static int hls_read_header(AVFormatContext *s)
{
    void *u = (s->flags & AVFMT_FLAG_CUSTOM_IO) ? NULL : s->pb;
    HLSContext *c = s->priv_data;
    int ret = 0, i;
    int highest_cur_seq_no = 0;

    c->ctx                = s;
    c->interrupt_callback = &s->interrupt_callback;
    c->strict_std_compliance = s->strict_std_compliance;

    c->first_packet = 1;
    c->first_timestamp = AV_NOPTS_VALUE;
    c->cur_timestamp = AV_NOPTS_VALUE;

    if (u) {
        // get the previous user agent & set back to null if string size is zero
        update_options(&c->user_agent, "user_agent", u);

        // get the previous cookies & set back to null if string size is zero
        update_options(&c->cookies, "cookies", u);

        // get the previous headers & set back to null if string size is zero
        update_options(&c->headers, "headers", u);

        // get the previous http proxt & set back to null if string size is zero
        update_options(&c->http_proxy, "http_proxy", u);
    }

    if ((ret = parse_playlist(c, s->filename, NULL, s->pb)) < 0)
        goto fail;

    if ((ret = save_avio_options(s)) < 0)
        goto fail;

    /* Some HLS servers don't like being sent the range header */
    av_dict_set(&c->avio_opts, "seekable", "0", 0);

    if (c->n_variants == 0) {
        av_log(NULL, AV_LOG_WARNING, "Empty playlist\n");
        ret = AVERROR_EOF;
        goto fail;
    }
    /* If the playlist only contained playlists (Master Playlist),
     * parse each individual playlist. */
    if (c->n_playlists > 1 || c->playlists[0]->n_segments == 0) {
        for (i = 0; i < c->n_playlists; i++) {
            struct playlist *pls = c->playlists[i];
            if ((ret = parse_playlist(c, pls->url, pls, NULL)) < 0)
                goto fail;
        }
    }

    if (c->variants[0]->playlists[0]->n_segments == 0) {
        av_log(NULL, AV_LOG_WARNING, "Empty playlist\n");
        ret = AVERROR_EOF;
        goto fail;
    }

    /* If this isn't a live stream, calculate the total duration of the
     * stream. */
    if (c->variants[0]->playlists[0]->finished) {
        int64_t duration = 0;
        for (i = 0; i < c->variants[0]->playlists[0]->n_segments; i++)
            duration += c->variants[0]->playlists[0]->segments[i]->duration;
        s->duration = duration;
    }

    /* Associate renditions with variants */
    for (i = 0; i < c->n_variants; i++) {
        struct variant *var = c->variants[i];

        if (var->audio_group[0])
            add_renditions_to_variant(c, var, AVMEDIA_TYPE_AUDIO, var->audio_group);
        if (var->video_group[0])
            add_renditions_to_variant(c, var, AVMEDIA_TYPE_VIDEO, var->video_group);
        if (var->subtitles_group[0])
            add_renditions_to_variant(c, var, AVMEDIA_TYPE_SUBTITLE, var->subtitles_group);
    }

    /* Create a program for each variant */
    for (i = 0; i < c->n_variants; i++) {
        struct variant *v = c->variants[i];
        AVProgram *program;

        program = av_new_program(s, i);
        if (!program)
            goto fail;
        av_dict_set_int(&program->metadata, "variant_bitrate", v->bandwidth, 0);
    }

    /* Select the starting segments */
    for (i = 0; i < c->n_playlists; i++) {
        struct playlist *pls = c->playlists[i];

        if (pls->n_segments == 0)
            continue;

        pls->cur_seq_no = select_cur_seq_no(c, pls);
        highest_cur_seq_no = FFMAX(highest_cur_seq_no, pls->cur_seq_no);
    }

