音视频入门基础：MPEG2-TS专题（17）——FFmpeg源码中，解析TS program map section的实现

一、引言

由《音视频入门基础：MPEG2-TS专题（16）——PMT简介》可以知道，PMT表（Program map table）由一个或多个段（Transport stream program map section，简称TS program map section，即组成PMT表的段）组成。当某个PMT表非常大时，只要接收到一个TS program map section的完整数据就可以进行解析，而不需要接收到完整的表时才开始解析工作，解析完各个TS program map section后再把这些信息汇总起来。FFmpeg源码中，通过pmt_cb函数解析TS program map section。

二、pmt_cb函数定义

pmt_cb函数定义在FFmpeg源码（本文演示用的FFmpeg源码版本为7.0.1）的源文件libavformat/mpegts.c中：

static void pmt_cb(MpegTSFilter *filter, const uint8_t *section, int section_len)
{MpegTSContext *ts = filter->u.section_filter.opaque;MpegTSSectionFilter *tssf = &filter->u.section_filter;struct Program old_program;SectionHeader h1, *h = &h1;PESContext *pes;AVStream *st;const uint8_t *p, *p_end, *desc_list_end;int program_info_length, pcr_pid, pid, stream_type;int desc_list_len;uint32_t prog_reg_desc = 0; /* registration descriptor */int stream_identifier = -1;struct Program *prg;int mp4_descr_count = 0;Mp4Descr mp4_descr[MAX_MP4_DESCR_COUNT] = { { 0 } };int i;av_log(ts->stream, AV_LOG_TRACE, "PMT: len %i\n", section_len);hex_dump_debug(ts->stream, section, section_len);p_end = section + section_len - 4;p = section;if (parse_section_header(h, &p, p_end) < 0)return;if (h->tid != PMT_TID)return;if (!h->current_next)return;if (skip_identical(h, tssf))return;av_log(ts->stream, AV_LOG_TRACE, "sid=0x%x sec_num=%d/%d version=%d tid=%d\n",h->id, h->sec_num, h->last_sec_num, h->version, h->tid);if (!ts->scan_all_pmts && ts->skip_changes)return;prg = get_program(ts, h->id);if (prg)old_program = *prg;elseclear_program(&old_program);if (ts->skip_unknown_pmt && !prg)return;if (prg && prg->nb_pids && prg->pids[0] != ts->current_pid)return;if (!ts->skip_clear)clear_avprogram(ts, h->id);clear_program(prg);add_pid_to_program(prg, ts->current_pid);pcr_pid = get16(&p, p_end);if (pcr_pid < 0)return;pcr_pid &= 0x1fff;add_pid_to_program(prg, pcr_pid);update_av_program_info(ts->stream, h->id, pcr_pid, h->version);av_log(ts->stream, AV_LOG_TRACE, "pcr_pid=0x%x\n", pcr_pid);program_info_length = get16(&p, p_end);if (program_info_length < 0)return;program_info_length &= 0xfff;while (program_info_length >= 2) {uint8_t tag, len;tag = get8(&p, p_end);len = get8(&p, p_end);av_log(ts->stream, AV_LOG_TRACE, "program tag: 0x%02x len=%d\n", tag, len);program_info_length -= 2;if (len > program_info_length)// something else is broken, exit the program_descriptors_loopbreak;program_info_length -= len;if (tag == IOD_DESCRIPTOR) {get8(&p, p_end); // scopeget8(&p, p_end); // labellen -= 2;mp4_read_iods(ts->stream, p, len, mp4_descr + mp4_descr_count,&mp4_descr_count, MAX_MP4_DESCR_COUNT);} else if (tag == REGISTRATION_DESCRIPTOR && len >= 4) {prog_reg_desc = bytestream_get_le32(&p);len -= 4;}p += len;}p += program_info_length;if (p >= p_end)goto out;// stop parsing after pmt, we found headerif (!ts->pkt)ts->stop_parse = 2;if (prg)prg->pmt_found = 1;for (i = 0; i < MAX_STREAMS_PER_PROGRAM; i++) {st = 0;pes = NULL;stream_type = get8(&p, p_end);if (stream_type < 0)break;pid = get16(&p, p_end);if (pid < 0)goto out;pid &= 0x1fff;if (pid == ts->current_pid)goto out;stream_identifier = parse_stream_identifier_desc(p, p_end) + 1;/* now create stream */if (ts->pids[pid] && ts->pids[pid]->type == MPEGTS_PES) {pes = ts->pids[pid]->u.pes_filter.opaque;if (ts->merge_pmt_versions && !pes->st) {st = find_matching_stream(ts, pid, h->id, stream_identifier, i, &old_program);if (st) {pes->st = st;pes->stream_type = stream_type;pes->merged_st = 1;}}if (!pes->st) {pes->st = avformat_new_stream(pes->stream, NULL);if (!pes->st)goto out;pes->st->id = pes->pid;}st = pes->st;} else if (is_pes_stream(stream_type, prog_reg_desc)) {if (ts->pids[pid])mpegts_close_filter(ts, ts->pids[pid]); // wrongly added sdt filter probablypes = add_pes_stream(ts, pid, pcr_pid);if (ts->merge_pmt_versions && pes && !pes->st) {st = find_matching_stream(ts, pid, h->id, stream_identifier, i, &old_program);if (st) {pes->st = st;pes->stream_type = stream_type;pes->merged_st = 1;}}if (pes && !pes->st) {st = avformat_new_stream(pes->stream, NULL);if (!st)goto out;st->id = pes->pid;}} else {int idx = ff_find_stream_index(ts->stream, pid);if (idx >= 0) {st = ts->stream->streams[idx];}if (ts->merge_pmt_versions && !st) {st = find_matching_stream(ts, pid, h->id, stream_identifier, i, &old_program);}if (!st) {st = avformat_new_stream(ts->stream, NULL);if (!st)goto out;st->id = pid;st->codecpar->codec_type = AVMEDIA_TYPE_DATA;if (stream_type == 0x86 && prog_reg_desc == AV_RL32("CUEI")) {mpegts_find_stream_type(st, stream_type, SCTE_types);mpegts_open_section_filter(ts, pid, scte_data_cb, ts, 1);}}}if (!st)goto out;if (pes && !pes->stream_type)mpegts_set_stream_info(st, pes, stream_type, prog_reg_desc);add_pid_to_program(prg, pid);if (prg) {prg->streams[i].idx = st->index;prg->streams[i].stream_identifier = stream_identifier;prg->nb_streams++;}av_program_add_stream_index(ts->stream, h->id, st->index);desc_list_len = get16(&p, p_end);if (desc_list_len < 0)goto out;desc_list_len &= 0xfff;desc_list_end  = p + desc_list_len;if (desc_list_end > p_end)goto out;for (;;) {if (ff_parse_mpeg2_descriptor(ts->stream, st, stream_type, &p,desc_list_end, mp4_descr,mp4_descr_count, pid, ts) < 0)break;if (pes && prog_reg_desc == AV_RL32("HDMV") &&stream_type == 0x83 && pes->sub_st) {av_program_add_stream_index(ts->stream, h->id,pes->sub_st->index);pes->sub_st->codecpar->codec_tag = st->codecpar->codec_tag;}}p = desc_list_end;}if (!ts->pids[pcr_pid])mpegts_open_pcr_filter(ts, pcr_pid);out:for (i = 0; i < mp4_descr_count; i++)av_free(mp4_descr[i].dec_config_descr);
}

