ffmpeg中av_read_frame函数分析

尊重原作者 转发请标明出处：https://blog.csdn.net/qq_36491519/article/details/86623746

av_read_frame是FFmpeg在拉流端用到的API，对于这个函数，很多时候在断网检测的时候会用到这个函数的返回值。有的人喜欢在返回值为连续负值时就判断为异常，从而断定为网络中断。其实这是一个不严谨的做法，而且我在实际应用中做了测试，发现在不同的平台下会发生各种情况，甚至在对于不同的电脑也会产生不同情况。大多数情况是返回负值的时间，即有时候立马返回负值，有时候要很久才返回负值。这就对断网检测来说不严谨。

其实最好的方法是在AVFormatContext中注册一个回调函数interrupt_callback，然后对这个回调函数进行实现。在回调函数中进行超时判断：use = av_gettime_relative() - timeout_start; 在这里timeout_start的刷新就可以在av_read_frame返回值为0时进行。

那问题就在于av_read_frame获取数据异常时为什么有时候返回负值快有时候返回负值慢那？而且没有规律可言。

这就得分析av_read_frame的实现了。

下面是av_read_frame的函数实现：

//尊重原作者 转发请标明出处：https://blog.csdn.net/qq_36491519/article/details/86623746
int av_read_frame(AVFormatContext *s, AVPacket *pkt)
{
    const int genpts = s->flags & AVFMT_FLAG_GENPTS;
    int eof = 0;
    int ret;
    AVStream *st;

    if (!genpts) {
        ret = s->internal->packet_buffer
              ? read_from_packet_buffer(&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 = read_from_packet_buffer(&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 = add_to_pktbuf(&s->internal->packet_buffer, pkt,
                            &s->internal->packet_buffer_end, 1);
        av_packet_unref(pkt);
        if (ret < 0)
            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;
}

这个函数里比较重要的第一个是三元运算。

 if (!genpts) {
        ret = s->internal->packet_buffer
              ? read_from_packet_buffer(&s->internal->packet_buffer,
                                        &s->internal->packet_buffer_end, pkt)
              : read_frame_internal(s, pkt);
        if (ret < 0)
            return ret;
        goto return_packet;
    }

判断不存在时间戳信息时用所示的两个函数来读数据，并直接跳转到返回部分。

接下来就是最主要部分，一个（；；）循环，在此循环里刚开始是对pts和dts的一些初始化和值的修改等。然后通过

read_from_packet_buffer读取数据，并跳转到返回部分。

最后是pkt1不存在时，

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

读取数据。

到这里大家有没有注意到在av_read_frame函数开头有两个函数read_from_packet_buffer和read_frame_internal，在（；；）里又是这两个函数，其他的都是判断之类的，所以在av_read_frame函数其实就是调了read_from_packet_buffer和read_frame_internal。

所以我们再来看看这两个函数，首先read_from_packet_buffer。

static int read_from_packet_buffer(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;
}

很简单的把pkt_buffer的值传给pkt.

对于read_frame_internal。

static int read_frame_internal(AVFormatContext *s, AVPacket *pkt)
{
    int ret = 0, i, got_packet = 0;
    AVDictionary *metadata = NULL;

    av_init_packet(pkt);

    while (!got_packet && !s->internal->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];

        /* update context if required */
        if (st->internal->need_context_update) {
            if (avcodec_is_open(st->internal->avctx)) {
                av_log(s, AV_LOG_DEBUG, "Demuxer context update while decoder is open, closing and trying to re-open\n");
                avcodec_close(st->internal->avctx);
                st->info->found_decoder = 0;
            }

            /* close parser, because it depends on the codec */
            if (st->parser && st->internal->avctx->codec_id != st->codecpar->codec_id) {
                av_parser_close(st->parser);
                st->parser = NULL;
            }

            ret = avcodec_parameters_to_context(st->internal->avctx, st->codecpar);
            if (ret < 0)
                return ret;

#if FF_API_LAVF_AVCTX
FF_DISABLE_DEPRECATION_WARNINGS
            /* update deprecated public codec context */
            ret = avcodec_parameters_to_context(st->codec, st->codecpar);
            if (ret < 0)
                return ret;
FF_ENABLE_DEPRECATION_WARNINGS
#endif

            st->internal->need_context_update = 0;
        }

        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=%"PRId64", 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->codecpar->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->codecpar->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, AV_NOPTS_VALUE, AV_NOPTS_VALUE);
            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;
            st->codecpar->sample_rate = st->internal->avctx->sample_rate;
            st->codecpar->bit_rate = st->internal->avctx->bit_rate;
            st->codecpar->channels = st->internal->avctx->channels;
            st->codecpar->channel_layout = st->internal->avctx->channel_layout;
            st->codecpar->codec_id = st->internal->avctx->codec_id;
        } else {
            /* free packet */
            av_packet_unref(&cur_pkt);
        }
        if (pkt->flags & AV_PKT_FLAG_KEY)
            st->skip_to_keyframe = 0;
        if (st->skip_to_keyframe) {
            av_packet_unref(&cur_pkt);
            if (got_packet) {
                *pkt = cur_pkt;
            }
            got_packet = 0;
        }
    }

    if (!got_packet && s->internal->parse_queue)
        ret = read_from_packet_buffer(&s->internal->parse_queue, &s->internal->parse_queue_end, pkt);

