在前面我们介绍了ffplay的总体架构和一些关键的数据结构。今天我们还是从这张图开始，主要介绍ffplay的读取线程部分。

从ffplay的main函数入口开始阅读源码，发现是在函数stream_open创建了资源读取线程，读取线程执行的函数是read_thread，所以要分析读取线程的工作内容，我们只需读懂函数read_thread即可。

下面是我加了注释的read_thread函数：

/**
 * 读取线程工作内容
 * @param arg
 * @return
 */
static int read_thread(void *arg)
{
    VideoState *is = arg;
    AVFormatContext *ic = NULL;
    int err, i, ret;
    int st_index[AVMEDIA_TYPE_NB];
    AVPacket *pkt = NULL;
    int64_t stream_start_time;
    int pkt_in_play_range = 0;
    const AVDictionaryEntry *t;
    // 互斥量，读取线程总不能一直读取吧，播放消费队列消费比不上读取的速度，队列满了就要陷入等待唤醒
    SDL_mutex *wait_mutex = SDL_CreateMutex();
    int scan_all_pmts_set = 0;
    int64_t pkt_ts;

    if (!wait_mutex) {
        av_log(NULL, AV_LOG_FATAL, "SDL_CreateMutex(): %sn", SDL_GetError());
        ret = AVERROR(ENOMEM);
        goto fail;
    }
    // 所有流的索引都设置成-1
    memset(st_index, -1, sizeof(st_index));
    is->eof = 0;
    // 分配pack
    pkt = av_packet_alloc();
    if (!pkt) {
        av_log(NULL, AV_LOG_FATAL, "Could not allocate packet.n");
        ret = AVERROR(ENOMEM);
        goto fail;
    }
    // 解封装上下文
    ic = avformat_alloc_context();
    if (!ic) {
        av_log(NULL, AV_LOG_FATAL, "Could not allocate context.n");
        ret = AVERROR(ENOMEM);
        goto fail;
    }
    // 回调函数，防止读取过程中阻塞时间过长
    ic->interrupt_callback.callback = decode_interrupt_cb;
    ic->interrupt_callback.opaque = is;
    if (!av_dict_get(format_opts, "scan_all_pmts", NULL, AV_DICT_MATCH_CASE)) {
        av_dict_set(&format_opts, "scan_all_pmts", "1", AV_DICT_DONT_OVERWRITE);
        scan_all_pmts_set = 1;
    }
    // 打开
    err = avformat_open_input(&ic, is->filename, is->iformat, &format_opts);
    if (err < 0) {
        print_error(is->filename, err);
        ret = -1;
        goto fail;
    }
    if (scan_all_pmts_set)
        av_dict_set(&format_opts, "scan_all_pmts", NULL, AV_DICT_MATCH_CASE);

    if ((t = av_dict_get(format_opts, "", NULL, AV_DICT_IGNORE_SUFFIX))) {
        av_log(NULL, AV_LOG_ERROR, "Option %s not found.n", t->key);
        ret = AVERROR_OPTION_NOT_FOUND;
        goto fail;
    }
    is->ic = ic;

    if (genpts)
        ic->flags |= AVFMT_FLAG_GENPTS;

    // 将其注入成一个变量
    av_format_inject_global_side_data(ic);

    if (find_stream_info) {
        AVDictionary **opts = setup_find_stream_info_opts(ic, codec_opts);
        int orig_nb_streams = ic->nb_streams;

        /*
         * 探测媒体类型，可得到当前文件的封装格式，音视频编码参数等信息
         * 调用该函数后得多的参数信息会比只调用avformat_open_input更为详细，
         * 其本质上是去做了decdoe packet获取信息的工作
         */
        err = avformat_find_stream_info(ic, opts);

        for (i = 0; i < orig_nb_streams; i++)
            av_dict_free(&opts[i]);
        av_freep(&opts);

        if (err < 0) {
            av_log(NULL, AV_LOG_WARNING,
                   "%s: could not find codec parametersn", is->filename);
            ret = -1;
            goto fail;
        }
    }