该函数的作用是：解析TS流中的TS program map section，提取出里面的属性。

形参filter：输出型参数，指向一个MpegTSFilter类型变量。执行pmt_cb函数后，(PESContext *)(((MpegTSContext *)(filter->u.section_filter.opaque))->pids[pid]->u.pes_filter.opaque)会得到从TS program map section中解析出来的属性。pid为该节目音频或视频流的PID。

形参section：输入型参数。存放一个TS program map section的数据，即“将一个或多个包含PMT表信息的transport packet（TS包）去掉它们TS Header和pointer_field后的有效数据组合起来后”的数据。

section_len：输入型参数。该TS program map section的长度，单位为字节。

返回值：无

三、pmt_cb函数的内部实现分析

pmt_cb函数中首先通过下面语句让指针p_end指向该TS program map section中有效数据的末尾，即CRC_32属性的开头；让指针p指向该TS program map section的开头：

     const uint8_t *p, *p_end;   
//...    p_end = section + section_len - 4;p     = section;

通过parse_section_header函数解析Section Header，这样指针h就会得到从Section Header中解析出来的属性。关于parse_section_header函数的用法可以参考：《音视频入门基础：MPEG2-TS专题（13）——FFmpeg源码中，解析Section Header的实现》：

    if (parse_section_header(h, &p, p_end) < 0)return;

宏PMT_TID定义如下：

#define PMT_TID         0x02 /* Program Map section */

判断SectionHeader中的table_id属性是否为PMT_TID（0x02），PMT表的table_id固定为0x02，如果不是，表示这不是PMT表，pmt_cb函数直接返回：

    if (h->tid != PMT_TID)return;

