旧版本的ffmpeg简单分析可参考雷神的博客：https://blog.csdn.net/leixiaohua1020/article/details/12677265

新版本的av_register_all函数定义如下：

void av_register_all(void)
{
    ff_thread_once(&av_format_next_init, av_format_init_next);
}

// 在libavformat/allformats.c中有定义
static AVOnce av_format_next_init = AV_ONCE_INIT;

// 在libavutil/thread.h中有定义
#define AV_ONCE_INIT 0

我们来看一下ff_thread_once的函数定义，如下：

static inline int ff_thread_once(char *control, void (*routine)(void))
{
    if (!*control) {
        routine();
        *control = 1;
    }
    return 0;
}

ff_thread_once这个函数的意义结合上面的定义，实际上就是运行一下routine这个函数，然后设置control的值为1，下次在进入的时候不再重新执行rountine这个函数，然后我们再来看一下rountine这个函数的定义，也即是av_format_init_next这个函数的定义。

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);
}

这个函数如今已经没什么用了，它的就是把你定义和系统初始化的编码器和解码器连接起来而已，然后就没有了。我们来看一下muxer_list的定义：

static const AVOutputFormat * const muxer_list[] = {
    &ff_a64_muxer,
    &ff_ac3_muxer,
    &ff_adts_muxer,
    &ff_adx_muxer,
    &ff_aiff_muxer,
    &ff_amr_muxer,
    &ff_apng_muxer,
    &ff_aptx_muxer,
    &ff_aptx_hd_muxer,
    &ff_asf_muxer,
    &ff_ass_muxer,
    &ff_ast_muxer,
    &ff_asf_stream_muxer,
    &ff_au_muxer,
    &ff_avi_muxer,
    &ff_avm2_muxer,
    &ff_avs2_muxer,
    &ff_bit_muxer,
    &ff_caf_muxer,
    &ff_cavsvideo_muxer,
    &ff_codec2_muxer,
    &ff_codec2raw_muxer,
    &ff_crc_muxer,
    &ff_dash_muxer,
    &ff_data_muxer,
    &ff_daud_muxer,
    &ff_dirac_muxer,
    &ff_dnxhd_muxer,
    &ff_dts_muxer,
    &ff_dv_muxer,
    &ff_eac3_muxer,
    &ff_f4v_muxer,
    &ff_ffmetadata_muxer,
    &ff_fifo_test_muxer,
    &ff_filmstrip_muxer,
    &ff_fits_muxer,
    &ff_flac_muxer,
    &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,
    &ff_hash_muxer,
    &ff_hds_muxer,
    &ff_hevc_muxer,
    &ff_hls_muxer,
    &ff_ico_muxer,
    &ff_ilbc_muxer,
    &ff_image2_muxer,
    &ff_image2pipe_muxer,
    &ff_ipod_muxer,
    &ff_ircam_muxer,
    &ff_ismv_muxer,
    &ff_ivf_muxer,
    &ff_jacosub_muxer,
    &ff_latm_muxer,
    &ff_lrc_muxer,
    &ff_m4v_muxer,
    &ff_md5_muxer,
    &ff_matroska_muxer,
    &ff_matroska_audio_muxer,
    &ff_microdvd_muxer,
    &ff_mjpeg_muxer,
    &ff_mlp_muxer,
    &ff_mmf_muxer,
    &ff_mov_muxer,
    &ff_mp2_muxer,
    &ff_mp3_muxer,
    &ff_mp4_muxer,
    &ff_mpeg1system_muxer,
    &ff_mpeg1vcd_muxer,
    &ff_mpeg1video_muxer,
    &ff_mpeg2dvd_muxer,
    &ff_mpeg2svcd_muxer,
    &ff_mpeg2video_muxer,
    &ff_mpeg2vob_muxer,
    &ff_mpegts_muxer,