    if (ic->pb)
        ic->pb->eof_reached = 0; // FIXME hack, ffplay maybe should not use avio_feof() to test for the end

    if (seek_by_bytes < 0)
        seek_by_bytes = !!(ic->iformat->flags & AVFMT_TS_DISCONT) && strcmp("ogg", ic->iformat->name);

    is->max_frame_duration = (ic->iformat->flags & AVFMT_TS_DISCONT) ? 10.0 : 3600.0;

    if (!window_title && (t = av_dict_get(ic->metadata, "title", NULL, 0)))
        window_title = av_asprintf("%s - %s", t->value, input_filename);

    /* if seeking requested, we execute it */
    // 起始播放时间处理
    if (start_time != AV_NOPTS_VALUE) {
        int64_t timestamp;

        timestamp = start_time;
        /* add the stream start time */
        if (ic->start_time != AV_NOPTS_VALUE)
            timestamp += ic->start_time;
        // 跳到播放位置
        ret = avformat_seek_file(ic, -1, INT64_MIN, timestamp, INT64_MAX, 0);
        if (ret < 0) {
            av_log(NULL, AV_LOG_WARNING, "%s: could not seek to position %0.3fn",
                    is->filename, (double)timestamp / AV_TIME_BASE);
        }
    }

    // 是否是时时流
    is->realtime = is_realtime(ic);

    if (show_status)
        // 打印媒体信息
        av_dump_format(ic, 0, is->filename, 0);

    // 流索引相关
    for (i = 0; i < ic->nb_streams; i++) {
        AVStream *st = ic->streams[i];
        enum AVMediaType type = st->codecpar->codec_type;
        st->discard = AVDISCARD_ALL;
        if (type >= 0 && wanted_stream_spec[type] && st_index[type] == -1)
            if (avformat_match_stream_specifier(ic, st, wanted_stream_spec[type]) > 0)
                st_index[type] = i;
    }
    for (i = 0; i < AVMEDIA_TYPE_NB; i++) {
        if (wanted_stream_spec[i] && st_index[i] == -1) {
            av_log(NULL, AV_LOG_ERROR, "Stream specifier %s does not match any %s streamn", wanted_stream_spec[i], av_get_media_type_string(i));
            st_index[i] = INT_MAX;
        }
    }

    if (!video_disable)
        // 查找视频流索引
        st_index[AVMEDIA_TYPE_VIDEO] =
            av_find_best_stream(ic, AVMEDIA_TYPE_VIDEO,
                                st_index[AVMEDIA_TYPE_VIDEO], -1, NULL, 0);
    if (!audio_disable)
        st_index[AVMEDIA_TYPE_AUDIO] =
            av_find_best_stream(ic, AVMEDIA_TYPE_AUDIO,
                                st_index[AVMEDIA_TYPE_AUDIO],
                                st_index[AVMEDIA_TYPE_VIDEO],
                                NULL, 0);
    if (!video_disable && !subtitle_disable)
        st_index[AVMEDIA_TYPE_SUBTITLE] =
            av_find_best_stream(ic, AVMEDIA_TYPE_SUBTITLE,
                                st_index[AVMEDIA_TYPE_SUBTITLE],
                                (st_index[AVMEDIA_TYPE_AUDIO] >= 0 ?
                                 st_index[AVMEDIA_TYPE_AUDIO] :
                                 st_index[AVMEDIA_TYPE_VIDEO]),
                                NULL, 0);

    // 更新视频信息初始化窗口参数
    is->show_mode = show_mode;
    if (st_index[AVMEDIA_TYPE_VIDEO] >= 0) {
        AVStream *st = ic->streams[st_index[AVMEDIA_TYPE_VIDEO]];
        AVCodecParameters *codecpar = st->codecpar;
        AVRational sar = av_guess_sample_aspect_ratio(ic, st, NULL);
        if (codecpar->width)
            set_default_window_size(codecpar->width, codecpar->height, sar);
    }

