static int init_input(AVFormatContext *s, const char *filename,AVDictionary **options)
{//...return av_probe_input_buffer2(s->pb, &s->iformat, filename,s, 0, s->format_probesize);
}

int av_probe_input_buffer2(AVIOContext *pb, AVInputFormat **fmt,const char *filename, void *logctx,unsigned int offset, unsigned int max_probe_size)
{//...//把AVFormatContext和具体的AVInputFormat联系起来了*fmt = av_probe_input_format2(&pd, 1, &score);//...
}

//遍历所有的Demuxer
while ((fmt1 = av_demuxer_iterate(&i))) {if (!is_opened == !(fmt1->flags & AVFMT_NOFILE) && strcmp(fmt1->name, "image2"))continue;score = 0;if (fmt1->read_probe) {score = fmt1->read_probe(&lpd);if (score)av_log(NULL, AV_LOG_TRACE, "Probing %s score:%d size:%d\n", fmt1->name, score, lpd.buf_size);if (fmt1->extensions && av_match_ext(lpd.filename, fmt1->extensions)) {switch (nodat) {case NO_ID3:score = FFMAX(score, 1);break;case ID3_GREATER_PROBE:case ID3_ALMOST_GREATER_PROBE:score = FFMAX(score, AVPROBE_SCORE_EXTENSION / 2 - 1);break;case ID3_GREATER_MAX_PROBE:score = FFMAX(score, AVPROBE_SCORE_EXTENSION);break;}}} else if (fmt1->extensions) {if (av_match_ext(lpd.filename, fmt1->extensions))score = AVPROBE_SCORE_EXTENSION;}if (av_match_name(lpd.mime_type, fmt1->mime_type)) {if (AVPROBE_SCORE_MIME > score) {av_log(NULL, AV_LOG_DEBUG, "Probing %s score:%d increased to %d due to MIME type\n", fmt1->name, score, AVPROBE_SCORE_MIME);score = AVPROBE_SCORE_MIME;}}if (score > score_max) {score_max = score;fmt       = (AVInputFormat*)fmt1;} else if (score == score_max)fmt = NULL;}

const AVInputFormat *av_demuxer_iterate(void **opaque)
{static const uintptr_t size = sizeof(demuxer_list)/sizeof(demuxer_list[0]) - 1;uintptr_t i = (uintptr_t)*opaque;const AVInputFormat *f = NULL;if (i < size) {//遍历demuxer_list中的所有demuxerf = demuxer_list[i];} else if (outdev_list) {f = indev_list[i - size];}if (f)*opaque = (void*)(i + 1);return f;
}

print_enabled_components libavformat/demuxer_list.c AVInputFormat demuxer_list $DEMUXER_LIST

