摘要：下面是，讀取頭信息頭信息。猜測網絡部分至少在一開始就應當初始化好的，因此在的過程里面找，在中找到了。就先暫時分析到此吧。

這章要簡單分析下ijkplayer是如何從文件或網絡讀取數據源的。還是read_thread函數中的關鍵點avformat_open_input函數：

int avformat_open_input(AVFormatContext **ps, const char *filename,
                        AVInputFormat *fmt, AVDictionary **options)
{
    ......
    if (!s && !(s = avformat_alloc_context()))
        return AVERROR(ENOMEM);
    ......
    if ((ret = init_input(s, filename, &tmp)) < 0)
        goto fail;
    ......
}

avformat_alloc_context創建AVFormatContext結構實例，然后走init_input根據文件名稱初始化。有必要看看這個init_input：

static int init_input(AVFormatContext *s, const char *filename,
                      AVDictionary **options)
{
    int ret;
    AVProbeData pd = { filename, NULL, 0 };
    int score = AVPROBE_SCORE_RETRY;

    if (s->pb) {
        s->flags |= AVFMT_FLAG_CUSTOM_IO;
        if (!s->iformat)
            return av_probe_input_buffer2(s->pb, &s->iformat, filename,
                                         s, 0, s->format_probesize);
        else if (s->iformat->flags & AVFMT_NOFILE)
            av_log(s, AV_LOG_WARNING, "Custom AVIOContext makes no sense and "
                                      "will be ignored with AVFMT_NOFILE format.
");
        return 0;
    }

    if ((s->iformat && s->iformat->flags & AVFMT_NOFILE) ||
        (!s->iformat && (s->iformat = av_probe_input_format2(&pd, 0, &score))))
        return score;

    if ((ret = s->io_open(s, &s->pb, filename, AVIO_FLAG_READ | s->avio_flags, options)) < 0)
        return ret;

    if (s->iformat)
        return 0;
    return av_probe_input_buffer2(s->pb, &s->iformat, filename,
                                 s, 0, s->format_probesize);
}

這里的重點只有一個，就是av_probe_input_buffer2。那么他在干什么呢？代碼就不貼了，關鍵點就是avio_read。

int avio_read(AVIOContext *s, unsigned char *buf, int size)
{
    int len, size1;

    size1 = size;
    while (size > 0) {
        len = FFMIN(s->buf_end - s->buf_ptr, size);
        if (len == 0 || s->write_flag) {
            if((s->direct || size > s->buffer_size) && !s->update_checksum) {
                // bypass the buffer and read data directly into buf
                if(s->read_packet)
                    len = s->read_packet(s->opaque, buf, size);

                if (len <= 0) {
                    /* do not modify buffer if EOF reached so that a seek back can
                    be done without rereading data */
                    s->eof_reached = 1;
                    if(len<0)
                        s->error= len;
                    break;
                } else {
                    s->pos += len;
                    s->bytes_read += len;
                    size -= len;
                    buf += len;
                    // reset the buffer
                    s->buf_ptr = s->buffer;
                    s->buf_end = s->buffer/* + len*/;
                }
            } else {
                fill_buffer(s);
                len = s->buf_end - s->buf_ptr;
                if (len == 0)
                    break;
            }
        } else {
            memcpy(buf, s->buf_ptr, len);
            buf += len;
            s->buf_ptr += len;
            size -= len;
        }
    }
    if (size1 == size) {
        if (s->error)      return s->error;
        if (avio_feof(s))  return AVERROR_EOF;
    }
    return size1 - size;
}

根據傳遞的要讀取的大小循環讀取內容，前面的都是如果緩存里面有，先從緩存讀取，后面的else里面的fill_buffer是個關鍵點。

static void fill_buffer(AVIOContext *s)
{
    int max_buffer_size = s->max_packet_size ?
                          s->max_packet_size : IO_BUFFER_SIZE;
    uint8_t *dst        = s->buf_end - s->buffer + max_buffer_size < s->buffer_size ?
                          s->buf_end : s->buffer;
    int len             = s->buffer_size - (dst - s->buffer);

    /* can"t fill the buffer without read_packet, just set EOF if appropriate */
    if (!s->read_packet && s->buf_ptr >= s->buf_end)
        s->eof_reached = 1;

    /* no need to do anything if EOF already reached */
    if (s->eof_reached)
        return;

    if (s->update_checksum && dst == s->buffer) {
        if (s->buf_end > s->checksum_ptr)
            s->checksum = s->update_checksum(s->checksum, s->checksum_ptr,
                                             s->buf_end - s->checksum_ptr);
        s->checksum_ptr = s->buffer;
    }

