FFmpeg源代码简单学习- -avformat_open_input

 部分转载自：https://blog.csdn.net/leixiaohua1020/article/details/44064715

//参数ps包含一切媒体相关的上下文结构，有它就有了一切，本函数如果打开媒体成功，
//会返回一个AVFormatContext的实例．
//参数filename是媒体文件名或URL．  文件=url
//参数fmt是要打开的媒体格式的操作结构，因为是读，所以是inputFormat．此处可以
//传入一个调用者定义的inputFormat，对应命令行中的 -f xxx段，如果指定了它，
//在打开文件中就不会探测文件的实际格式了，以它为准了．
//参数options是对某种格式的一些操作，是为了在命令行中可以对不同的格式传入
//特殊的操作参数而建的， 为了了解流程，完全可以无视它．

int avformat_open_input(AVFormatContext **ps,
        const char *filename,
        AVInputFormat *fmt,
        AVDictionary **options)
{
    AVFormatContext *s = *ps;9
    int ret = 0;
    AVFormatParameters ap = { { 0 } };
    AVDictionary *tmp = NULL;

    //创建上下文结构
    //若未创建成功，则报错
    if (!s && !(s = avformat_alloc_context()))
        return AVERROR(ENOMEM);
    //如果用户指定了输入格式，直接使用它
      //-f xxx
      //对应于用户指定的格式
    if (fmt)
        s->iformat = fmt;

    //忽略
    if (options)
        av_dict_copy(&tmp, *options, 0);

    if ((ret = av_opt_set_dict(s, &tmp)) < 0)
        goto fail;

    //打开输入媒体（如果需要的话），初始化所有与媒体读写有关的结构们，比如
    //AVIOContext，AVInputFormat等等
      //init_input包含结构体的初始化函数
    if ((ret = init_input(s, filename)) < 0)
        goto fail;

    //执行完此函数后，s->pb和s->iformat都已经指向了有效实例．pb是用于读写数据的，它
    //把媒体数据当做流来读写，不管是什么媒体格式，而iformat把pb读出来的流按某种媒体格
    //式进行分析，也就是说pb在底层，iformat在上层．

    //很多静态图像文件格式，都被当作一个格式处理，比如要打开.jpeg文件，需要的格式
    //名为image2．此处还不是很了解具体细节，作不得准哦．
    /* check filename in case an image number is expected */
    if (s->iformat->flags & AVFMT_NEEDNUMBER) {
        if (!av_filename_number_test(filename)) {
            ret = AVERROR(EINVAL);
            goto fail;
        }
    }

    s->duration = s->start_time = AV_NOPTS_VALUE;
    //上下文中保存下文件名
    av_strlcpy(s->filename, filename, sizeof(s->filename));

    /* allocate private data */
    //为当前格式分配私有数据，主要用于某格式的读写操作时所用的私有结构．
    //此结构的大小在定义AVInputFormat时已指定了．
    if (s->iformat->priv_data_size > 0) {
        if (!(s->priv_data = av_mallocz(s->iformat->priv_data_size))) {
            ret = AVERROR(ENOMEM);
            goto fail;
        }
        //这个可以先不必管它
        if (s->iformat->priv_class) {
            *(const AVClass**) s->priv_data = s->iformat->priv_class;
            av_opt_set_defaults(s->priv_data);
            if ((ret = av_opt_set_dict(s->priv_data, &tmp)) < 0)
                goto fail;
        }
    }

    /* e.g. AVFMT_NOFILE formats will not have a AVIOContext */
    //从mp3文件中读ID3数据并保存之．
    if (s->pb)
        ff_id3v2_read(s, ID3v2_DEFAULT_MAGIC);

    //读一下媒体的头部，在read_header()中主要是做某种格式的初始化工作，比如填充自己的
    //私有结构，根据流的数量分配流结构并初始化，把文件指针指向数据区开始处等．
    if (!(s->flags & AVFMT_FLAG_PRIV_OPT) && s->iformat->read_header)
        if ((ret = s->iformat->read_header(s, &ap)) < 0)
            goto fail;

    //保存数据区开始的位置
    if (!(s->flags & AVFMT_FLAG_PRIV_OPT) && s->pb && !s->data_offset)
        s->data_offset = avio_tell(s->pb);

    s->raw_packet_buffer_remaining_size = RAW_PACKET_BUFFER_SIZE;

    if (options) {
        av_dict_free(options);
        *options = tmp;
    }
    *ps = s;
    //执行成功
    return 0;

    //执行失败
    fail: av_dict_free(&tmp);
    if (s->pb && !(s->flags & AVFMT_FLAG_CUSTOM_IO))
        avio_close(s->pb);
    avformat_free_context(s);
    *ps = NULL;
    return ret;
}


