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【FFMPEG源码分析】从ffplay源码摸清ffmpeg框架(二)

news 2025/9/16 4:55:53

demux模块

从前面一篇文章中可以得知，demux模块的使用方法大致如下:

分配AVFormatContext
通过avformat_open_input(…)传入AVFormatContext指针和文件路径，启动demux
通过av_read_frame(…) 从AVFormatContext中读取demux后的audio/video/subtitle数据包AVPacket

AVFormatContext *ic = avformat_alloc_context();
avformat_open_input(&ic, filename, null, null);
while(1) {AVPacket *pkt = av_packet_alloc();av_read_frame(ic, pkt);..... use pkt data to do something.........;
}

在阅读源码之前，我们先提几个问题，再顺着问题阅读源码:

AVFormatContext如何下载数据 ?
如何匹配到具体的demuxer ？
demuxer的模板是什么样的？如何新增一个demuxer ?
demuxer是如何驱动起来的？

下面我们分别看下avformat_alloc_context(…), avformat_open_input(…), av_read_frame(…)分别做了什么？在文章结尾看看能否回答上面这几个问题。

avformat_alloc_context

ffmpeg\libavformat\option.c
AVFormatContext *avformat_alloc_context(void)
{FFFormatContext *const si = av_mallocz(sizeof(*si));AVFormatContext *s;s = &si->pub;s->av_class = &av_format_context_class;s->io_open  = io_open_default;s->io_close = ff_format_io_close_default;s->io_close2= io_close2_default;av_opt_set_defaults(s);si->pkt = av_packet_alloc();si->parse_pkt = av_packet_alloc();si->shortest_end = AV_NOPTS_VALUE;return s;
}

上面代码中比较重要的函数是io_open，这个函数会创建AVIOContext数据下载模块。

int (*io_open)(struct AVFormatContext *s, AVIOContext **pb, const char *url,int flags, AVDictionary **options);

avformat_open_input

ffmpeg\libavformat\demux.c
int avformat_open_input(AVFormatContext **ps, const char *filename,const AVInputFormat *fmt, AVDictionary **options)
{AVFormatContext *s = *ps;FFFormatContext *si;AVDictionary *tmp = NULL;ID3v2ExtraMeta *id3v2_extra_meta = NULL;int ret = 0;//如果外部没有传入AVFormatContext，则在此分配if (!s && !(s = avformat_alloc_context()))return AVERROR(ENOMEM);FFFormatContext *si = ffformatcontext(s);//如果外部有传入AVInputFormat，则直接使用，否则后面init_input中会进行分配if (fmt)s->iformat = fmt;..........................................;//重点函数，下面会深入细节进行分析if ((ret = init_input(s, filename, &tmp)) < 0)goto fail;s->probe_score = ret;.......................;//下面这一段代码比较关键，AVInputFormat分配之后，再来详细描述if (s->iformat->priv_data_size > 0) {if (!(s->priv_data = av_mallocz(s->iformat->priv_data_size))) {ret = AVERROR(ENOMEM);goto fail;}if (s->iformat->priv_class) {*(const AVClass **) s->priv_data = s->iformat->priv_class;av_opt_set_defaults(s->priv_data);if ((ret = av_opt_set_dict(s->priv_data, &tmp)) < 0)goto fail;}}...........................;//关键函数，用于下载第一笔数据和demux第一笔数据if (s->iformat->read_header)if ((ret = s->iformat->read_header(s)) < 0) {if (s->iformat->flags_internal & FF_FMT_INIT_CLEANUP)goto close;goto fail;}}.......................;//更新codec信息，后面会详细讲解update_stream_avctx(s);......................;return 0;
}

通过上面代码可以梳理出有以下关键步骤:

init_input() 执行后会找到数据下载具体的模块AVIOContext和具体的demux模块AVInputFormat，并进行初始化。
分配AVInputFormat中的priv_data_size，这个是指各各demux模块中私有的一个context结构图，如: ts格式demuxer中的struct MpegTSContext，mov格式demuxer中的struct MOVContext等
AVInputFormat中的read_header() 会开始下载第一笔数据和demux第一笔数据
update_stream_avctx(…) 获取并更新codec信息
下面深入分析下上面的四个步骤.

