mpegts代码分析
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因为TS流的复用和解复用是通过一个结构体 AVInputFormat 传递给解复用器的。所以重点分析该结构体提供的外部接口：
AVInputFormat mpegtsraw_demuxer = { "mpegtsraw", NULL_IF_CONFIG_SMALL("MPEG-2 raw transport stream format"), sizeof(MpegTSContext), NULL, mpegts_read_header, mpegts_raw_read_packet, mpegts_read_close, read_seek, mpegts_get_pcr, .flags = AVFMT_SHOW_IDS|AVFMT_TS_DISCONT,};1、mpegts_read_header()：
功能：判断是TS流的类型，建立TS流的业务信息表SDT表和PAD表
static int mpegts_read_header(AVFormatContext *s, AVFormatParameters *ap){ MpegTSContext *ts = s->priv_data; ByteIOContext *pb = s->pb; uint8_t buf[5*1024]; int len; int64_t pos; if (ap) { ts->mpeg2ts_compute_pcr = ap->mpeg2ts_compute_pcr; if(ap->mpeg2ts_raw){ av_log(s, AV_LOG_ERROR, "use mpegtsraw_demuxer!\n"); return -1; } }/* read the first 1024 bytes to get packet size */pos = url_ftell(pb); //保存流的当前位置，便于检测操作完成后恢复到原来的位置,这样在播放的时候就不会浪费一段流。 len = get_buffer(pb, buf, sizeof(buf));////读取一段流来检测TS包的大小 if (len != sizeof(buf)) goto fail; ts->raw_packet_size = get_packet_size(buf, sizeof(buf)); //得到TS包的pack的size。在get_packedt_size中buf的大小必须大于1024，否则直接返回-1。得到TS流包的大小，通常是188bytes,我目前见过的都是188个字节的。
//TS包的大小有三种：
//1) 通常情况下的188字节
//2)日本弄了个192Bytes的DVH-S格式
//3)在188Bytes的基础上，加上16Bytes的FEC(前向纠错),也就是204bytes
/*static int get_packet_size(const uint8_t *buf, int size){ int score, fec_score, dvhs_score;//Must have 1024 byte if (size < (TS_FEC_PACKET_SIZE * 5 + 1)) return -1; score = analyze(buf, size, TS_PACKET_SIZE, NULL); //通过在size长度内进行对比，获得一个0x47在在间隔packet_size出现的最大概率，以下类似 dvhs_score = analyze(buf, size, TS_DVHS_PACKET_SIZE, NULL); fec_score= analyze(buf, size, TS_FEC_PACKET_SIZE, NULL);// av_log(NULL, AV_LOG_DEBUG, "score: %d, dvhs_score: %d, fec_score: %d \n", score, dvhs_score, fec_score); if (score > fec_score && score > dvhs_score) return TS_PACKET_SIZE; //188 else if(dvhs_score > score && dvhs_score > fec_score) return TS_DVHS_PACKET_SIZE;//192 else if(score < fec_score && dvhs_score < fec_score) return TS_FEC_PACKET_SIZE; //204 else return -1;}*/
/* static int analyze(const uint8_t *buf, int size, int packet_size, int *index){ int stat[packet_size]; int i; int x=0; int best_score=0; memset(stat, 0, packet_size*sizeof(int)); for(x=i=0; i<size-3; i++){ if(buf[i] == 0x47 && !(buf[i+1] & 0x80) && (buf[i+3] & 0x30)){ stat[x]++; //sta[x]记录了在x位置上(这是在0~packet_size范围内)，0x47出现的次数。记住x是每到paket_size就清零的。 if(stat[x] > best_score){ best_score= stat[x]; //best_score存储了0x47在某个位置上出现的最大的次数 if(index) *index= x; } } x++; if(x == packet_size) x= 0; } return best_score;} */if (ts->raw_packet_size <= 0) goto fail; ts->stream = s;//auto_guess = 1, 则在handle_packet的函数中只要发现一个PES的pid就 //建立该PES的stream //auto_guess = 0, 则忽略。
//auto_guess主要作用是用来在TS流中没有业务信息时，如果被设置成了1的话，
//那么就会将任何一个PID的流当做媒体流建立对应的PES数据结构。 //在mpegts_read_header函数的过程中发现了PES的pid，但 //是不建立对应的流,只是分析PSI信息。 //相关的代码见handle_packet函数的下面的代码： // tss = ts->pids[pid]; //if (ts->auto_guess && tss == NULL && is_start) { // add_pes_stream(ts, pid, -1, 0); // tss = ts->pids[pid]; //}
ts->auto_guess = 0; if (s->iformat == &mpegts_demuxer) { //若当前输入的流为TS流 /* normal demux */ /* first do a scaning to get all the services */ url_fseek(pb, pos, SEEK_SET);//挂载解析SDT表的回调函数到ts->pids变量上,这样在handle_packet函数中根据对应的pid找到对应处理回调函数mpegts_scan_sdt(ts);//这儿调用了mpegts_open_section_filter(ts, SDT_PID,sdt_cb, ts, 1)，这个函数将在后面进行分析。//同上，只是挂上PAT表解析的回调函数 mpegts_set_service(ts);//这儿调用了mpegts_open_section_filter(ts, PAT_PID,pat_cb, ts, 1)函数 //探测一段流，便于检测出SDT，PAT，PMT表 handle_packets(ts, s->probesize); //这儿调用了handle_packet(ts, packet)函数 /* if could not find service, enable auto_guess *///打开add pes stream的标志，这样在handle_packet函数中发现了pes的
