音视频入门基础：MPEG2-TS专题（12）—— FFmpeg源码中，使用Section把各个transport packet组合起来的实现

一、引言

从《音视频入门基础：MPEG2-TS专题（9）——FFmpeg源码中，解码TS Header的实现》可以知道：FFmpeg源码中使用handle_packet函数来处理一个transport packet（TS包），该函数的前半部分实现解析一个transport packet的TS Header。而在解析完TS Header后，handle_packet函数内部会调用write_section_data函数来把各个transport packet组合起来，并调用对应的解析PSI/SI表的方法：

/* handle one TS packet */
static int handle_packet(MpegTSContext *ts, const uint8_t *packet, int64_t pos)
{
//...
    /* if past the end of packet, ignore */
    p_end = packet + TS_PACKET_SIZE;
    if (p >= p_end || !has_payload)
        return 0;

    if (pos >= 0) {
        av_assert0(pos >= TS_PACKET_SIZE);
        ts->pos47_full = pos - TS_PACKET_SIZE;
    }

    if (tss->type == MPEGTS_SECTION) {
        if (is_start) {
            /* pointer field present */
            len = *p++;
            if (len > p_end - p)
                return 0;
            if (len && cc_ok) {
                /* write remaining section bytes */
                write_section_data(ts, tss,
                                   p, len, 0);
                /* check whether filter has been closed */
                if (!ts->pids[pid])
                    return 0;
            }
            p += len;
            if (p < p_end) {
                write_section_data(ts, tss,
                                   p, p_end - p, 1);
            }
        } else {
            if (cc_ok) {
                write_section_data(ts, tss,
                                   p, p_end - p, 0);
            }
        }

        // stop find_stream_info from waiting for more streams
        // when all programs have received a PMT
        if (ts->stream->ctx_flags & AVFMTCTX_NOHEADER && ts->scan_all_pmts <= 0) {
            int i;
            for (i = 0; i < ts->nb_prg; i++) {
                if (!ts->prg[i].pmt_found)
                    break;
            }
            if (i == ts->nb_prg && ts->nb_prg > 0) {
                av_log(ts->stream, AV_LOG_DEBUG, "All programs have pmt, headers found\n");
                ts->stream->ctx_flags &= ~AVFMTCTX_NOHEADER;
            }
        }

    } 
//...
}

上述代码中，首先让指针p_end指向该transport packet的末尾：

    /* if past the end of packet, ignore */
    p_end = packet + TS_PACKET_SIZE;

如果已经读取到了该transport packet的末尾（p >= p_end）或者载荷不存在（!has_payload），handle_packet函数直接返回，不继续进行处理：

    if (p >= p_end || !has_payload)
        return 0;

如果该transport packet不是PES分组，是Section数据（tss->type == MPEGTS_SECTION），并且TS Header中的payload_unit_start_indicator属性的值为1（is_start为真），表示该transport packet是一个Section的首包，这时TS Header后面还会有一个长度为1字节的pointer_field属性，通过语句：len = *p++读取该pointer_field属性，让指针p指向该transport packet的有效数据。根据有没有pointer_field属性和pointer_field的值是多少，调用write_section_data函数并传入不同参数：

    if (tss->type == MPEGTS_SECTION) {
        if (is_start) {
            /* pointer field present */
            len = *p++;
            if (len > p_end - p)
                return 0;
            if (len && cc_ok) {
                /* write remaining section bytes */
                write_section_data(ts, tss,
                                   p, len, 0);
                /* check whether filter has been closed */
                if (!ts->pids[pid])
                    return 0;
            }
            p += len;
            if (p < p_end) {
                write_section_data(ts, tss,
                                   p, p_end - p, 1);
            }
        }
    //...
}

二、write_section_data函数

（一）write_section_data函数的定义

​​​​write_section_data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：

/**
 *  Assemble PES packets out of TS packets, and then call the "section_cb"
 *  function when they are complete.
 */
static void write_section_data(MpegTSContext *ts, MpegTSFilter *tss1,
                               const uint8_t *buf, int buf_size, int is_start)
{
    MpegTSSectionFilter *tss = &tss1->u.section_filter;
    uint8_t *cur_section_buf = NULL;
    int len, offset;

    if (is_start) {
        memcpy(tss->section_buf, buf, buf_size);
        tss->section_index = buf_size;
        tss->section_h_size = -1;
        tss->end_of_section_reached = 0;
    } else {
        if (tss->end_of_section_reached)
            return;
        len = MAX_SECTION_SIZE - tss->section_index;
        if (buf_size < len)
            len = buf_size;
        memcpy(tss->section_buf + tss->section_index, buf, len);
        tss->section_index += len;
    }

    offset = 0;
    cur_section_buf = tss->section_buf;
    while (cur_section_buf - tss->section_buf < MAX_SECTION_SIZE && cur_section_buf[0] != 0xff) {
        /* compute section length if possible */
        if (tss->section_h_size == -1 && tss->section_index - offset >= 3) {
            len = (AV_RB16(cur_section_buf + 1) & 0xfff) + 3;
            if (len > MAX_SECTION_SIZE)
                return;
            tss->section_h_size = len;
        }