init_input

ffmpeg\libavformat\demux.c
static int init_input(AVFormatContext *s, const char *filename,AVDictionary **options)
{int ret;AVProbeData pd = { filename, NULL, 0 };int score = AVPROBE_SCORE_RETRY;//使用外部AVIOContext模块，我们不关注这种caseif (s->pb) {........................;return 0;}//这一步因为score不够会获取iformat失败if ((s->iformat && s->iformat->flags & AVFMT_NOFILE) ||(!s->iformat && (s->iformat = av_probe_input_format2(&pd, 0, &score))))return score;//获取AVIOContext s->pbif ((ret = s->io_open(s, &s->pb, filename, AVIO_FLAG_READ | s->avio_flags, options)) < 0)return ret;if (s->iformat)return 0;//在这里重新获取iformatreturn av_probe_input_buffer2(s->pb, &s->iformat, filename,s, 0, s->format_probesize);
}

下面来看下io_open和av_probe_input_buffer2的实现。
io_open赋值的位置:

ffmpeg\libavformat\option.c
AVFormatContext *avformat_alloc_context(void)
{FFFormatContext *const si = av_mallocz(sizeof(*si));AVFormatContext *s;..............;s->io_open  = io_open_default;..............;
}

再来看看io_open_default的具体实现

ffmpeg\libavformat\option.c
static int io_open_default(AVFormatContext *s, AVIOContext **pb,const char *url, int flags, AVDictionary **options)
{..............................;return ffio_open_whitelist(pb, url, flags, &s->interrupt_callback, options, s->protocol_whitelist, s->protocol_blacklist);
}ffmpeg\libavformat\aviobuf.c
int ffio_open_whitelist(AVIOContext **s, const char *filename, int flags,const AVIOInterruptCB *int_cb, AVDictionary **options,const char *whitelist, const char *blacklist)
{URLContext *h;//获取到URLContextffurl_open_whitelist(&h, filename, flags, int_cb, options, whitelist, blacklist, NULL);//获取到AVIOContext ffio_fdopen(s, h);return 0;
}ffmpeg\libavformat\avio.c
int ffurl_open_whitelist(URLContext **puc, const char *filename, int flags,const AVIOInterruptCB *int_cb, AVDictionary **options,const char *whitelist, const char* blacklist,URLContext *parent)
{AVDictionary *tmp_opts = NULL;AVDictionaryEntry *e;ffurl_alloc(puc, filename, flags, int_cb);........set some options .......;ffurl_connect(*puc, options);.............;
}
// ffurl_alloc  ---------------  start  ----------------------
ffmpeg\libavformat\avio.c
int ffurl_alloc(URLContext **puc, const char *filename, int flags,const AVIOInterruptCB *int_cb)
{const URLProtocol *p = NULL;//通过url链接找到对应的下载数据的protocolp = url_find_protocol(filename);if (p)return url_alloc_for_protocol(puc, p, filename, flags, int_cb);*puc = NULL;return AVERROR_PROTOCOL_NOT_FOUND;
}ffmpeg\libavformat\avio.c
static const struct URLProtocol *url_find_protocol(const char *filename)
{const URLProtocol **protocols;char proto_str[128], proto_nested[128], *ptr;size_t proto_len = strspn(filename, URL_SCHEME_CHARS);int i;//通过filename字符串找到proto_str，即是什么协议if (filename[proto_len] != ':' &&(strncmp(filename, "subfile,", 8) || !strchr(filename + proto_len + 1, ':')) ||is_dos_path(filename))strcpy(proto_str, "file");elseav_strlcpy(proto_str, filename,FFMIN(proto_len + 1, sizeof(proto_str)));av_strlcpy(proto_nested, proto_str, sizeof(proto_nested));if ((ptr = strchr(proto_nested, '+')))*ptr = '\0';//获取到配置好的所有URLProtocol列表//ffmpeg通过config来配置支持哪些protocol，编译之前config时会生成libavformat/protocol_list.c//里面会定义一个静态的全局数组url_protocols//static const URLProtocol * const url_protocols[] = {//              &ff_http_protocol,//              &ff_https_protocol,//              &ff_tcp_protocol,//              &ff_tls_protocol,//              NULL };protocols = ffurl_get_protocols(NULL, NULL);if (!protocols)return NULL;for (i = 0; protocols[i]; i++) {const URLProtocol *up = protocols[i];//通过URLProtocol的name字段与proto_str进行匹配//如: const URLProtocol ff_http_protocol = {//                         .name  = "http",//                         ........