    /* open the streams */
    // 打开视频流，内不创建了解码线程
    if (st_index[AVMEDIA_TYPE_AUDIO] >= 0) {
        stream_component_open(is, st_index[AVMEDIA_TYPE_AUDIO]);
    }

    ret = -1;
    // 打开音频流
    if (st_index[AVMEDIA_TYPE_VIDEO] >= 0) {
        ret = stream_component_open(is, st_index[AVMEDIA_TYPE_VIDEO]);
    }
    if (is->show_mode == SHOW_MODE_NONE)
        is->show_mode = ret >= 0 ? SHOW_MODE_VIDEO : SHOW_MODE_RDFT;

    // 打开字幕流
    if (st_index[AVMEDIA_TYPE_SUBTITLE] >= 0) {
        stream_component_open(is, st_index[AVMEDIA_TYPE_SUBTITLE]);
    }

    if (is->video_stream < 0 && is->audio_stream < 0) {
        av_log(NULL, AV_LOG_FATAL, "Failed to open file '%s' or configure filtergraphn",
               is->filename);
        ret = -1;
        goto fail;
    }

    if (infinite_buffer < 0 && is->realtime)
        infinite_buffer = 1;

    // 循环读取
    for (;;) {
        // 是否退出了
        if (is->abort_request)
            break;
        // 更新是否暂停了
        if (is->paused != is->last_paused) {
            is->last_paused = is->paused;
            if (is->paused)
                is->read_pause_return = av_read_pause(ic);
            else
                av_read_play(ic);
        }
#if CONFIG_RTSP_DEMUXER || CONFIG_MMSH_PROTOCOL
        if (is->paused &&
                (!strcmp(ic->iformat->name, "rtsp") ||
                 (ic->pb && !strncmp(input_filename, "mmsh:", 5)))) {
            /* wait 10 ms to avoid trying to get another packet */
            /* XXX: horrible */
            SDL_Delay(10);
            continue;
        }
#endif
        // 是否seek了
        if (is->seek_req) {
            int64_t seek_target = is->seek_pos;
            int64_t seek_min    = is->seek_rel > 0 ? seek_target - is->seek_rel + 2: INT64_MIN;
            int64_t seek_max    = is->seek_rel < 0 ? seek_target - is->seek_rel - 2: INT64_MAX;
// FIXME the +-2 is due to rounding being not done in the correct direction in generation
//      of the seek_pos/seek_rel variables

            ret = avformat_seek_file(is->ic, -1, seek_min, seek_target, seek_max, is->seek_flags);
            if (ret < 0) {
                av_log(NULL, AV_LOG_ERROR,
                       "%s: error while seekingn", is->ic->url);
            } else {
                // seek了要刷新待解码包队列
                if (is->audio_stream >= 0)
                    packet_queue_flush(&is->audioq);
                if (is->subtitle_stream >= 0)
                    packet_queue_flush(&is->subtitleq);
                if (is->video_stream >= 0)
                    packet_queue_flush(&is->videoq);
                if (is->seek_flags & AVSEEK_FLAG_BYTE) {
                   set_clock(&is->extclk, NAN, 0);
                } else {
                   set_clock(&is->extclk, seek_target / (double)AV_TIME_BASE, 0);
                }
            }
            // seek复位
            is->seek_req = 0;
            is->queue_attachments_req = 1;
            is->eof = 0;
            if (is->paused)
                step_to_next_frame(is);
        }
        // 检测video是否为attached_pic
        if (is->queue_attachments_req) {
            // attached_pic 附带的图片。比如说一些MP3，AAC音频文件附带的专辑封面，所以需要注意的是音频文件不一定只存在音频流本身
            if (is->video_st && is->video_st->disposition & AV_DISPOSITION_ATTACHED_PIC) {
                if ((ret = av_packet_ref(pkt, &is->video_st->attached_pic)) < 0)
                    goto fail;
                packet_queue_put(&is->videoq, pkt);
                packet_queue_put_nullpacket(&is->videoq, pkt, is->video_stream);
            }
            is->queue_attachments_req = 0;
        }