#define AVPROBE_PADDING_SIZE 32             ///< extra allocated bytes at the end of the probe buffer
#define AVPROBE_SCORE_EXTENSION  50 ///< score for file extension
#define AVPROBE_SCORE_MIME       75 ///< score for file mime type
#define AVPROBE_SCORE_MAX       100 ///< maximum score

AVInputFormat *av_probe_input_format3(AVProbeData *pd, int is_opened,
                                      int *score_ret)
{
    AVProbeData lpd = *pd;
    AVInputFormat *fmt1 = NULL, *fmt;
    int score, nodat = 0, score_max = 0;
    const static uint8_t zerobuffer[AVPROBE_PADDING_SIZE];

    if (!lpd.buf)
        lpd.buf = zerobuffer;

    if (lpd.buf_size > 10 && ff_id3v2_match(lpd.buf, ID3v2_DEFAULT_MAGIC)) {
        int id3len = ff_id3v2_tag_len(lpd.buf);
        if (lpd.buf_size > id3len + 16) {
            lpd.buf      += id3len;
            lpd.buf_size -= id3len;
        } else if (id3len >= PROBE_BUF_MAX) {
            nodat = 2;
        } else
            nodat = 1;
    }

    fmt = NULL;

    //循环调用函数av_iformat_next
    while ((fmt1 = av_iformat_next(fmt1))) {
        if (!is_opened == !(fmt1->flags & AVFMT_NOFILE) && strcmp(fmt1->name, "image2"))
            continue;
        score = 0;
        if (fmt1->read_probe) {//如果包含read_probe，则调用
            score = fmt1->read_probe(&lpd);//获取分数
            if (fmt1->extensions && av_match_ext(lpd.filename, fmt1->extensions)) {
            	//如果扩展名存在，并且使用av_match_ext来比较扩展名和lpd的文件名
            	//FFMAX分数取较大值
                if      (nodat == 0) score = FFMAX(score, 1);
                else if (nodat == 1) score = FFMAX(score, AVPROBE_SCORE_EXTENSION / 2 - 1);
                else                 score = FFMAX(score, AVPROBE_SCORE_EXTENSION);
            }
        } else if (fmt1->extensions) {
            if (av_match_ext(lpd.filename, fmt1->extensions))
                score = AVPROBE_SCORE_EXTENSION;
        }
        if (av_match_name(lpd.mime_type, fmt1->mime_type))
            score = FFMAX(score, AVPROBE_SCORE_MIME);
        if (score > score_max) {
            score_max = score;
            fmt       = fmt1;
        } else if (score == score_max)
            fmt = NULL;
    }
    if (nodat == 1)
        score_max = FFMIN(AVPROBE_SCORE_EXTENSION / 2 - 1, score_max);
    *score_ret = score_max;

    return fmt;
}





int av_probe_input_buffer2(AVIOContext *pb, AVInputFormat **fmt,
                          const char *filename, void *logctx,
                          unsigned int offset, unsigned int max_probe_size)
{
    AVProbeData pd = { filename ? filename : "" };
    uint8_t *buf = NULL;
    int ret = 0, probe_size, buf_offset = 0;
    int score = 0;
    int ret2;

 //max_probe_size的处理
    if (!max_probe_size)
        max_probe_size = PROBE_BUF_MAX;
    else if (max_probe_size < PROBE_BUF_MIN) {
        av_log(logctx, AV_LOG_ERROR,
               "Specified probe size value %u cannot be < %u\n", max_probe_size, PROBE_BUF_MIN);
        return AVERROR(EINVAL);
    }

    if (offset >= max_probe_size)
        return AVERROR(EINVAL);

    if (pb->av_class) {
        uint8_t *mime_type_opt = NULL;
        av_opt_get(pb, "mime_type", AV_OPT_SEARCH_CHILDREN, &mime_type_opt);
        pd.mime_type = (const char *)mime_type_opt;
    }

//忽略
#if 0
    if (!*fmt && pb->av_class && av_opt_get(pb, "mime_type", AV_OPT_SEARCH_CHILDREN, &mime_type) >= 0 && mime_type) {
        if (!av_strcasecmp(mime_type, "audio/aacp")) {
            *fmt = av_find_input_format("aac");
        }
        av_freep(&mime_type);
    }
#endif

 //逐步增大probe_size
    //如果直到max时仍然未读取到fmt，则退出
    for (probe_size = PROBE_BUF_MIN; probe_size <= max_probe_size && !*fmt;
         probe_size = FFMIN(probe_size << 1,
                            FFMAX(max_probe_size, probe_size + 1))) {
        score = probe_size < max_probe_size ? AVPROBE_SCORE_RETRY : 0;