//     }// const URLProtocol ff_tcp_protocol = {//     .name                = "tcp",//     ........// }if (!strcmp(proto_str, up->name)) {av_freep(&protocols);return up;}if (up->flags & URL_PROTOCOL_FLAG_NESTED_SCHEME &&!strcmp(proto_nested, up->name)) {av_freep(&protocols);return up;}}av_freep(&protocols);if (av_strstart(filename, "https:", NULL) || av_strstart(filename, "tls:", NULL))av_log(NULL, AV_LOG_WARNING, "https protocol not found, recompile FFmpeg with ""openssl, gnutls or securetransport enabled.\n");return NULL;
}得到URLProtocol后再生成URLContext
static int url_alloc_for_protocol(URLContext **puc, const URLProtocol *up,const char *filename, int flags,const AVIOInterruptCB *int_cb)
{URLContext *uc;int err;...................;//分配URLContextuc = av_mallocz(sizeof(URLContext) + strlen(filename) + 1);..................;uc->av_class = &ffurl_context_class;uc->filename = (char *)&uc[1];strcpy(uc->filename, filename);uc->prot            = up;uc->flags           = flags;uc->is_streamed     = 0; /* default = not streamed */uc->max_packet_size = 0; /* default: stream file */if (up->priv_data_size) {//分配网络协议模块自己的context结构体，如struct TCPContext，struct HTTPContextuc->priv_data = av_mallocz(up->priv_data_size);...................;if (up->priv_data_class) {char *start;*(const AVClass **)uc->priv_data = up->priv_data_class;av_opt_set_defaults(uc->priv_data);..................;}}if (int_cb)uc->interrupt_callback = *int_cb;*puc = uc;return 0;
....................;
}
至此URLContext和URLProtocol都已经得到了。其关系为URLContext.prot为其对应的URLProtocol
// ffurl_alloc  ---------------  end ----------------------
得到URLContext后再看看ffurl_connect做了什么?
// ffurl_connect---------------  start ----------------------
int ffurl_connect(URLContext *uc, AVDictionary **options)
{..............................;//调用具体的protocol开始下载数据err =uc->prot->url_open2 ? uc->prot->url_open2(uc,uc->filename,uc->flags,options) :uc->prot->url_open(uc, uc->filename, uc->flags);.......................;return 0;
}
// ffurl_connect---------------  end ----------------------
再来看看ffio_fdopen 如何分配AVIOContext
int ffio_fdopen(AVIOContext **s, URLContext *h)
{uint8_t *buffer = NULL;buffer = av_malloc(buffer_size);*s = avio_alloc_context(buffer, buffer_size, h->flags & AVIO_FLAG_WRITE, h,(int (*)(void *, uint8_t *, int))  ffurl_read,(int (*)(void *, uint8_t *, int))  ffurl_write,(int64_t (*)(void *, int64_t, int))ffurl_seek);(*s)->protocol_whitelist = av_strdup(h->protocol_whitelist);(*s)->protocol_blacklist = av_strdup(h->protocol_blacklist);if(h->prot) {(*s)->read_pause = (int (*)(void *, int))h->prot->url_read_pause;(*s)->read_seek  =(int64_t (*)(void *, int, int64_t, int))h->prot->url_read_seek;if (h->prot->url_read_seek)(*s)->seekable |= AVIO_SEEKABLE_TIME;}((FFIOContext*)(*s))->short_seek_get = (int (*)(void *))ffurl_get_short_seek;(*s)->av_class = &ff_avio_class;return 0;
}

io_open_default总结下其主要工作:

通过播放文件的链接获取到具体的协议如:http/https/tcp等，然后在libavformat/protocol_list.c中定义一个静态的全局数组url_protocols遍历，通过协议名称匹配到对应的URLProtocol
分配生成URLContext，并通过URLProtocol.priv_data_size分配具体协议的context，如:struct TCPContext，struct HTTPContext,并将URLProtocol赋值给URLContext.proto字段。
取URLContext得后，调用ffurl_connect(…) 调用URLProtocol.url_open()进行初始化准备下载数据
通过ffio_fdopen(…) 分配AVIOContext， AVIOContext.opaque = URLContext
最终将AVIOContext赋值给AVFormatContext.pb 字段

下面再来看看av_probe_input_buffer2(…)函数如何找到具体的demux模块

int av_probe_input_buffer2(AVIOContext *pb, const AVInputFormat **fmt,const char *filename, void *logctx,unsigned int offset, unsigned int max_probe_size)
{AVProbeData pd = { filename ? filename : "" };uint8_t *buf = NULL;int ret = 0, probe_size, buf_offset = 0;int score = 0;int ret2;//从AVIOContext 中获取mime_typeif (pb->av_class) {uint8_t *mime_type_opt = NULL;char *semi;av_opt_get(pb, "mime_type", AV_OPT_SEARCH_CHILDREN, &mime_type_opt);pd.mime_type = (const char *)mime_type_opt;semi = pd.mime_type ? strchr(pd.mime_type, ';') : NULL;if (semi) {*semi = '\0';}}for (probe_size = PROBE_BUF_MIN; probe_size <= max_probe_size && !*fmt;probe_size = FFMIN(probe_size << 1,FFMAX(max_probe_size, probe_size + 1))) {score = probe_size < max_probe_size ? AVPROBE_SCORE_RETRY : 0;/* Read probe data. */if ((ret = av_reallocp(&buf, probe_size + AVPROBE_PADDING_SIZE)) < 0)goto fail;//读取probe数据if ((ret = avio_read(pb, buf + buf_offset,probe_size - buf_offset)) < 0) {.......................;}.................;/* Guess file format. */*fmt = av_probe_input_format2(&pd, 1, &score);...................;}.....................;
}const AVInputFormat *av_probe_input_format2(const AVProbeData *pd,int is_opened, int *score_max)
{int score_ret;const AVInputFormat *fmt = av_probe_input_format3(pd, is_opened, &score_ret);if (score_ret > *score_max) {*score_max = score_ret;return fmt;} elsereturn NULL;
}const AVInputFormat *av_probe_input_format3(const AVProbeData *pd,int is_opened, int *score_ret)
{AVProbeData lpd = *pd;const AVInputFormat *fmt1 = NULL;const AVInputFormat *fmt = NULL;int score, score_max = 0;//通过从AVIOContext中读取的probe buffer来判断nodat值if (lpd.buf_size > 10 && ff_id3v2_match(lpd.buf, ID3v2_DEFAULT_MAGIC)) {if (lpd.buf_size > id3len + 16) {nodat = ID3_ALMOST_GREATER_PROBE;} else if (id3len >= PROBE_BUF_MAX) {nodat = ID3_GREATER_MAX_PROBE;} elsenodat = ID3_GREATER_PROBE;}//遍历所有的demuxer，获取得分最高的一个while ((fmt1 = av_demuxer_iterate(&i))) {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)) {score = AVPROBE_SCORE_MIME;}}if (score > score_max) {score_max = score;fmt       = fmt1;} else if (score == score_max)fmt = NULL;}if (nodat == ID3_GREATER_PROBE)score_max = FFMIN(AVPROBE_SCORE_EXTENSION / 2 - 1, score_max);*score_ret = score_max;return fmt;
}