    /* make buffer smaller in case it ended up large after probing */
    if (s->read_packet && s->orig_buffer_size && s->buffer_size > s->orig_buffer_size) {
        if (dst == s->buffer) {
            int ret = ffio_set_buf_size(s, s->orig_buffer_size);
            if (ret < 0)
                av_log(s, AV_LOG_WARNING, "Failed to decrease buffer size
");

            s->checksum_ptr = dst = s->buffer;
        }
        av_assert0(len >= s->orig_buffer_size);
        len = s->orig_buffer_size;
    }

    if (s->read_packet)
        len = s->read_packet(s->opaque, dst, len);
    else
        len = 0;
    if (len <= 0) {
        /* do not modify buffer if EOF reached so that a seek back can
           be done without rereading data */
        s->eof_reached = 1;
        if (len < 0)
            s->error = len;
    } else {
        s->pos += len;
        s->buf_ptr = dst;
        s->buf_end = dst + len;
        s->bytes_read += len;
    }
}

通過read_packet去讀取數據包。
好了，回到avformat_open_input里繼續看：

if (!s->protocol_whitelist && s->pb && s->pb->protocol_whitelist) {
        s->protocol_whitelist = av_strdup(s->pb->protocol_whitelist);
        if (!s->protocol_whitelist) {
            ret = AVERROR(ENOMEM);
            goto fail;
        }
    }

    if (!s->protocol_blacklist && s->pb && s->pb->protocol_blacklist) {
        s->protocol_blacklist = av_strdup(s->pb->protocol_blacklist);
        if (!s->protocol_blacklist) {
            ret = AVERROR(ENOMEM);
            goto fail;
        }
    }

    if (s->format_whitelist && av_match_list(s->iformat->name, s->format_whitelist, ",") <= 0) {
        av_log(s, AV_LOG_ERROR, "Format not on whitelist "%s"
", s->format_whitelist);
        ret = AVERROR(EINVAL);
        goto fail;
    }
    ......
    if (s->pb)
        ff_id3v2_read(s, ID3v2_DEFAULT_MAGIC, &id3v2_extra_meta, 0);
    ......
    if ((ret = avformat_queue_attached_pictures(s)) < 0)
        goto fail;

    if (!(s->flags&AVFMT_FLAG_PRIV_OPT) && s->pb && !s->internal->data_offset)
        s->internal->data_offset = avio_tell(s->pb);

    s->internal->raw_packet_buffer_remaining_size = RAW_PACKET_BUFFER_SIZE;

    update_stream_avctx(s);
    ......

這一塊我認為是在判斷協議和格式的黑白名單，是否在支持的范圍內。下面是ff_id3v2_read，讀取id3v2頭信息頭信息。這個查了下是mp3的頭信息，因此不再具體看了。下面就是avformat_queue_attached_pictures和update_stream_avctx了，將數據包加入緩存并更新隊列。然后基本上就沒有了，那么到底網絡的讀取在什么地方呢？我們只是看到都是從流讀取的，好吧，需要查下網絡的部分了。
猜測網絡部分至少在一開始就應當初始化好的，因此在init的過程里面找，在ffp_global_init中找到了avformat_network_init。

int avformat_network_init(void)
{
#if CONFIG_NETWORK
    int ret;
    ff_network_inited_globally = 1;
    if ((ret = ff_network_init()) < 0)
        return ret;
    if ((ret = ff_tls_init()) < 0)
        return ret;
#endif
    return 0;
}

ff_network_init和ff_tls_init，逐個看。

int ff_network_init(void)
{
#if HAVE_WINSOCK2_H
    WSADATA wsaData;
#endif

    if (!ff_network_inited_globally)
        av_log(NULL, AV_LOG_WARNING, "Using network protocols without global "
                                     "network initialization. Please use "
                                     "avformat_network_init(), this will "
                                     "become mandatory later.
");
#if HAVE_WINSOCK2_H
    if (WSAStartup(MAKEWORD(1,1), &wsaData))
        return 0;
#endif
    return 1;
}

看到了吧，除了ff_network_inited_globally全局變量用來標記是否已經初始化過之外，就是標準的socket了，沒什么可看的了。

int ff_tls_init(void)
{
#if CONFIG_TLS_OPENSSL_PROTOCOL
    int ret;
    if ((ret = ff_openssl_init()) < 0)
        return ret;
#endif
#if CONFIG_TLS_GNUTLS_PROTOCOL
    ff_gnutls_init();
#endif
    return 0;
}

初始化openssl等。
就先暫時分析到此吧。