        if (tss->section_h_size != -1 &&
            tss->section_index >= offset + tss->section_h_size) {
            int crc_valid = 1;
            tss->end_of_section_reached = 1;

            if (tss->check_crc) {
                crc_valid = !av_crc(av_crc_get_table(AV_CRC_32_IEEE), -1, cur_section_buf, tss->section_h_size);
                if (tss->section_h_size >= 4)
                    tss->crc = AV_RB32(cur_section_buf + tss->section_h_size - 4);

                if (crc_valid) {
                    ts->crc_validity[ tss1->pid ] = 100;
                }else if (ts->crc_validity[ tss1->pid ] > -10) {
                    ts->crc_validity[ tss1->pid ]--;
                }else
                    crc_valid = 2;
            }
            if (crc_valid) {
                tss->section_cb(tss1, cur_section_buf, tss->section_h_size);
                if (crc_valid != 1)
                    tss->last_ver = -1;
            }

            cur_section_buf += tss->section_h_size;
            offset += tss->section_h_size;
            tss->section_h_size = -1;
        } else {
            tss->section_h_size = -1;
            tss->end_of_section_reached = 0;
            break;
        }
    }
}

该函数的作用是：把各个transport packet组合起来，并调用对应的解析PSI/SI表的方法。

形参ts：既是输入型参数也是输出型参数，指向一个MpegTSContext类型变量。

形参tss1：既是输入型参数也是输出型参数，指向一个MpegTSFilter类型变量。

形参buf：指针，输入型参数，指向某个transport packet去掉TS Header和pointer_field后的有效数据。

形参buf_size：输入型参数，该transport packet有效数据的长度，单位为字节。

形参is_start：输入型参数，该transport packet是否为一个Section的首包。值为1表示是，值为0表示否。

返回值：无

（二）write_section_data函数的内部实现

write_section_data函数中，首先判断该transport packet是否为一个Section的首包。如果是，通过语句：memcpy(tss->section_buf, buf, buf_size）将该transport packet去掉TS Header和pointer_field后的有效数据拷贝到tss->section_buf中。tss->section_buf存放一个Section的数据，一个Section可能包含一个或多个transport packet。tss->section_index为该Section的累计长度，通过语句：tss->section_index = buf_size让该Section的累计长度等于该transport packet有效数据的长度：

    if (is_start) {
        memcpy(tss->section_buf, buf, buf_size);
        tss->section_index = buf_size;
        tss->section_h_size = -1;
        tss->end_of_section_reached = 0;
    }

如果该transport packet不是一个Section的首包，并且还未到达该Section的末尾，通过语句：memcpy(tss->section_buf + tss->section_index, buf, len)将该transport packet的有效数据拼接到tss->section_buf的末尾。通过语句：tss->section_index += len让该Section的累计长度增加：

    if (is_start) {
    //...
    } else {
        if (tss->end_of_section_reached)
            return;
        len = MAX_SECTION_SIZE - tss->section_index;
        if (buf_size < len)
            len = buf_size;
        memcpy(tss->section_buf + tss->section_index, buf, len);
        tss->section_index += len;
    }

读取该Section的Section Header中的section_length属性，section_length属性的值加3就是整个Section的长度，将整个Section的长度赋值给变量len和tss->section_h_size：

        /* compute section length if possible */
        if (tss->section_h_size == -1 && tss->section_index - offset >= 3) {
            len = (AV_RB16(cur_section_buf + 1) & 0xfff) + 3;
            if (len > MAX_SECTION_SIZE)
                return;
            tss->section_h_size = len;
        }

如果已经读取到该Section的末尾，并且需要检查CRC校验（tss->check_crc为真），通过语句：crc_valid = !av_crc(av_crc_get_table(AV_CRC_32_IEEE), -1, cur_section_buf, tss->section_h_size)判断该Section的CRC校验是否正确（关于av_crc函数用法可以参考：《FFmpeg源码中，计算CRC校验的实现》）。通过语句：tss->crc = AV_RB32(cur_section_buf + tss->section_h_size - 4)获取到该Section的CRC校验：

        if (tss->section_h_size != -1 &&
            tss->section_index >= offset + tss->section_h_size) {
            int crc_valid = 1;
            tss->end_of_section_reached = 1;

            if (tss->check_crc) {
                crc_valid = !av_crc(av_crc_get_table(AV_CRC_32_IEEE), -1, cur_section_buf, tss->section_h_size);
                if (tss->section_h_size >= 4)
                    tss->crc = AV_RB32(cur_section_buf + tss->section_h_size - 4);

                if (crc_valid) {
                    ts->crc_validity[ tss1->pid ] = 100;
                }else if (ts->crc_validity[ tss1->pid ] > -10) {
                    ts->crc_validity[ tss1->pid ]--;
                }else
                    crc_valid = 2;
            }

如果CRC校验正确，根据Section类型调用对应的解析PSI/SI表的方法。tss->section_cb是函数指针，指向不同PSI/SI表的解析函数，比如SDT表对应的解析函数是sdt_cb，PAT表对应的解析函数是pat_cb，PMT表对应的解析函数是pmt_cb：

            if (crc_valid) {
                tss->section_cb(tss1, cur_section_buf, tss->section_h_size);
                if (crc_valid != 1)
                    tss->last_ver = -1;
            }