从上面的代码逻辑可以大致看出，选取demuxer的逻辑:

通过ibavformat/demuxer_list.c中配置定义的全局数组demuxer_list,如

static const AVInputFormat * const demuxer_list[] = {
&ff_flac_demuxer,
&ff_hls_demuxer,
&ff_matroska_demuxer,
&ff_mov_demuxer,
&ff_mp3_demuxer,
&ff_mpegts_demuxer,
&ff_ogg_demuxer,
&ff_wav_demuxer,
NULL };

从AVIOContext中获取到数据的prop如: mime_type, 后缀名以及其他prop与每一个AVInputFormat进行匹配，获取一个得分最高的AVInputFormat

AVFormatContext中的priv_data_size

再来重新看下avformat_open_input()中这段代码的含义

   // s 为AVFormatContext, s->iformat为AVInputFormat类型if (s->iformat->priv_data_size > 0) {if (!(s->priv_data = av_mallocz(s->iformat->priv_data_size))) {ret = AVERROR(ENOMEM);goto fail;}if (s->iformat->priv_class) {*(const AVClass **) s->priv_data = s->iformat->priv_class;av_opt_set_defaults(s->priv_data);if ((ret = av_opt_set_dict(s->priv_data, &tmp)) < 0)goto fail;}}

以ff_mpegts_demuxer为例子，s->iformat->priv_data_size为struct MpegTSContext，因此av_mallocz(s->iformat->priv_data_size)实际分配了一个struct MpegTSContext。
*(const AVClass **) s->priv_data = s->iformat->priv_class;
这句话的实际含义是MpegTSContext.class = s->iformat->priv_class, 即为AVClass mpegts_class

从这里可以看出MpegTSContext 与AVInputFormat之间的关系，AVInputFormat提供demuxer统一接口，MpegTSContext为demuxer接口提供不同的操作上下文。

struct MpegTSContext {const AVClass *class; //这个成员必须是第一个/* user data */AVFormatContext *stream;/** raw packet size, including FEC if present */int raw_packet_size;...........;
};const AVInputFormat ff_mpegts_demuxer = {.name           = "mpegts",.long_name      = NULL_IF_CONFIG_SMALL("MPEG-TS (MPEG-2 Transport Stream)"),.priv_data_size = sizeof(MpegTSContext),.read_probe     = mpegts_probe,.read_header    = mpegts_read_header,.read_packet    = mpegts_read_packet,.read_close     = mpegts_read_close,.read_timestamp = mpegts_get_dts,.flags          = AVFMT_SHOW_IDS | AVFMT_TS_DISCONT,.priv_class     = &mpegts_class,
};static const AVClass mpegts_class = {.class_name = "mpegts demuxer",.item_name  = av_default_item_name,.option     = options,.version    = LIBAVUTIL_VERSION_INT,
};

AVInputFormat中的read_header

下面再来看看avformat_open_input(…)中的read_header干了什么?

if (s->iformat->read_header)//s 为AVFormatContext, s->iformat为AVInputFormatif ((ret = s->iformat->read_header(s)) < 0) {if (s->iformat->flags_internal & FF_FMT_INIT_CLEANUP)goto close;goto fail;}const AVInputFormat ff_mpegts_demuxer = {.name           = "mpegts",.........................;.read_header    = mpegts_read_header,.read_packet    = mpegts_read_packet,..................;
};

以ff_mpegts_demuxer为例子，看看read_header具体做了什么?