static const AVInputFormat * const demuxer_list[] = {&ff_aa_demuxer,&ff_aac_demuxer,&ff_ac3_demuxer,&ff_acm_demuxer,&ff_act_demuxer,&ff_adf_demuxer,&ff_adp_demuxer,&ff_ads_demuxer,&ff_adx_demuxer,&ff_aea_demuxer,&ff_afc_demuxer,&ff_aiff_demuxer,&ff_aix_demuxer,&ff_amr_demuxer,&ff_amrnb_demuxer,&ff_amrwb_demuxer,&ff_anm_demuxer,&ff_apc_demuxer,&ff_ape_demuxer,&ff_apng_demuxer,&ff_aptx_demuxer,&ff_aptx_hd_demuxer,&ff_aqtitle_demuxer,&ff_asf_demuxer,&ff_asf_o_demuxer,&ff_ass_demuxer,&ff_ast_demuxer,&ff_au_demuxer,&ff_avi_demuxer,&ff_avr_demuxer,&ff_avs_demuxer,&ff_bethsoftvid_demuxer,&ff_bfi_demuxer,&ff_bintext_demuxer,&ff_bink_demuxer,&ff_bit_demuxer,&ff_bmv_demuxer,&ff_bfstm_demuxer,&ff_brstm_demuxer,&ff_boa_demuxer,&ff_c93_demuxer,&ff_caf_demuxer,&ff_cavsvideo_demuxer,&ff_cdg_demuxer,&ff_cdxl_demuxer,&ff_cine_demuxer,&ff_codec2_demuxer,&ff_codec2raw_demuxer,&ff_concat_demuxer,&ff_data_demuxer,&ff_daud_demuxer,&ff_dcstr_demuxer,&ff_dfa_demuxer,&ff_dirac_demuxer,&ff_dnxhd_demuxer,&ff_dsf_demuxer,&ff_dsicin_demuxer,&ff_dss_demuxer,&ff_dts_demuxer,&ff_dtshd_demuxer,&ff_dv_demuxer,&ff_dvbsub_demuxer,&ff_dvbtxt_demuxer,&ff_dxa_demuxer,&ff_ea_demuxer,&ff_ea_cdata_demuxer,&ff_eac3_demuxer,&ff_epaf_demuxer,&ff_ffmetadata_demuxer,&ff_filmstrip_demuxer,&ff_fits_demuxer,&ff_flac_demuxer,&ff_flic_demuxer,&ff_flv_demuxer,&ff_live_flv_demuxer,&ff_fourxm_demuxer,&ff_frm_demuxer,&ff_fsb_demuxer,&ff_g722_demuxer,&ff_g723_1_demuxer,&ff_g726_demuxer,&ff_g726le_demuxer,&ff_g729_demuxer,&ff_gdv_demuxer,&ff_genh_demuxer,&ff_gif_demuxer,&ff_gsm_demuxer,&ff_gxf_demuxer,&ff_h261_demuxer,&ff_h263_demuxer,&ff_h264_demuxer,&ff_hevc_demuxer,&ff_hls_demuxer,&ff_hnm_demuxer,&ff_ico_demuxer,&ff_idcin_demuxer,&ff_idf_demuxer,&ff_iff_demuxer,&ff_ilbc_demuxer,&ff_image2_demuxer,&ff_image2pipe_demuxer,&ff_image2_alias_pix_demuxer,&ff_image2_brender_pix_demuxer,&ff_ingenient_demuxer,&ff_ipmovie_demuxer,&ff_ircam_demuxer,&ff_iss_demuxer,&ff_iv8_demuxer,&ff_ivf_demuxer,&ff_ivr_demuxer,&ff_jacosub_demuxer,&ff_jv_demuxer,&ff_lmlm4_demuxer,&ff_loas_demuxer,&ff_lrc_demuxer,&ff_lvf_demuxer,&ff_lxf_demuxer,&ff_m4v_demuxer,&ff_matroska_demuxer,&ff_mgsts_demuxer,&ff_microdvd_demuxer,&ff_mjpeg_demuxer,&ff_mjpeg_2000_demuxer,&ff_mlp_demuxer,&ff_mlv_demuxer,&ff_mm_demuxer,&ff_mmf_demuxer,&ff_mov_demuxer,&ff_mp3_demuxer,&ff_mpc_demuxer,&ff_mpc8_demuxer,&ff_mpegps_demuxer,&ff_mpegts_demuxer,&ff_mpegtsraw_demuxer,&ff_mpegvideo_demuxer,&ff_mpjpeg_demuxer,&ff_mpl2_demuxer,&ff_mpsub_demuxer,&ff_msf_demuxer,&ff_msnwc_tcp_demuxer,&ff_mtaf_demuxer,&ff_mtv_demuxer,&ff_musx_demuxer,&ff_mv_demuxer,&ff_mvi_demuxer,&ff_mxf_demuxer,&ff_mxg_demuxer,&ff_nc_demuxer,&ff_nistsphere_demuxer,&ff_nsp_demuxer,&ff_nsv_demuxer,&ff_nut_demuxer,&ff_nuv_demuxer,&ff_ogg_demuxer,&ff_oma_demuxer,&ff_paf_demuxer,&ff_pcm_alaw_demuxer,&ff_pcm_mulaw_demuxer,&ff_pcm_f64be_demuxer,&ff_pcm_f64le_demuxer,&ff_pcm_f32be_demuxer,&ff_pcm_f32le_demuxer,&ff_pcm_s32be_demuxer,&ff_pcm_s32le_demuxer,&ff_pcm_s24be_demuxer,&ff_pcm_s24le_demuxer,&ff_pcm_s16be_demuxer,&ff_pcm_s16le_demuxer,&ff_pcm_s8_demuxer,&ff_pcm_u32be_demuxer,&ff_pcm_u32le_demuxer,&ff_pcm_u24be_demuxer,&ff_pcm_u24le_demuxer,&ff_pcm_u16be_demuxer,&ff_pcm_u16le_demuxer,&ff_pcm_u8_demuxer,&ff_pjs_demuxer,&ff_pmp_demuxer,&ff_pva_demuxer,&ff_pvf_demuxer,&ff_qcp_demuxer,&ff_r3d_demuxer,&ff_rawvideo_demuxer,&ff_realtext_demuxer,&ff_redspark_demuxer,&ff_rl2_demuxer,&ff_rm_demuxer,&ff_roq_demuxer,&ff_rpl_demuxer,&ff_rsd_demuxer,&ff_rso_demuxer,&ff_rtp_demuxer,&ff_rtsp_demuxer,&ff_s337m_demuxer,&ff_sami_demuxer,&ff_sap_demuxer,&ff_sbc_demuxer,&ff_sbg_demuxer,&ff_scc_demuxer,&ff_sdp_demuxer,&ff_sdr2_demuxer,&ff_sds_demuxer,&ff_sdx_demuxer,&ff_segafilm_demuxer,&ff_shorten_demuxer,&ff_siff_demuxer,&ff_sln_demuxer,&ff_smacker_demuxer,&ff_smjpeg_demuxer,&ff_smush_demuxer,&ff_sol_demuxer,&ff_sox_demuxer,&ff_spdif_demuxer,&ff_srt_demuxer,&ff_str_demuxer,&ff_stl_demuxer,&ff_subviewer1_demuxer,&ff_subviewer_demuxer,&ff_sup_demuxer,&ff_svag_demuxer,&ff_swf_demuxer,&ff_tak_demuxer,&ff_tedcaptions_demuxer,&ff_thp_demuxer,&ff_threedostr_demuxer,&ff_tiertexseq_demuxer,&ff_tmv_demuxer,&ff_truehd_demuxer,&ff_tta_demuxer,&ff_txd_demuxer,&ff_tty_demuxer,&ff_ty_demuxer,&ff_v210_demuxer,&ff_v210x_demuxer,&ff_vag_demuxer,&ff_vc1_demuxer,&ff_vc1t_demuxer,&ff_vivo_demuxer,&ff_vmd_demuxer,&ff_vobsub_demuxer,&ff_voc_demuxer,&ff_vpk_demuxer,&ff_vplayer_demuxer,&ff_vqf_demuxer,&ff_w64_demuxer,&ff_wav_demuxer,&ff_wc3_demuxer,&ff_webm_dash_manifest_demuxer,&ff_webvtt_demuxer,&ff_wsaud_demuxer,&ff_wsd_demuxer,&ff_wsvqa_demuxer,&ff_wtv_demuxer,&ff_wve_demuxer,&ff_wv_demuxer,&ff_xa_demuxer,&ff_xbin_demuxer,&ff_xmv_demuxer,&ff_xvag_demuxer,&ff_xwma_demuxer,&ff_yop_demuxer,&ff_yuv4mpegpipe_demuxer,&ff_image_bmp_pipe_demuxer,&ff_image_dds_pipe_demuxer,&ff_image_dpx_pipe_demuxer,&ff_image_exr_pipe_demuxer,&ff_image_j2k_pipe_demuxer,&ff_image_jpeg_pipe_demuxer,&ff_image_jpegls_pipe_demuxer,&ff_image_pam_pipe_demuxer,&ff_image_pbm_pipe_demuxer,&ff_image_pcx_pipe_demuxer,&ff_image_pgmyuv_pipe_demuxer,&ff_image_pgm_pipe_demuxer,&ff_image_pictor_pipe_demuxer,&ff_image_png_pipe_demuxer,&ff_image_ppm_pipe_demuxer,&ff_image_psd_pipe_demuxer,&ff_image_qdraw_pipe_demuxer,&ff_image_sgi_pipe_demuxer,&ff_image_svg_pipe_demuxer,&ff_image_sunrast_pipe_demuxer,&ff_image_tiff_pipe_demuxer,&ff_image_webp_pipe_demuxer,&ff_image_xpm_pipe_demuxer,&ff_image_xwd_pipe_demuxer,NULL };