    &ff_mpjpeg_muxer,
    &ff_mxf_muxer,
    &ff_mxf_d10_muxer,
    &ff_mxf_opatom_muxer,
    &ff_null_muxer,
    &ff_nut_muxer,
    &ff_oga_muxer,
    &ff_ogg_muxer,
    &ff_ogv_muxer,
    &ff_oma_muxer,
    &ff_opus_muxer,
    &ff_pcm_alaw_muxer,
    &ff_pcm_mulaw_muxer,
    &ff_pcm_vidc_muxer,
    &ff_pcm_f64be_muxer,
    &ff_pcm_f64le_muxer,
    &ff_pcm_f32be_muxer,
    &ff_pcm_f32le_muxer,
    &ff_pcm_s32be_muxer,
    &ff_pcm_s32le_muxer,
    &ff_pcm_s24be_muxer,
    &ff_pcm_s24le_muxer,
    &ff_pcm_s16be_muxer,
    &ff_pcm_s16le_muxer,
    &ff_pcm_s8_muxer,
    &ff_pcm_u32be_muxer,
    &ff_pcm_u32le_muxer,
    &ff_pcm_u24be_muxer,
    &ff_pcm_u24le_muxer,
    &ff_pcm_u16be_muxer,
    &ff_pcm_u16le_muxer,
    &ff_pcm_u8_muxer,
    &ff_psp_muxer,
    &ff_rawvideo_muxer,
    &ff_rm_muxer,
    &ff_roq_muxer,
    &ff_rso_muxer,
    &ff_rtp_muxer,
    &ff_rtp_mpegts_muxer,
    &ff_rtsp_muxer,
    &ff_sap_muxer,
    &ff_sbc_muxer,
    &ff_scc_muxer,
    &ff_segafilm_muxer,
    &ff_segment_muxer,
    &ff_stream_segment_muxer,
    &ff_singlejpeg_muxer,
    &ff_smjpeg_muxer,
    &ff_smoothstreaming_muxer,
    &ff_sox_muxer,
    &ff_spx_muxer,
    &ff_spdif_muxer,
    &ff_srt_muxer,
    &ff_sup_muxer,
    &ff_swf_muxer,
    &ff_tee_muxer,
    &ff_tg2_muxer,
    &ff_tgp_muxer,
    &ff_mkvtimestamp_v2_muxer,
    &ff_truehd_muxer,
    &ff_tta_muxer,
    &ff_uncodedframecrc_muxer,
    &ff_vc1_muxer,
    &ff_vc1t_muxer,
    &ff_voc_muxer,
    &ff_w64_muxer,
    &ff_wav_muxer,
    &ff_webm_muxer,
    &ff_webm_dash_manifest_muxer,
    &ff_webm_chunk_muxer,
    &ff_webp_muxer,
    &ff_webvtt_muxer,
    &ff_wtv_muxer,
    &ff_wv_muxer,
    &ff_yuv4mpegpipe_muxer,
    NULL };

随便找其中一个出来看。

AVOutputFormat ff_h264_muxer = {
    .name              = "h264",
    .long_name         = NULL_IF_CONFIG_SMALL("raw H.264 video"),
    .extensions        = "h264,264",
    .audio_codec       = AV_CODEC_ID_NONE,
    .video_codec       = AV_CODEC_ID_H264,
    .write_header      = force_one_stream,
    .write_packet      = ff_raw_write_packet,
    .check_bitstream   = h264_check_bitstream,
    .flags             = AVFMT_NOTIMESTAMPS,
};

可以看出现在解码器和编码器的初始化都是通过定义全局变量来初始化的。与原来相比，唯一的改变就是使用数组替换链表完成解码器和编码器的初始化。

总结，现在的av_register_all函数已经不再需要，以前，我们使用ffmpeg的时候这个函数永远是第一个被使用的，而现在，我们并不需要这个函数，同理，对于其他几个函数，如下：

av_register_input_format(), av_register_output_format(),av_register_all(), av_iformat_next(), av_oformat_next()

都已经被取代或者被移除。