static int mpegts_read_header(AVFormatContext *s)
{MpegTSContext *ts = s->priv_data;AVIOContext *pb   = s->pb;.................;if (s->iformat == &ff_mpegts_demuxer) {seek_back(s, pb, pos);mpegts_open_section_filter(ts, SDT_PID, sdt_cb, ts, 1);mpegts_open_section_filter(ts, PAT_PID, pat_cb, ts, 1);mpegts_open_section_filter(ts, EIT_PID, eit_cb, ts, 1);handle_packets(ts, probesize / ts->raw_packet_size);/* if could not find service, enable auto_guess */ts->auto_guess = 1;av_log(ts->stream, AV_LOG_TRACE, "tuning done\n");s->ctx_flags |= AVFMTCTX_NOHEADER;} else {.......................;}seek_back(s, pb, pos);return 0;
}

具体再看下handle_packets函数，

static int handle_packets(MpegTSContext *ts, int64_t nb_packets)
{AVFormatContext *s = ts->stream;uint8_t packet[TS_PACKET_SIZE + AV_INPUT_BUFFER_PADDING_SIZE];const uint8_t *data;int64_t packet_num;int ret = 0;..............................;ts->stop_parse = 0;packet_num = 0;memset(packet + TS_PACKET_SIZE, 0, AV_INPUT_BUFFER_PADDING_SIZE);for (;;) {packet_num++;if (nb_packets != 0 && packet_num >= nb_packets ||ts->stop_parse > 1) {ret = AVERROR(EAGAIN);break;}if (ts->stop_parse > 0)break;ret = read_packet(s, packet, ts->raw_packet_size, &data);if (ret != 0)break;ret = handle_packet(ts, data, avio_tell(s->pb));finished_reading_packet(s, ts->raw_packet_size);if (ret != 0)break;}ts->last_pos = avio_tell(s->pb);return ret;
}
主要看下read_packet 和 handle_packet```c
static int read_packet(AVFormatContext *s, uint8_t *buf, int raw_packet_size,const uint8_t **data)
{*AVIOContext *pb = s->pb*;int len;for (;;) {len = ffio_read_indirect(pb, buf, TS_PACKET_SIZE, data);if (len != TS_PACKET_SIZE)return len < 0 ? len : AVERROR_EOF;/* check packet sync byte */if ((*data)[0] != 0x47) {/* find a new packet start */if (mpegts_resync(s, raw_packet_size, *data) < 0)return AVERROR(EAGAIN);elsecontinue;} else {break;}}return 0;
}
int ffio_read_indirect(AVIOContext *s, unsigned char *buf, int size, const unsigned char **data)
{if (s->buf_end - s->buf_ptr >= size && !s->write_flag) {*data = s->buf_ptr;s->buf_ptr += size;return size;} else {*data = buf;return avio_read(s, buf, size);}
}