//pid，就会自动建立该pes的stream。 ts->auto_guess = 1;#ifdef DEBUG_SI av_log(ts->stream, AV_LOG_DEBUG, "tuning done\n");#endif s->ctx_flags |= AVFMTCTX_NOHEADER; } else { AVStream *st; int pcr_pid, pid, nb_packets, nb_pcrs, ret, pcr_l; int64_t pcrs[2], pcr_h; int packet_count[2]; uint8_t packet[TS_PACKET_SIZE]; /* only read packets */ st = av_new_stream(s, 0); if (!st) goto fail; av_set_pts_info(st, 60, 1, 27000000); // st->codec->codec_type = CODEC_TYPE_DATA; st->codec->codec_id = CODEC_ID_MPEG2TS; /* we iterate until we find two PCRs to estimate the bitrate */ pcr_pid = -1; nb_pcrs = 0; nb_packets = 0; for(;;) { ret = read_packet(s->pb, packet, ts->raw_packet_size); if (ret < 0) return -1; pid = AV_RB16(packet + 1) & 0x1fff; if ((pcr_pid == -1 || pcr_pid == pid) && parse_pcr(&pcr_h, &pcr_l, packet) == 0) { pcr_pid = pid; packet_count[nb_pcrs] = nb_packets; pcrs[nb_pcrs] = pcr_h * 300 + pcr_l; nb_pcrs++; if (nb_pcrs >= 2) break; } nb_packets++; } /* NOTE1: the bitrate is computed without the FEC */ /* NOTE2: it is only the bitrate of the start of the stream */ ts->pcr_incr = (pcrs[1] - pcrs[0]) / (packet_count[1] - packet_count[0]); ts->cur_pcr = pcrs[0] - ts->pcr_incr * packet_count[0]; s->bit_rate = (TS_PACKET_SIZE * 8) * 27e6 / ts->pcr_incr; st->codec->bit_rate = s->bit_rate; st->start_time = ts->cur_pcr;#if 0 av_log(ts->stream, AV_LOG_DEBUG, "start=%0.3f pcr=%0.3f incr=%d\n", st->start_time / 1000000.0, pcrs[0] / 27e6, ts->pcr_incr);#endif } //恢复到检测前的位置。 url_fseek(pb, pos, SEEK_SET); return 0; fail: return -1;}2、下面接着分析上面注释时候的函数mpegts_open_section_filter(MpegTSContext *ts, unsigned int pid , SectionCallback *section_cb, void *opaque, int check_crc):
函数的功能：将pid值为pid的滤波器挂载在ts->PID[pid]上,并设置filter的相关参数
struct MpegTSContext { /* user data */ AVFormatContext *stream; /** raw packet size, including FEC if present */ int raw_packet_size; int pos47; /** if true, all pids are analyzed to find streams */ int auto_guess; /** compute exact PCR for each transport stream packet */ int mpeg2ts_compute_pcr; int64_t cur_pcr; /**< used to estimate the exact PCR */ int pcr_incr; /**< used to estimate the exact PCR */ /* data needed to handle file based ts */ /** stop parsing loop */ int stop_parse; /** packet containing Audio/Video data */ AVPacket *pkt; /******************************************/ /* private mpegts data */ /* scan context */ /** structure to keep track of Program->pids mapping */ unsigned int nb_prg; struct Program *prg; /** filters for various streams specified by PMT + for the PAT and PMT */ MpegTSFilter *pids[NB_PID_MAX]; //这儿每一个PID都有一个filter滤波器};static MpegTSFilter *mpegts_open_section_filter(MpegTSContext *ts, unsigned int pid, SectionCallback *section_cb, void *opaque, int check_crc){ MpegTSFilter *filter; MpegTSSectionFilter *sec;#ifdef DEBUG_SI av_log(ts->stream, AV_LOG_DEBUG, "Filter: pid=0x%x\n", pid);#endif if (pid >= NB_PID_MAX || ts->pids[pid]) return NULL; filter = av_mallocz(sizeof(MpegTSFilter)); //给filter分配空间，挂载到MpegTSContext的pids上，就是该实例 if (!filter) return NULL; ts->pids[pid] = filter; //挂载filter filter->type = MPEGTS_SECTION; filter->pid = pid; //需要截获的PID值 filter->last_cc = -1; sec = &filter->u.section_filter; //设置filter相关的参数，因为业务信息表的分析的单位是段，
