FFMPeg代码分析：av_read_frame()函数的内部构造
上文中贴出了av_read_frame()函数的实现，现在更细致地分析一下其内部的实现流程。
av_read_frame()开始后，通常会调用read_frame_internal(s, pkt)函数：
static int read_frame_internal(AVFormatContext *s, AVPacket *pkt)
{
int ret = 0, i, got_packet = 0;
av_init_packet(pkt);
while (!got_packet && !s->parse_queue) {
AVStream *st;
AVPacket cur_pkt;
/* read next packet */
ret = ff_read_packet(s, &cur_pkt);
if (ret < 0) {
if (ret == AVERROR(EAGAIN))
return ret;
/* flush the parsers */
for(i = 0; i < s->nb_streams; i++) {
st = s->streams[i];
if (st->parser && st->need_parsing)
parse_packet(s, NULL, st->index);
}
/* all remaining packets are now in parse_queue =>
* really terminate parsing */
break;
}
ret = 0;
st = s->streams[cur_pkt.stream_index];
if (cur_pkt.pts != AV_NOPTS_VALUE &&
cur_pkt.dts != AV_NOPTS_VALUE &&
cur_pkt.pts < cur_pkt.dts) {
av_log(s, AV_LOG_WARNING, "Invalid timestamps stream=%d, pts=%s, dts=%s, size=%d\n",
cur_pkt.stream_index,
av_ts2str(cur_pkt.pts),
av_ts2str(cur_pkt.dts),
cur_pkt.size);
}
if (s->debug & FF_FDEBUG_TS)
av_log(s, AV_LOG_DEBUG, "ff_read_packet stream=%d, pts=%s, dts=%s, size=%d, duration=%d, flags=%d\n",
cur_pkt.stream_index,
av_ts2str(cur_pkt.pts),
av_ts2str(cur_pkt.dts),
cur_pkt.size,
cur_pkt.duration,
cur_pkt.flags);
if (st->need_parsing && !st->parser && !(s->flags & AVFMT_FLAG_NOPARSE)) {
st->parser = av_parser_init(st->codec->codec_id);
if (!st->parser) {
av_log(s, AV_LOG_VERBOSE, "parser not found for codec "
"%s, packets or times may be invalid.\n",
avcodec_get_name(st->codec->codec_id));
/* no parser available: just output the raw packets */
st->need_parsing = AVSTREAM_PARSE_NONE;
} else if(st->need_parsing == AVSTREAM_PARSE_HEADERS) {
st->parser->flags |= PARSER_FLAG_COMPLETE_FRAMES;
} else if(st->need_parsing == AVSTREAM_PARSE_FULL_ONCE) {
st->parser->flags |= PARSER_FLAG_ONCE;
} else if(st->need_parsing == AVSTREAM_PARSE_FULL_RAW) {
st->parser->flags |= PARSER_FLAG_USE_CODEC_TS;
}
}
if (!st->need_parsing || !st->parser) {
/* no parsing needed: we just output the packet as is */
*pkt = cur_pkt;
compute_pkt_fields(s, st, NULL, pkt);
if ((s->iformat->flags & AVFMT_GENERIC_INDEX) &&
(pkt->flags & AV_PKT_FLAG_KEY) && pkt->dts != AV_NOPTS_VALUE) {
ff_reduce_index(s, st->index);
av_add_index_entry(st, pkt->pos, pkt->dts, 0, 0, AVINDEX_KEYFRAME);
}
got_packet = 1;
} else if (st->discard < AVDISCARD_ALL) {
if ((ret = parse_packet(s, &cur_pkt, cur_pkt.stream_index)) < 0)
return ret;
} else {
/* free packet */
av_free_packet(&cur_pkt);
}
if (pkt->flags & AV_PKT_FLAG_KEY)
st->skip_to_keyframe = 0;
if (st->skip_to_keyframe) {
av_free_packet(&cur_pkt);
if (got_packet) {
*pkt = cur_pkt;
}
got_packet = 0;
}
}
if (!got_packet && s->parse_queue)
ret = read_from_packet_buffer(&s->parse_queue, &s->parse_queue_end, pkt);
if(s->debug & FF_FDEBUG_TS)
av_log(s, AV_LOG_DEBUG, "read_frame_internal stream=%d, pts=%s, dts=%s, size=%d, duration=%d, flags=%d\n",
pkt->stream_index,
av_ts2str(pkt->pts),