//libavformat/avidec.c
static const char avi_headers[][8] = {{ 'R', 'I', 'F', 'F', 'A', 'V', 'I', ' '  },{ 'R', 'I', 'F', 'F', 'A', 'V', 'I', 'X'  },{ 'R', 'I', 'F', 'F', 'A', 'V', 'I', 0x19 },{ 'O', 'N', '2', ' ', 'O', 'N', '2', 'f'  },{ 'R', 'I', 'F', 'F', 'A', 'M', 'V', ' '  },{ 0 }
};
static int avi_probe(AVProbeData *p)
{int i;/* check file header */for (i = 0; avi_headers[i][0]; i++)if (AV_RL32(p->buf    ) == AV_RL32(avi_headers[i]    ) &&AV_RL32(p->buf + 8) == AV_RL32(avi_headers[i] + 4))return AVPROBE_SCORE_MAX;return 0;
}

rm/rmvb文件的特征码是.RMF、.ra，下面是FFmpeg中rm探测函数：

//libavformat/rmdec.c
static int rm_probe(AVProbeData *p)
{/* check file header */if ((p->buf[0] == '.' && p->buf[1] == 'R' &&p->buf[2] == 'M' && p->buf[3] == 'F' &&p->buf[4] == 0 && p->buf[5] == 0) ||(p->buf[0] == '.' && p->buf[1] == 'r' &&p->buf[2] == 'a' && p->buf[3] == 0xfd))return AVPROBE_SCORE_MAX;elsereturn 0;
}

flv文件的特征码是FLV，下面是FFmpeg中flv探测函数：

//libavformat/flvdec.c
static int probe(AVProbeData *p, int live)
{const uint8_t *d = p->buf;unsigned offset = AV_RB32(d + 5);if (d[0] == 'F' &&d[1] == 'L' &&d[2] == 'V' &&d[3] < 5 && d[5] == 0 &&offset + 100 < p->buf_size &&offset > 8) {int is_live = !memcmp(d + offset + 40, "NGINX RTMP", 10);if (live == is_live)return AVPROBE_SCORE_MAX;}return 0;
}static int flv_probe(AVProbeData *p)
{return probe(p, 0);
}

