpeg源码分析：av_register_all()和avcodec_register_all()

一、av_register_all()

二、avcodec_register_all()