    if (ret >= 0) {
        AVStream *st = s->streams[pkt->stream_index];
        int discard_padding = 0;
        if (st->first_discard_sample && pkt->pts != AV_NOPTS_VALUE) {
            int64_t pts = pkt->pts - (is_relative(pkt->pts) ? RELATIVE_TS_BASE : 0);
            int64_t sample = ts_to_samples(st, pts);
            int duration = ts_to_samples(st, pkt->duration);
            int64_t end_sample = sample + duration;
            if (duration > 0 && end_sample >= st->first_discard_sample &&
                sample < st->last_discard_sample)
                discard_padding = FFMIN(end_sample - st->first_discard_sample, duration);
        }
        if (st->start_skip_samples && (pkt->pts == 0 || pkt->pts == RELATIVE_TS_BASE))
            st->skip_samples = st->start_skip_samples;
        if (st->skip_samples || discard_padding) {
            uint8_t *p = av_packet_new_side_data(pkt, AV_PKT_DATA_SKIP_SAMPLES, 10);
            if (p) {
                AV_WL32(p, st->skip_samples);
                AV_WL32(p + 4, discard_padding);
                av_log(s, AV_LOG_DEBUG, "demuxer injecting skip %d / discard %d\n", st->skip_samples, discard_padding);
            }
            st->skip_samples = 0;
        }

        if (st->inject_global_side_data) {
            for (i = 0; i < st->nb_side_data; i++) {
                AVPacketSideData *src_sd = &st->side_data[i];
                uint8_t *dst_data;

                if (av_packet_get_side_data(pkt, src_sd->type, NULL))
                    continue;

                dst_data = av_packet_new_side_data(pkt, src_sd->type, src_sd->size);
                if (!dst_data) {
                    av_log(s, AV_LOG_WARNING, "Could not inject global side data\n");
                    continue;
                }

                memcpy(dst_data, src_sd->data, src_sd->size);
            }
            st->inject_global_side_data = 0;
        }

#if FF_API_LAVF_MERGE_SD
FF_DISABLE_DEPRECATION_WARNINGS
        if (!(s->flags & AVFMT_FLAG_KEEP_SIDE_DATA))
            av_packet_merge_side_data(pkt);
FF_ENABLE_DEPRECATION_WARNINGS
#endif
    }

    av_opt_get_dict_val(s, "metadata", AV_OPT_SEARCH_CHILDREN, &metadata);
    if (metadata) {
        s->event_flags |= AVFMT_EVENT_FLAG_METADATA_UPDATED;
        av_dict_copy(&s->metadata, metadata, 0);
        av_dict_free(&metadata);
        av_opt_set_dict_val(s, "metadata", NULL, AV_OPT_SEARCH_CHILDREN);
    }

#if FF_API_LAVF_AVCTX
    update_stream_avctx(s);
#endif

    if (s->debug & FF_FDEBUG_TS)
        av_log(s, AV_LOG_DEBUG,
               "read_frame_internal stream=%d, pts=%s, dts=%s, "
               "size=%d, duration=%"PRId64", flags=%d\n",
               pkt->stream_index,
               av_ts2str(pkt->pts),
               av_ts2str(pkt->dts),
               pkt->size, pkt->duration, pkt->flags);

    return ret;
}

这个函数比较复杂。我们来分析一下。

在开始的while循环里，调用了ff_read_packet函数，这个函数里又是一个（；；），无限循环套无限循环，

int ff_read_packet(AVFormatContext *s, AVPacket *pkt)
{
    int ret, i, err;
    AVStream *st;

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

        if (pktl) {
            *pkt = pktl->pkt;
            st   = s->streams[pkt->stream_index];
            if (s->internal->raw_packet_buffer_remaining_size <= 0)
                if ((err = probe_codec(s, st, NULL)) < 0)
                    return err;
            if (st->request_probe <= 0) {
                s->internal->raw_packet_buffer                 = pktl->next;
                s->internal->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) {
            /* Some demuxers return FFERROR_REDO when they consume
               data and discard it (ignored streams, junk, extradata).
               We must re-call the demuxer to get the real packet. */
            if (ret == FFERROR_REDO)
                continue;
            if (!pktl || ret == AVERROR(EAGAIN))
                return ret;
            for (i = 0; i < s->nb_streams; i++) {
                st = s->streams[i];
                if (st->probe_packets || st->request_probe > 0)
                    if ((err = probe_codec(s, st, NULL)) < 0)
                        return err;
                av_assert0(st->request_probe <= 0);
            }
            continue;
        }

        if (!pkt->buf) {
            AVPacket tmp = { 0 };
            ret = av_packet_ref(&tmp, pkt);
            if (ret < 0)
                return ret;
            *pkt = tmp;
        }

        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_packet_unref(pkt);
            continue;
        }

        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];

        if (update_wrap_reference(s, st, pkt->stream_index, pkt) && st->pts_wrap_behavior == AV_PTS_WRAP_SUB_OFFSET) {
            // correct first time stamps to negative values
            if (!is_relative(st->first_dts))
                st->first_dts = wrap_timestamp(st, st->first_dts);
            if (!is_relative(st->start_time))
                st->start_time = wrap_timestamp(st, st->start_time);
            if (!is_relative(st->cur_dts))
                st->cur_dts = wrap_timestamp(st, st->cur_dts);
        }

        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;

        err = add_to_pktbuf(&s->internal->raw_packet_buffer, pkt,
                            &s->internal->raw_packet_buffer_end, 0);
        if (err)
            return err;
        s->internal->raw_packet_buffer_remaining_size -= pkt->size;

        if ((err = probe_codec(s, st, pkt)) < 0)
            return err;
    }
}

比较重要的的是一个函数指针s->iformat->read_packet(s, pkt)，在它里面进行了数据的读取，到这里主要的获取数据函数就分析完了。

希望对大家有用。

尊重原作者 转发请标明出处：https://blog.csdn.net/qq_36491519/article/details/86623746