mkv的特征码就是EBML_Header_ID和已知的DocType，下面是FFmpeg的mkv探测函数：

//libavformat/matroskadec.c
/* top-level master-IDs */
#define EBML_ID_HEADER             0x1A45DFA3
static int matroska_probe(AVProbeData *p)
{uint64_t total = 0;int len_mask = 0x80, size = 1, n = 1, i;/* EBML header? */if (AV_RB32(p->buf) != EBML_ID_HEADER)return 0;/* length of header */total = p->buf[4];while (size <= 8 && !(total & len_mask)) {size++;len_mask >>= 1;}if (size > 8)return 0;total &= (len_mask - 1);while (n < size)total = (total << 8) | p->buf[4 + n++];/* Does the probe data contain the whole header? */if (p->buf_size < 4 + size + total)return 0;/* The header should contain a known document type. For now,* we don't parse the whole header but simply check for the* availability of that array of characters inside the header.* Not fully fool-proof, but good enough. */for (i = 0; i < FF_ARRAY_ELEMS(matroska_doctypes); i++) {size_t probelen = strlen(matroska_doctypes[i]);if (total < probelen)continue;for (n = 4 + size; n <= 4 + size + total - probelen; n++)if (!memcmp(p->buf + n, matroska_doctypes[i], probelen))return AVPROBE_SCORE_MAX;}// probably valid EBML header but no recognized doctypereturn AVPROBE_SCORE_EXTENSION;
}

ASF/WMV文件的特征码就是ASF_HEADER_GUID，下面是FFmpeg中asf探测函数：

//libavformat/asfdec_f.c
const ff_asf_guid ff_asf_header = {0x30, 0x26, 0xB2, 0x75, 0x8E, 0x66, 0xCF, 0x11, 0xA6, 0xD9, 0x00, 0xAA, 0x00, 0x62, 0xCE, 0x6C
};
static int asf_probe(AVProbeData *pd)
{/* check file header */if (!ff_guidcmp(pd->buf, &ff_asf_header))return AVPROBE_SCORE_MAX;elsereturn 0;
}

MP4文件是由不同BOX构成的，需要根据box type来探测实际类型，FFmpeg中mp4探测函数（也包括mov,m4a,3gp,3g2）如下：

//libavformat/mov.c
static int mov_probe(AVProbeData *p)
{int64_t offset;uint32_t tag;int score = 0;int moov_offset = -1;/* check file header */offset = 0;for (;;) {/* ignore invalid offset */if ((offset + 8) > (unsigned int)p->buf_size)break;tag = AV_RL32(p->buf + offset + 4);switch(tag) {/* check for obvious tags */case MKTAG('m','o','o','v'):moov_offset = offset + 4;case MKTAG('m','d','a','t'):case MKTAG('p','n','o','t'): /* detect movs with preview pics like ew.mov and april.mov */case MKTAG('u','d','t','a'): /* Packet Video PVAuthor adds this and a lot of more junk */case MKTAG('f','t','y','p'):if (AV_RB32(p->buf+offset) < 8 &&(AV_RB32(p->buf+offset) != 1 ||offset + 12 > (unsigned int)p->buf_size ||AV_RB64(p->buf+offset + 8) == 0)) {score = FFMAX(score, AVPROBE_SCORE_EXTENSION);} else if (tag == MKTAG('f','t','y','p') &&(   AV_RL32(p->buf + offset + 8) == MKTAG('j','p','2',' ')|| AV_RL32(p->buf + offset + 8) == MKTAG('j','p','x',' '))) {score = FFMAX(score, 5);} else {score = AVPROBE_SCORE_MAX;}offset = FFMAX(4, AV_RB32(p->buf+offset)) + offset;break;/* those are more common words, so rate then a bit less */case MKTAG('e','d','i','w'): /* xdcam files have reverted first tags */case MKTAG('w','i','d','e'):case MKTAG('f','r','e','e'):case MKTAG('j','u','n','k'):case MKTAG('p','i','c','t'):score  = FFMAX(score, AVPROBE_SCORE_MAX - 5);offset = FFMAX(4, AV_RB32(p->buf+offset)) + offset;break;case MKTAG(0x82,0x82,0x7f,0x7d):case MKTAG('s','k','i','p'):case MKTAG('u','u','i','d'):case MKTAG('p','r','f','l'):/* if we only find those cause probedata is too small at least rate them */score  = FFMAX(score, AVPROBE_SCORE_EXTENSION);offset = FFMAX(4, AV_RB32(p->buf+offset)) + offset;break;default:offset = FFMAX(4, AV_RB32(p->buf+offset)) + offset;}}if(score > AVPROBE_SCORE_MAX - 50 && moov_offset != -1) {/* moov atom in the header - we should make sure that this is not a* MOV-packed MPEG-PS */offset = moov_offset;while(offset < (p->buf_size - 16)){ /* Sufficient space *//* We found an actual hdlr atom */if(AV_RL32(p->buf + offset     ) == MKTAG('h','d','l','r') &&AV_RL32(p->buf + offset +  8) == MKTAG('m','h','l','r') &&AV_RL32(p->buf + offset + 12) == MKTAG('M','P','E','G')){av_log(NULL, AV_LOG_WARNING, "Found media data tag MPEG indicating this is a MOV-packed MPEG-PS.\n");/* We found a media handler reference atom describing an* MPEG-PS-in-MOV, return a* low score to force expanding the probe window until* mpegps_probe finds what it needs */return 5;}else/* Keep looking */offset+=2;}}return score;
}