//所以该filter的类型是MPEGTS_SECTION sec->section_cb = section_cb; //设置filter回调处理函数 sec->opaque = opaque; sec->section_buf = av_malloc(MAX_SECTION_SIZE); //分配段数据处理的缓冲区，调用handle_packet函数后会调用
//write_section_data将ts包中的业务信息表的数据存储在这儿，
//直到一个段收集完成才交付上面注册的回调函数处理 sec->check_crc = check_crc; if (!sec->section_buf) { av_free(filter); return NULL; } return filter;}3、函数：handle_packets(MpegTSContext *ts, int nb_packets)
static int handle_packets(MpegTSContext *ts, int nb_packets){ AVFormatContext *s = ts->stream; ByteIOContext *pb = s->pb; uint8_t packet[TS_PACKET_SIZE]; int packet_num, ret; ts->stop_parse = 0; packet_num = 0; for(;;) { if (ts->stop_parse>0) break; packet_num++; if (nb_packets != 0 && packet_num >= nb_packets) break;//resrech syn (0x47),cpy one ts pkt to packet. ret = read_packet(pb, packet, ts->raw_packet_size); if (ret != 0) return ret; handle_packet(ts, packet); } return 0;}read_packet()函数：读取一个packet到buf中:
static int read_packet(ByteIOContext *pb, uint8_t *buf, int raw_packet_size){ int skip, len; for(;;) { len = get_buffer(pb, buf, TS_PACKET_SIZE); //读一个包的长度到buf中 if (len != TS_PACKET_SIZE) return AVERROR(EIO); /* check paquet sync byte */ if (buf[0] != 0x47) { //如果当前的数据没有同步，则需要重新同步后，在读数据到buf中。 /* find a new packet start */ url_fseek(pb, -TS_PACKET_SIZE, SEEK_CUR); //移动指针 if (mpegts_resync(pb) < 0) //重新进行同步，即将指针指向packet的头部 return AVERROR_INVALIDDATA; else continue; } else { skip = raw_packet_size - TS_PACKET_SIZE; if (skip > 0) url_fskip(pb, skip); //跳过一个包的长度，即指向下一个包 break; } } return 0;}handle_packet()函数：
收集PID的信息
/* handle one TS packet */static void handle_packet(MpegTSContext *ts, const uint8_t *packet){ AVFormatContext *s = ts->stream; MpegTSFilter *tss; int len, pid, cc, cc_ok, afc, is_start; const uint8_t *p, *p_end; int64_t pos; pid = AV_RB16(packet + 1) & 0x1fff; //获得当前包的PID的值 if(pid && discard_pid(ts, pid)) // return; //PES or PSI is_start = packet[1] & 0x40; tss = ts->pids[pid]; if (ts->auto_guess && tss == NULL && is_start) { add_pes_stream(ts, pid, -1, 0); tss = ts->pids[pid]; } if (!tss) //仅当PID为SDT或PAT时，才继续处理，否则直接返回 return; /* continuity check (currently not used) */ cc = (packet[3] & 0xf); cc_ok = (tss->last_cc < 0) || ((((tss->last_cc + 1) & 0x0f) == cc)); tss->last_cc = cc; /* skip adaptation field */ afc = (packet[3] >> 4) & 3; p = packet + 4; if (afc == 0) /* reserved value */ return; if (afc == 2) /* adaptation field only */ return; if (afc == 3) { /* skip adapation field */ p += p[0] + 1; } /* if past the end of packet, ignore */ p_end = packet + TS_PACKET_SIZE; if (p >= p_end) return; pos = url_ftell(ts->stream->pb); ts->pos47= pos % ts->raw_packet_size; if (tss->type == MPEGTS_SECTION) { if (is_start) { /* pointer field present */ len = *p++; //pointer_field指针：当一个分段开始时，其值为0。当pay_unit_start_indicator为1时，传输流有效负载的第一字节应该包含此指针。如果一个传输流中没有一个分段开始，则pay_unit_start_indicator为0，且没有pointer_field字段。 if (p + len > p_end) return; if (len && cc_ok) { /* write remaining section bytes */ write_section_data(s, tss, p, len, 0); /* check whether filter has been closed */ if (!ts->pids[pid]) return; } p += len; if (p < p_end) { write_section_data(s, tss, p, p_end - p, 1); } } else { if (cc_ok) { write_section_data(s, tss, p, p_end - p, 0); } } } else { // Note: The position here points actually behind the current packet. tss->u.pes_filter.pes_cb(tss, p, p_end - p, is_start, pos - ts->raw_packet_size); }}
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