        /* if the queue are full, no need to read more */
        // 队列是否有足够的包
        if (infinite_buffer<1 &&
              (is->audioq.size + is->videoq.size + is->subtitleq.size > MAX_QUEUE_SIZE
            || (stream_has_enough_packets(is->audio_st, is->audio_stream, &is->audioq) &&
                stream_has_enough_packets(is->video_st, is->video_stream, &is->videoq) &&
                stream_has_enough_packets(is->subtitle_st, is->subtitle_stream, &is->subtitleq)))) {
            /* wait 10 ms */
            SDL_LockMutex(wait_mutex);
            SDL_CondWaitTimeout(is->continue_read_thread, wait_mutex, 10);
            SDL_UnlockMutex(wait_mutex);
            continue;
        }
        // 播放完了，看是否需要重播
        if (!is->paused &&
            (!is->audio_st || (is->auddec.finished == is->audioq.serial && frame_queue_nb_remaining(&is->sampq) == 0)) &&
            (!is->video_st || (is->viddec.finished == is->videoq.serial && frame_queue_nb_remaining(&is->pictq) == 0))) {
            if (loop != 1 && (!loop || --loop)) {
                // 这里的重播就不用再次从新走一次逻辑了，直接seek就好了
                stream_seek(is, start_time != AV_NOPTS_VALUE ? start_time : 0, 0, 0);
            } else if (autoexit) {
                ret = AVERROR_EOF;
                goto fail;
            }
        }
        // 读取一个包
        ret = av_read_frame(ic, pkt);
        if (ret < 0) {
            // 读到末尾了，放入空包，以便后续冲刷解码器
            if ((ret == AVERROR_EOF || avio_feof(ic->pb)) && !is->eof) {
                if (is->video_stream >= 0)
                    packet_queue_put_nullpacket(&is->videoq, pkt, is->video_stream);
                if (is->audio_stream >= 0)
                    packet_queue_put_nullpacket(&is->audioq, pkt, is->audio_stream);
                if (is->subtitle_stream >= 0)
                    packet_queue_put_nullpacket(&is->subtitleq, pkt, is->subtitle_stream);
                is->eof = 1;
            }
            if (ic->pb && ic->pb->error) {
                if (autoexit)
                    goto fail;
                else
                    break;
            }
            // 休息一会，但是可以被唤醒，有需求你就去干
            SDL_LockMutex(wait_mutex);
            SDL_CondWaitTimeout(is->continue_read_thread, wait_mutex, 10);
            SDL_UnlockMutex(wait_mutex);
            continue;
        } else {
            is->eof = 0;
        }
        // 将包放入队列前先检查这个包是否在播放的时间之内
        /* check if packet is in play range specified by user, then queue, otherwise discard */
        stream_start_time = ic->streams[pkt->stream_index]->start_time;
        pkt_ts = pkt->pts == AV_NOPTS_VALUE ? pkt->dts : pkt->pts;
        pkt_in_play_range = duration == AV_NOPTS_VALUE ||
                (pkt_ts - (stream_start_time != AV_NOPTS_VALUE ? stream_start_time : 0)) *
                av_q2d(ic->streams[pkt->stream_index]->time_base) -
                (double)(start_time != AV_NOPTS_VALUE ? start_time : 0) / 1000000
                <= ((double)duration / 1000000);
        // 将包放入队列
        if (pkt->stream_index == is->audio_stream && pkt_in_play_range) {
            packet_queue_put(&is->audioq, pkt);
        } else if (pkt->stream_index == is->video_stream && pkt_in_play_range
                   && !(is->video_st->disposition & AV_DISPOSITION_ATTACHED_PIC)) {
            packet_queue_put(&is->videoq, pkt);
        } else if (pkt->stream_index == is->subtitle_stream && pkt_in_play_range) {
            packet_queue_put(&is->subtitleq, pkt);
        } else {
            av_packet_unref(pkt);
        }
    }

    ret = 0;
 fail:
    if (ic && !is->ic)
        avformat_close_input(&ic);

    av_packet_free(&pkt);
    if (ret != 0) {
        SDL_Event event;

        event.type = FF_QUIT_EVENT;
        event.user.data1 = is;
        SDL_PushEvent(&event);
    }
    SDL_DestroyMutex(wait_mutex);
    return 0;
}