        /* Read probe data. */
        if ((ret = av_reallocp(&buf, probe_size + AVPROBE_PADDING_SIZE)) < 0)
            goto fail;
          //调用avio_read函数进行
        if ((ret = avio_read(pb, buf + buf_offset,
                             probe_size - buf_offset)) < 0) {
            /* Fail if error was not end of file, otherwise, lower score. */
            if (ret != AVERROR_EOF)
                goto fail;

            score = 0;
            ret   = 0;          /* error was end of file, nothing read */
        }
        buf_offset += ret;
        if (buf_offset < offset)
            continue;
        pd.buf_size = buf_offset - offset;
        pd.buf = &buf[offset];

        memset(pd.buf + pd.buf_size, 0, AVPROBE_PADDING_SIZE);

        /* Guess file format. */
        //判断格式
        *fmt = av_probe_input_format2(&pd, 1, &score);
        if (*fmt) {
            /* This can only be true in the last iteration. */
            if (score <= AVPROBE_SCORE_RETRY) {//如果分数过低
                av_log(logctx, AV_LOG_WARNING,
                       "Format %s detected only with low score of %d, "
                       "misdetection possible!\n", (*fmt)->name, score);
            } else
            //输出格式、probe_size、分数
                av_log(logctx, AV_LOG_DEBUG,
                       "Format %s probed with size=%d and score=%d\n",
                       (*fmt)->name, probe_size, score);
#if 0
            FILE *f = fopen("probestat.tmp", "ab");
            fprintf(f, "probe_size:%d format:%s score:%d filename:%s\n", probe_size, (*fmt)->name, score, filename);
            fclose(f);
#endif
        }
    }

    if (!*fmt)//无效数据
        ret = AVERROR_INVALIDDATA;


//fail处理
fail:
    /* Rewind. Reuse probe buffer to avoid seeking. */
    ret2 = ffio_rewind_with_probe_data(pb, &buf, buf_offset);
    if (ret >= 0)
        ret = ret2;

    av_freep(&pd.mime_type);
    return ret < 0 ? ret : score;
}




//flv头部文件读取
static int flv_read_header(AVFormatContext *s)
{
    int flags;
    FLVContext *flv = s->priv_data;
    int offset;
    int pre_tag_size = 0;

    /* Actual FLV data at 0xe40000 in KUX file */
    //如果为kux文件
    if(!strcmp(s->iformat->name, "kux"))
        avio_skip(s->pb, 0xe40000);

    avio_skip(s->pb, 4);
    flags = avio_r8(s->pb);

//
    flv->missing_streams = flags & (FLV_HEADER_FLAG_HASVIDEO | FLV_HEADER_FLAG_HASAUDIO);

    s->ctx_flags |= AVFMTCTX_NOHEADER;

    offset = avio_rb32(s->pb);
    avio_seek(s->pb, offset, SEEK_SET);

    /* Annex E. The FLV File Format
     * E.3 TheFLVFileBody
     *     Field               Type    Comment
     *     PreviousTagSize0    UI32    Always 0
     * */
    pre_tag_size = avio_rb32(s->pb);
    if (pre_tag_size) {
        av_log(s, AV_LOG_WARNING, "Read FLV header error, input file is not a standard flv format, first PreviousTagSize0 always is 0\n");
    }

    s->start_time = 0;
    flv->sum_flv_tag_size = 0;
    flv->last_keyframe_stream_index = -1;

    return 0;
}





static AVStream *create_stream(AVFormatContext *s, int codec_type)
{
    FLVContext *flv   = s->priv_data;
    //调用avformat_new_stream函数创建音视频流
    AVStream *st = avformat_new_stream(s, NULL);
    if (!st)//未成功创建
        return NULL;
    st->codecpar->codec_type = codec_type;
    if (s->nb_streams>=3 ||(   s->nb_streams==2
                           && s->streams[0]->codecpar->codec_type != AVMEDIA_TYPE_SUBTITLE
                           && s->streams[1]->codecpar->codec_type != AVMEDIA_TYPE_SUBTITLE
                           && s->streams[0]->codecpar->codec_type != AVMEDIA_TYPE_DATA
                           && s->streams[1]->codecpar->codec_type != AVMEDIA_TYPE_DATA))
        s->ctx_flags &= ~AVFMTCTX_NOHEADER;
    if (codec_type == AVMEDIA_TYPE_AUDIO) {
        st->codecpar->bit_rate = flv->audio_bit_rate;
        flv->missing_streams &= ~FLV_HEADER_FLAG_HASAUDIO;
    }
    if (codec_type == AVMEDIA_TYPE_VIDEO) {
        st->codecpar->bit_rate = flv->video_bit_rate;
        flv->missing_streams &= ~FLV_HEADER_FLAG_HASVIDEO;
        st->avg_frame_rate = flv->framerate;
    }


    avpriv_set_pts_info(st, 32, 1, 1000); /* 32 bit pts in ms */
    flv->last_keyframe_stream_index = s->nb_streams - 1;
    add_keyframes_index(s);
    return st;
}