    /* Open the demuxer for each playlist */
    for (i = 0; i < c->n_playlists; i++) {
        struct playlist *pls = c->playlists[i];
        AVInputFormat *in_fmt = NULL;

        if (!(pls->ctx = avformat_alloc_context())) {
            ret = AVERROR(ENOMEM);
            goto fail;
        }

        if (pls->n_segments == 0)
            continue;

        pls->index  = i;
        pls->needed = 1;
        pls->parent = s;

        /*
         * If this is a live stream and this playlist looks like it is one segment
         * behind, try to sync it up so that every substream starts at the same
         * time position (so e.g. avformat_find_stream_info() will see packets from
         * all active streams within the first few seconds). This is not very generic,
         * though, as the sequence numbers are technically independent.
         */
        if (!pls->finished && pls->cur_seq_no == highest_cur_seq_no - 1 &&
            highest_cur_seq_no < pls->start_seq_no + pls->n_segments) {
            pls->cur_seq_no = highest_cur_seq_no;
        }

        pls->read_buffer = av_malloc(INITIAL_BUFFER_SIZE);
        if (!pls->read_buffer){
            ret = AVERROR(ENOMEM);
            avformat_free_context(pls->ctx);
            pls->ctx = NULL;
            goto fail;
        }
        ffio_init_context(&pls->pb, pls->read_buffer, INITIAL_BUFFER_SIZE, 0, pls,
                          read_data, NULL, NULL);
        pls->pb.seekable = 0;
        ret = av_probe_input_buffer(&pls->pb, &in_fmt, pls->segments[0]->url,
                                    NULL, 0, 0);
        if (ret < 0) {
            /* Free the ctx - it isn't initialized properly at this point,
             * so avformat_close_input shouldn't be called. If
             * avformat_open_input fails below, it frees and zeros the
             * context, so it doesn't need any special treatment like this. */
            av_log(s, AV_LOG_ERROR, "Error when loading first segment '%s'\n", pls->segments[0]->url);
            avformat_free_context(pls->ctx);
            pls->ctx = NULL;
            goto fail;
        }
        pls->ctx->pb       = &pls->pb;
        pls->ctx->io_open  = nested_io_open;
        pls->ctx->flags   |= s->flags & ~AVFMT_FLAG_CUSTOM_IO;

        if ((ret = ff_copy_whiteblacklists(pls->ctx, s)) < 0)
            goto fail;

        ret = avformat_open_input(&pls->ctx, pls->segments[0]->url, in_fmt, NULL);
        if (ret < 0)
            goto fail;

        if (pls->id3_deferred_extra && pls->ctx->nb_streams == 1) {
            ff_id3v2_parse_apic(pls->ctx, &pls->id3_deferred_extra);
            avformat_queue_attached_pictures(pls->ctx);
            ff_id3v2_free_extra_meta(&pls->id3_deferred_extra);
            pls->id3_deferred_extra = NULL;
        }

        if (pls->is_id3_timestamped == -1)
            av_log(s, AV_LOG_WARNING, "No expected HTTP requests have been made\n");

        /*
         * For ID3 timestamped raw audio streams we need to detect the packet
         * durations to calculate timestamps in fill_timing_for_id3_timestamped_stream(),
         * but for other streams we can rely on our user calling avformat_find_stream_info()
         * on us if they want to.
         */
        if (pls->is_id3_timestamped) {
            ret = avformat_find_stream_info(pls->ctx, NULL);
            if (ret < 0)
                goto fail;
        }

        pls->has_noheader_flag = !!(pls->ctx->ctx_flags & AVFMTCTX_NOHEADER);

        /* Create new AVStreams for each stream in this playlist */
        ret = update_streams_from_subdemuxer(s, pls);
        if (ret < 0)
            goto fail;

        add_metadata_from_renditions(s, pls, AVMEDIA_TYPE_AUDIO);
        add_metadata_from_renditions(s, pls, AVMEDIA_TYPE_VIDEO);
        add_metadata_from_renditions(s, pls, AVMEDIA_TYPE_SUBTITLE);
    }

    update_noheader_flag(s);

    return 0;
fail:
    hls_close(s);
    return ret;
}