读取线程准备工作

1、创建读取线程互斥量SDL_CreateMutex

2、分配AVPacket，av_packet_alloc

3、分配解封装上下文avformat_alloc_context

4、设置回调函数ic->interrupt_callback.callback

5、打开媒体文件avformat_open_input

6、avformat_find_stream_info探测媒体信息

7、是否需要起始seek播放avformat_seek_file

8、获取流索引

9、stream_component_open初始化相关解码器组件

在ffplay读取线程的准备工作中，我们发现一个检查是否是实时流的方法，它是这样子判断的：

static int is_realtime(AVFormatContext *s)
{
    if(   !strcmp(s->iformat->name, "rtp")
       || !strcmp(s->iformat->name, "rtsp")
       || !strcmp(s->iformat->name, "sdp")
    )
        return 1;

    if(s->pb && (   !strncmp(s->url, "rtp:", 4)
                 || !strncmp(s->url, "udp:", 4)
                )
    )
        return 1;
    return 0;
}

for循环读取

1、首先通过大内总管VideoState判断是否请求了退出

2、根据是否暂停了控制RTSP流以及SDL显示

3、处理seek操作

在这个过程中会调用函数packet_queue_flush刷新待解码包队列，然后将队列的播放序列serial加1，这就是播放序列的妙用之处。

4、检查队列中是否有足够的包，stream_has_enough_packets，如果队列满了则读取线程最多等待10ms，但是可以被唤醒。

5、播放完了，判断是否需要进行重播

    // 播放完了，看是否需要重播
        if (!is->paused &&
            (!is->audio_st || (is->auddec.finished == is->audioq.serial && frame_queue_nb_remaining(&is->sampq) == 0)) &&
            (!is->video_st || (is->viddec.finished == is->videoq.serial && frame_queue_nb_remaining(&is->pictq) == 0))) {
            if (loop != 1 && (!loop || --loop)) {
                // 这里的重播就不用再次从新走一次逻辑了，直接seek就好了
                stream_seek(is, start_time != AV_NOPTS_VALUE ? start_time : 0, 0, 0);
            } else if (autoexit) {
                ret = AVERROR_EOF;
                goto fail;
            }
        }

6、av_read_frame读取一个资源包

如果读到了文件末尾则增加一个空包packet_queue_put_nullpacket，以便于后续在解码线程用于冲刷解码器，这样就能将解码器内部缓存的数据全部取出。

如果读取到的不是空包，则判断该包的pts是否处于有效播放时间内，如果不在则丢弃，如果在则放入队列packet_queue_put。

    // 将包放入队列前先检查这个包是否在播放的时间之内
        /* check if packet is in play range specified by user, then queue, otherwise discard */
        stream_start_time = ic->streams[pkt->stream_index]->start_time;
        pkt_ts = pkt->pts == AV_NOPTS_VALUE ? pkt->dts : pkt->pts;
        pkt_in_play_range = duration == AV_NOPTS_VALUE ||
                (pkt_ts - (stream_start_time != AV_NOPTS_VALUE ? stream_start_time : 0)) *
                av_q2d(ic->streams[pkt->stream_index]->time_base) -
                (double)(start_time != AV_NOPTS_VALUE ? start_time : 0) / 1000000
                <= ((double)duration / 1000000);
        // 将包放入队列
        if (pkt->stream_index == is->audio_stream && pkt_in_play_range) {
            packet_queue_put(&is->audioq, pkt);
        } else if (pkt->stream_index == is->video_stream && pkt_in_play_range
                   && !(is->video_st->disposition & AV_DISPOSITION_ATTACHED_PIC)) {
            packet_queue_put(&is->videoq, pkt);
        } else if (pkt->stream_index == is->subtitle_stream && pkt_in_play_range) {
            packet_queue_put(&is->subtitleq, pkt);
        } else {
            av_packet_unref(pkt);
        }