判断SectionHeader中的current_next_indicator属性的值，如果值为1，表示发送的TS program map section为当前有效的，pmt_cb函数继续往下执行；值为0表示发送的该Section尚未有效并且下一个Section将生效，pmt_cb函数直接返回：

    if (!h->current_next)return;

h->id是该TS program map section的program_number属性。从《音视频入门基础：MPEG2-TS专题（15）——FFmpeg源码中，解析Program association section的实现》可以知道，FFmpeg源码解析完PAT表的Section后，ts->stream->programs会得到从PAT表的Section中解析出来的属性。所以在pmt_cb函数中通过语句：prg = get_program(ts, h->id)，让变量prg拿到之前保存在ts->stream->programs中的该节目对应的program_number属性和program_map_PID属性的信息：

prg = get_program(ts, h->id);if (prg)old_program = *prg;elseclear_program(&old_program);if (ts->skip_unknown_pmt && !prg)return;if (prg && prg->nb_pids && prg->pids[0] != ts->current_pid)return;if (!ts->skip_clear)clear_avprogram(ts, h->id);clear_program(prg);add_pid_to_program(prg, ts->current_pid);

读取TS program map section中的PCR_PID属性（PCR所在transport packet的PID），赋值给变量pcr_pid：

    pcr_pid = get16(&p, p_end);if (pcr_pid < 0)return;pcr_pid &= 0x1fff;

将上述得到的PCR_PID属性赋值给ts->stream->programs[i]->pcr_pid，将version_number属性赋值给ts->stream->programs[i]->pmt_version，i为该节目是TS流中的第几个节目：

    add_pid_to_program(prg, pcr_pid);update_av_program_info(ts->stream, h->id, pcr_pid, h->version);

读取TS program map section中的program_info_length属性，赋值给变量program_info_length：

    program_info_length = get16(&p, p_end);if (program_info_length < 0)return;program_info_length &= 0xfff;

program_info_length属性不小于2，表示program_info_length属性之后存在节目描述信息，读取节目描述信息：

    while (program_info_length >= 2) {uint8_t tag, len;tag = get8(&p, p_end);len = get8(&p, p_end);av_log(ts->stream, AV_LOG_TRACE, "program tag: 0x%02x len=%d\n", tag, len);program_info_length -= 2;if (len > program_info_length)// something else is broken, exit the program_descriptors_loopbreak;program_info_length -= len;if (tag == IOD_DESCRIPTOR) {get8(&p, p_end); // scopeget8(&p, p_end); // labellen -= 2;mp4_read_iods(ts->stream, p, len, mp4_descr + mp4_descr_count,&mp4_descr_count, MAX_MP4_DESCR_COUNT);} else if (tag == REGISTRATION_DESCRIPTOR && len >= 4) {prog_reg_desc = bytestream_get_le32(&p);len -= 4;}p += len;}

循环读取TS program map section中的stream_type属性（媒体流的类型），赋值给变量stream_type：

    for (i = 0; i < MAX_STREAMS_PER_PROGRAM; i++) {st = 0;pes = NULL;stream_type = get8(&p, p_end);if (stream_type < 0)break;//...
}

循环读取TS program map section中的elementary_PID属性（节目的音频或视频PID），赋值给变量pid：

        pid = get16(&p, p_end);if (pid < 0)goto out;pid &= 0x1fff;

将上述得到的elementary_PID属性和PCR_PID属性保存到指针pes指向的PESContext结构中：

else if (is_pes_stream(stream_type, prog_reg_desc)) {if (ts->pids[pid])mpegts_close_filter(ts, ts->pids[pid]); // wrongly added sdt filter probablypes = add_pes_stream(ts, pid, pcr_pid);if (ts->merge_pmt_versions && pes && !pes->st) {st = find_matching_stream(ts, pid, h->id, stream_identifier, i, &old_program);if (st) {pes->st = st;pes->stream_type = stream_type;pes->merged_st = 1;}}if (pes && !pes->st) {st = avformat_new_stream(pes->stream, NULL);if (!st)goto out;st->id = pes->pid;}} 

将上述得到的stream_type属性保存到指针pes指向的PESContext结构中：

        if (pes && !pes->stream_type)mpegts_set_stream_info(st, pes, stream_type, prog_reg_desc);

将上述得到的elementary_PID属性保存到指针prg指向的内存中。由于prg等于((MpegTSContext *)(filter->u.section_filter.opaque))->prg，而上述代码中又存在：MpegTSContext *ts = filter->u.section_filter.opaque 以及 pes = ts->pids[pid]->u.pes_filter.opaque。所以执行pmt_cb函数后，(PESContext *)(((MpegTSContext *)(filter->u.section_filter.opaque))->pids[pid]->u.pes_filter.opaque)会得到从TS program map section中解析出来的属性，pid为该节目音频或视频流的PID：

        add_pid_to_program(prg, pid);if (prg) {prg->streams[i].idx = st->index;prg->streams[i].stream_identifier = stream_identifier;prg->nb_streams++;}