从上面的代码可以看出read_packet 是通过AVFormatContext.pb即AVIOContext 通过avio_read读取一个pkt。

handle_packet函数暂时还看不懂，先放着，后面再深入分析。

/* handle one TS packet */
static int handle_packet(MpegTSContext *ts, const uint8_t *packet, int64_t pos)
{MpegTSFilter *tss;int len, pid, cc, expected_cc, cc_ok, afc, is_start, is_discontinuity,has_adaptation, has_payload;const uint8_t *p, *p_end;pid = AV_RB16(packet + 1) & 0x1fff;is_start = packet[1] & 0x40;tss = ts->pids[pid];if (ts->auto_guess && !tss && is_start) {add_pes_stream(ts, pid, -1);tss = ts->pids[pid];}if (!tss)return 0;if (is_start)tss->discard = discard_pid(ts, pid);if (tss->discard)return 0;ts->current_pid = pid;afc = (packet[3] >> 4) & 3;if (afc == 0) /* reserved value */return 0;has_adaptation   = afc & 2;has_payload      = afc & 1;is_discontinuity = has_adaptation &&packet[4] != 0 && /* with length > 0 */(packet[5] & 0x80); /* and discontinuity indicated *//* continuity check (currently not used) */cc = (packet[3] & 0xf);expected_cc = has_payload ? (tss->last_cc + 1) & 0x0f : tss->last_cc;cc_ok = pid == 0x1FFF || // null packet PIDis_discontinuity ||tss->last_cc < 0 ||expected_cc == cc;tss->last_cc = cc;if (!cc_ok) {av_log(ts->stream, AV_LOG_DEBUG,"Continuity check failed for pid %d expected %d got %d\n",pid, expected_cc, cc);if (tss->type == MPEGTS_PES) {PESContext *pc = tss->u.pes_filter.opaque;pc->flags |= AV_PKT_FLAG_CORRUPT;}}if (packet[1] & 0x80) {av_log(ts->stream, AV_LOG_DEBUG, "Packet had TEI flag set; marking as corrupt\n");if (tss->type == MPEGTS_PES) {PESContext *pc = tss->u.pes_filter.opaque;pc->flags |= AV_PKT_FLAG_CORRUPT;}}p = packet + 4;if (has_adaptation) {int64_t pcr_h;int pcr_l;if (parse_pcr(&pcr_h, &pcr_l, packet) == 0)tss->last_pcr = pcr_h * 300 + pcr_l;/* skip adaptation field */p += p[0] + 1;}/* 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 PMTif (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) {int types = 0;for (i = 0; i < ts->stream->nb_streams; i++) {AVStream *st = ts->stream->streams[i];if (st->codecpar->codec_type >= 0)types |= 1<<st->codecpar->codec_type;}if ((types & (1<<AVMEDIA_TYPE_AUDIO) && types & (1<<AVMEDIA_TYPE_VIDEO)) || pos > 100000) {av_log(ts->stream, AV_LOG_DEBUG, "All programs have pmt, headers found\n");ts->stream->ctx_flags &= ~AVFMTCTX_NOHEADER;}}}} else {int ret;// Note: The position here points actually behind the current packet.if (tss->type == MPEGTS_PES) {if ((ret = tss->u.pes_filter.pes_cb(tss, p, p_end - p, is_start,pos - ts->raw_packet_size)) < 0)return ret;}}return 0;
}

update_stream_avctx

static int update_stream_avctx(AVFormatContext *s)
{int ret;for (unsigned i = 0; i < s->nb_streams; i++) {AVStream *const st  = s->streams[i];FFStream *const sti = ffstream(st);.......................;ret = avcodec_parameters_to_context(sti->avctx, st->codecpar);sti->need_context_update = 0;}return 0;
}
//将AVStream中的codecpar信息赋值给FFStream中的AVCodecContext
int avcodec_parameters_to_context(AVCodecContext *codec,const AVCodecParameters *par)
{int ret;codec->codec_type = par->codec_type;codec->codec_id   = par->codec_id;codec->codec_tag  = par->codec_tag;codec->bit_rate              = par->bit_rate;codec->bits_per_coded_sample = par->bits_per_coded_sample;codec->bits_per_raw_sample   = par->bits_per_raw_sample;codec->profile               = par->profile;codec->level                 = par->level;switch (par->codec_type) {case AVMEDIA_TYPE_VIDEO:codec->pix_fmt                = par->format;codec->width                  = par->width;.............;break;case AVMEDIA_TYPE_AUDIO:codec->sample_fmt       = par->format;codec->sample_rate      = par->sample_rate;codec->block_align      = par->block_align;codec->frame_size       = par->frame_size;..............;break;case AVMEDIA_TYPE_SUBTITLE:codec->width  = par->width;codec->height = par->height;break;}......................;return 0;
}

