ffmpeg开发系列--av_read_frame()

int av_read_frame(AVFormatContext *s, AVPacket *pkt){const int genpts = s->flags & AVFMT_FLAG_GENPTS;　//missing pts?int          eof = 0;int ret;AVStream *st;if (!genpts) {ret = s->packet_buffer ?read_from_packet_buffer(&s->packet_buffer, &s->packet_buffer_end, pkt) :read_frame_internal(s, pkt);if (ret < 0)return ret;goto return_packet;}for (;;) {AVPacketList *pktl = s->packet_buffer; //packet_buffer是AVPacketList*if (pktl) {AVPacket *next_pkt = &pktl->pkt;if (next_pkt->dts != AV_NOPTS_VALUE) {int wrap_bits = s->streams[next_pkt->stream_index]->pts_wrap_bits;// last dts seen for this stream. if any of packets following// current one had no dts, we will set this to AV_NOPTS_VALUE.int64_t last_dts = next_pkt->dts;while (pktl && next_pkt->pts == AV_NOPTS_VALUE) {if (pktl->pkt.stream_index == next_pkt->stream_index &&(av_compare_mod(next_pkt->dts, pktl->pkt.dts, 2LL << (wrap_bits - 1)) < 0)) {if (av_compare_mod(pktl->pkt.pts, pktl->pkt.dts, 2LL << (wrap_bits - 1))) { //not b framenext_pkt->pts = pktl->pkt.dts;}if (last_dts != AV_NOPTS_VALUE) {// Once last dts was set to AV_NOPTS_VALUE, we don't change it.last_dts = pktl->pkt.dts;}}pktl = pktl->next;}if (eof && next_pkt->pts == AV_NOPTS_VALUE && last_dts != AV_NOPTS_VALUE) {// Fixing the last reference frame had none pts issue (For MXF etc).// We only do this when// 1. eof.// 2. we are not able to resolve a pts value for current packet.// 3. the packets for this stream at the end of the files had valid dts.next_pkt->pts = last_dts + next_pkt->duration;}pktl = s->packet_buffer;}/* read packet from packet buffer, if there is data */if (!(next_pkt->pts == AV_NOPTS_VALUE &&next_pkt->dts != AV_NOPTS_VALUE && !eof)) {ret = read_from_packet_buffer(&s->packet_buffer,&s->packet_buffer_end, pkt);goto return_packet;}}ret = read_frame_internal(s, pkt);if (ret < 0) {if (pktl && ret != AVERROR(EAGAIN)) {eof = 1;continue;} elsereturn ret;}if (av_dup_packet(add_to_pktbuf(&s->packet_buffer, pkt,&s->packet_buffer_end)) < 0)return AVERROR(ENOMEM);}return_packet:st = s->streams[pkt->stream_index];if (st->skip_samples) {uint8_t *p = av_packet_new_side_data(pkt, AV_PKT_DATA_SKIP_SAMPLES, 10);AV_WL32(p, st->skip_samples);av_log(s, AV_LOG_DEBUG, "demuxer injecting skip %d\n", st->skip_samples);st->skip_samples = 0;}if ((s->iformat->flags & AVFMT_GENERIC_INDEX) && pkt->flags & AV_PKT_FLAG_KEY) {ff_reduce_index(s, st->index);av_add_index_entry(st, pkt->pos, pkt->dts, 0, 0, AVINDEX_KEYFRAME);}if (is_relative(pkt->dts))pkt->dts -= RELATIVE_TS_BASE;if (is_relative(pkt->pts))pkt->pts -= RELATIVE_TS_BASE;return ret;}