av_ts2str(pkt->dts),
pkt->size,
pkt->duration,
pkt->flags);
return ret;
}首先调用av_init_packet(pkt)对pkt进行初始化：
void av_init_packet(AVPacket *pkt)
{
pkt->pts = AV_NOPTS_VALUE;
pkt->dts = AV_NOPTS_VALUE;
pkt->pos = -1;
pkt->duration = 0;
pkt->convergence_duration = 0;
pkt->flags = 0;
pkt->stream_index = 0;
#if FF_API_DESTRUCT_PACKET
FF_DISABLE_DEPRECATION_WARNINGS
pkt->destruct = NULL;
FF_ENABLE_DEPRECATION_WARNINGS
#endif
pkt->buf = NULL;
pkt->side_data = NULL;
pkt->side_data_elems = 0;
}该函数将pts、dts设为AV_NOPTS_VALUE，将pos初始化为-1，将其他参数设为0或空值；随后在一个while循环中，调用ff_read_packet函数读取数据：
int ff_read_packet(AVFormatContext *s, AVPacket *pkt)
{
int ret, i, err;
AVStream *st;
for(;;){
AVPacketList *pktl = s->raw_packet_buffer;
if (pktl) {
*pkt = pktl->pkt;
st = s->streams[pkt->stream_index];
if (s->raw_packet_buffer_remaining_size <= 0) {
if ((err = probe_codec(s, st, NULL)) < 0)
return err;
}
if(st->request_probe <= 0){
s->raw_packet_buffer = pktl->next;
s->raw_packet_buffer_remaining_size += pkt->size;
av_free(pktl);
return 0;
}
}
pkt->data = NULL;
pkt->size = 0;
av_init_packet(pkt);
ret= s->iformat->read_packet(s, pkt);
if (ret < 0) {
if (!pktl || ret == AVERROR(EAGAIN))
return ret;
for (i = 0; i < s->nb_streams; i++) {
st = s->streams[i];
if (st->probe_packets) {
if ((err = probe_codec(s, st, NULL)) < 0)
return err;
}
av_assert0(st->request_probe <= 0);
}
continue;
}
if ((s->flags & AVFMT_FLAG_DISCARD_CORRUPT) &&
(pkt->flags & AV_PKT_FLAG_CORRUPT)) {
av_log(s, AV_LOG_WARNING,
"Dropped corrupted packet (stream = %d)\n",
pkt->stream_index);
av_free_packet(pkt);
continue;
}
if(!(s->flags & AVFMT_FLAG_KEEP_SIDE_DATA))
av_packet_merge_side_data(pkt);
if(pkt->stream_index >= (unsigned)s->nb_streams){
av_log(s, AV_LOG_ERROR, "Invalid stream index %d\n", pkt->stream_index);
continue;
}
st= s->streams[pkt->stream_index];
pkt->dts = wrap_timestamp(st, pkt->dts);
pkt->pts = wrap_timestamp(st, pkt->pts);
force_codec_ids(s, st);
/* TODO: audio: time filter; video: frame reordering (pts != dts) */
if (s->use_wallclock_as_timestamps)
pkt->dts = pkt->pts = av_rescale_q(av_gettime(), AV_TIME_BASE_Q, st->time_base);
if(!pktl && st->request_probe <= 0)
return ret;
add_to_pktbuf(&s->raw_packet_buffer, pkt, &s->raw_packet_buffer_end);
s->raw_packet_buffer_remaining_size -= pkt->size;
if ((err = probe_codec(s, st, pkt)) < 0)
return err;
}
}该函数从AVFormatContext指针格式的媒体文件句柄中读取待解码数据，并储存于AVPacket实例中。当读取成功时返回0，读取失败则返回AVERROR值。
该函数调用av_init_packet对参数pkg初始化，并调用iformat的方法read_packet读取数据。在本demo中，AVFormatContext实例中iformat成员为ff_mov_demuxer，如下所示：
AVInputFormat ff_mov_demuxer = {
.name = "mov,mp4,m4a,3gp,3g2,mj2",
.long_name = NULL_IF_CONFIG_SMALL("QuickTime / MOV"),
.priv_data_size = sizeof(MOVContext),
.read_probe = mov_probe,
.read_header = mov_read_header,
.read_packet = mov_read_packet,
.read_close = mov_read_close,
.read_seek = mov_read_seek,
.priv_class = &mov_class,
.flags = AVFMT_NO_BYTE_SEEK,
};其read_packet指针指向mov_read_packet函数，这个函数的内容也比较长，暂时就不贴了，有时间慢慢分析。