对于像TS这种流媒体格式，通常通过0x47同步字节和pid判断，FFmpeg中ts的探测函数如下：

//libavformat/mpegts.c
static int analyze(const uint8_t *buf, int size, int packet_size,int probe)
{int stat[TS_MAX_PACKET_SIZE];int stat_all = 0;int i;int best_score = 0;memset(stat, 0, packet_size * sizeof(*stat));for (i = 0; i < size - 3; i++) {if (buf[i] == 0x47) {int pid = AV_RB16(buf+1) & 0x1FFF;int asc = buf[i + 3] & 0x30;if (!probe || pid == 0x1FFF || asc) {int x = i % packet_size;stat[x]++;stat_all++;if (stat[x] > best_score) {best_score = stat[x];}}}}return best_score - FFMAX(stat_all - 10*best_score, 0)/10;
}
static int mpegts_probe(AVProbeData *p)
{const int size = p->buf_size;int maxscore = 0;int sumscore = 0;int i;int check_count = size / TS_FEC_PACKET_SIZE;
#define CHECK_COUNT 10
#define CHECK_BLOCK 100if (!check_count)return 0;for (i = 0; i<check_count; i+=CHECK_BLOCK) {int left = FFMIN(check_count - i, CHECK_BLOCK);int score      = analyze(p->buf + TS_PACKET_SIZE     *i, TS_PACKET_SIZE     *left, TS_PACKET_SIZE     , 1);int dvhs_score = analyze(p->buf + TS_DVHS_PACKET_SIZE*i, TS_DVHS_PACKET_SIZE*left, TS_DVHS_PACKET_SIZE, 1);int fec_score  = analyze(p->buf + TS_FEC_PACKET_SIZE *i, TS_FEC_PACKET_SIZE *left, TS_FEC_PACKET_SIZE , 1);score = FFMAX3(score, dvhs_score, fec_score);sumscore += score;maxscore = FFMAX(maxscore, score);}sumscore = sumscore * CHECK_COUNT / check_count;maxscore = maxscore * CHECK_COUNT / CHECK_BLOCK;ff_dlog(0, "TS score: %d %d\n", sumscore, maxscore);if        (check_count > CHECK_COUNT && sumscore > 6) {return AVPROBE_SCORE_MAX   + sumscore - CHECK_COUNT;} else if (check_count >= CHECK_COUNT && sumscore > 6) {return AVPROBE_SCORE_MAX/2 + sumscore - CHECK_COUNT;} else if (check_count >= CHECK_COUNT && maxscore > 6) {return AVPROBE_SCORE_MAX/2 + sumscore - CHECK_COUNT;} else if (sumscore > 6) {return 2;} else {return 0;}
}

参考文章：

https://www.cnblogs.com/tocy/p/media_container_9-probe-utility.html