av_read_frame

int av_read_frame(AVFormatContext *s, AVPacket *pkt)
{FFFormatContext *const si = ffformatcontext(s);const int genpts = s->flags & AVFMT_FLAG_GENPTS;int eof = 0;int ret;AVStream *st;.........................;for (;;) {.....................;read_frame_internal(s, pkt);.....................;}........................;return ret;
}
static int read_frame_internal(AVFormatContext *s, AVPacket *pkt)
{FFFormatContext *const si = ffformatcontext(s);int ret, got_packet = 0;AVDictionary *metadata = NULL;while (!got_packet && !si->parse_queue.head) {AVStream *st;FFStream *sti;/* read next packet */ret = ff_read_packet(s, pkt);..............................;}return ret;
}int ff_read_packet(AVFormatContext *s, AVPacket *pkt)
{FFFormatContext *const si = ffformatcontext(s);int err;for (;;) {PacketListEntry *pktl = si->raw_packet_buffer.head;AVStream *st;FFStream *sti;const AVPacket *pkt1;...................;err = s->iformat->read_packet(s, pkt);......................;}......................;
}
以MpegTSContext为例子,看看pkt是如何存储，如何被读出来的```c
static int mpegts_read_packet(AVFormatContext *s, AVPacket *pkt)
{MpegTSContext *ts = s->priv_data;int ret, i;pkt->size = -1;ts->pkt = pkt;//这个函数中会做具体的demux动作ret = handle_packets(ts, 0);..........................;return ret;
}
static int handle_packets(MpegTSContext *ts, int64_t nb_packets)
{AVFormatContext *s = ts->stream;uint8_t packet[TS_PACKET_SIZE + AV_INPUT_BUFFER_PADDING_SIZE];const uint8_t *data;int64_t packet_num;int ret = 0;..........................;ts->stop_parse = 0;packet_num = 0;memset(packet + TS_PACKET_SIZE, 0, AV_INPUT_BUFFER_PADDING_SIZE);for (;;) {packet_num++;if (nb_packets != 0 && packet_num >= nb_packets ||ts->stop_parse > 1) {ret = AVERROR(EAGAIN);break;}if (ts->stop_parse > 0)break;//从网络io中读取未demux的数据，存储在data变量中ret = read_packet(s, packet, ts->raw_packet_size, &data);if (ret != 0)break;//在此函数中将data中的数据demux之后，存放至MpegTSContext.pktret = handle_packet(ts, data, avio_tell(s->pb));finished_reading_packet(s, ts->raw_packet_size);if (ret != 0)break;}ts->last_pos = avio_tell(s->pb);return ret;
}

问题解答

AVFormatContext如何下载数据 ?
下图中蓝色线路图表示数据下载过程，即通过AVIOContext下载。
如何匹配到具体的demuxer ？
读取media数据开头一段数据，遍历libavformat/demuxer_list.c中配置定义的全局数组demuxer_list，将此数据通过每一个demuer的read_probe(…)解析匹配得到一个socre或者通过mime_type和文件后缀名得到socre，取socre最高的一个作为当前数据的demuxer模块。
demuxer的模板是什么样的？如何新增一个demuxer ?
在libavformat/demuxer_list.c中增加一个ff_yxyts_demuxer
如:

static const AVInputFormat * const demuxer_list[] = {
&ff_flac_demuxer,
&ff_hls_demuxer,
&ff_yxyts_demuxer,
NULL };
struct YxyTSContext {const AVClass *class; //这个成员必须是第一个/* user data */AVFormatContext *stream;/** raw packet size, including FEC if present */int raw_packet_size;...........;
};
const AVInputFormat ff_yxyts_demuxer = {.name           = "mpegts",.long_name      = NULL_IF_CONFIG_SMALL("MPEG-TS (MPEG-2 Transport Stream)"),.priv_data_size = sizeof(YxyTSContext),.read_probe     = yxyts_probe,.read_header    = yxyts_read_header,.read_packet    = yxyts_read_packet,.read_close     = yxyts_read_close,.read_timestamp = yxyts_get_dts,.flags          = AVFMT_SHOW_IDS | AVFMT_TS_DISCONT,.priv_class     = &yxyts_class,
};

demuxer是如何驱动起来的？
从下图中可以看出，需要上层应用主动调用av_read_frame(…)获取源数据并demux，然后将demux出来的audio/video的info信息存入AVStream中，具体的audio/video数据通过AVPacket返回给上层应用。