av_read_frame的作用是读取一帧视频数据或者读取多帧音频数据，读取的数据都是待解码的数据，该函数的流程如下所示：

函数的源码如下所示：

int av_read_frame(AVFormatContext *s, AVPacket *pkt)
{
    const int genpts = s->flags & AVFMT_FLAG_GENPTS;
    int eof = 0;
    int ret;
    AVStream *st;
    //不自动生成时间戳,走此flow,好像一般不走这里
    if (!genpts) {
    	// ffmpeg内部读取的时候其实是将解析出来的一帧帧数据放入一个队列中，所以此处先检查解析包队列中是否有数据，
    	// 如果有，直接读取，否则调用真正的读取函数。
    	// 此处为何要使用缓存队列而不是直接读取，因为在解析的时候有可能一次解析N个帧，
    	// 如mpegts 一个PES中可能包含2帧，所以解析出来的帧直接放入队列中。
        ret = s->internal->packet_buffer
              ? ff_packet_list_get(&s->internal->packet_buffer,
                                        &s->internal->packet_buffer_end, pkt)
              : read_frame_internal(s, pkt);
        if (ret < 0)
            return ret;
        goto return_packet;
    }

    for (;;) {
        AVPacketList *pktl = s->internal->packet_buffer;

        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;
                av_assert2(wrap_bits <= 64);
                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, 2ULL << (wrap_bits - 1)) < 0) {
                        if (av_compare_mod(pktl->pkt.pts, pktl->pkt.dts, 2ULL << (wrap_bits - 1))) {
                            // not B-frame
                            next_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->internal->packet_buffer;
            }

            /* read packet from packet buffer, if there is data */
            st = s->streams[next_pkt->stream_index];
            if (!(next_pkt->pts == AV_NOPTS_VALUE && st->discard < AVDISCARD_ALL &&
                  next_pkt->dts != AV_NOPTS_VALUE && !eof)) {
                ret = ff_packet_list_get(&s->internal->packet_buffer,
                                               &s->internal->packet_buffer_end, pkt);
                goto return_packet;
            }
        }

        ret = read_frame_internal(s, pkt);
        if (ret < 0) {
            if (pktl && ret != AVERROR(EAGAIN)) {
                eof = 1;
                continue;
            } else
                return ret;
        }

        ret = ff_packet_list_put(&s->internal->packet_buffer,
                                 &s->internal->packet_buffer_end,
                                 pkt, 0);
        if (ret < 0) {
            av_packet_unref(pkt);
            return ret;
        }
    }

return_packet:

    st = s->streams[pkt->stream_index];
    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;
}

该函数中主要调用了两个函数，如果paketList中有数据，则调用ff_packet_list_get直接从list中读取一帧数据，如果没有则调用read_frame_internal重新读取一帧数据放到list中。
ff_packet_list_get函数比较简单，直接把buf的指针置为pktl->next，其源码如下所示：

int ff_packet_list_get(AVPacketList **pkt_buffer,
                       AVPacketList **pkt_buffer_end,
                       AVPacket      *pkt)
{
    AVPacketList *pktl;
    av_assert0(*pkt_buffer);
    pktl        = *pkt_buffer;
    *pkt        = pktl->pkt;
    *pkt_buffer = pktl->next;
    if (!pktl->next)
        *pkt_buffer_end = NULL;
    av_freep(&pktl);
    return 0;
}

read_frame_internal 在ffmpeg中实现了将format格式的packet,最终转换成一帧帧的es流packet，并解析填充了packet的pts,dts等信息，为最终解码提供了重要的数据,read_frame_internal,调用ff_read_packet，该函数最终调用s->iformat->read_packet指针函数，也就是说不同的farmat的read_packet函数也不相同，具体实现可以到libavformat目录下对应的源文件查看。

这次分析暂时到这里，分析ffmpeg源码感觉像无限套娃，看来要好好提高代码阅读能力啊。

参考博客

https://blog.csdn.net/fallenink/article/details/8307104
https://blog.csdn.net/leixiaohua1020/article/details/12678577
https://blog.csdn.net/qq_39683826/article/details/115767168
https://blog.csdn.net/yihuanyihuan/article/details/88528439