ffmpeg学习：avformat_open_input函数

ffmpeg版本4.1

1、概述

int avformat_open_input(AVFormatContext **ps, const char *url, AVInputFormat *fmt, AVDictionary **options);
函数打开输入流，url指向打开的流的路径，如本地文件路径或rtsp地址，fmt参数强制指定输入流格式。

 

2、源码分析

 

int avformat_open_input(AVFormatContext **ps, const char *filename,
                        AVInputFormat *fmt, AVDictionary **options)
{
    AVFormatContext *s = *ps;
    int i, ret = 0;
    AVDictionary *tmp = NULL;
    ID3v2ExtraMeta *id3v2_extra_meta = NULL;

    if (!s && !(s = avformat_alloc_context())) //传递的AVFormatContext结构体指针为空时动态申请AVFormatContext空间
        return AVERROR(ENOMEM);
    if (!s->av_class) {
        av_log(NULL, AV_LOG_ERROR, "Input context has not been properly allocated by avformat_alloc_context() and is not NULL either\n");
        return AVERROR(EINVAL);
    }
    if (fmt)//指定输入格式的条件下赋值AVFormatContext结构体的iformat成员变量
        s->iformat = fmt;

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

    if (s->pb) // must be before any goto fail
        s->flags |= AVFMT_FLAG_CUSTOM_IO;//用户指定了数据访问方式，通过该方式可以实现内存访问功能

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

    if (!(s->url = av_strdup(filename ? filename : ""))) {//指定文件名的情况下记录打开的文件名到url，url是指针，对文件名长度无限制
        ret = AVERROR(ENOMEM);
        goto fail;
    }

#if FF_API_FORMAT_FILENAME
FF_DISABLE_DEPRECATION_WARNINGS
    av_strlcpy(s->filename, filename ? filename : "", sizeof(s->filename));//不推荐的保存文件名方式，文件名最长1024
FF_ENABLE_DEPRECATION_WARNINGS
#endif
    if ((ret = init_input(s, filename, &tmp)) < 0)//读取输入文件，遍历demuxer_list,找出得分最高的，赋值给s->iformat
        goto fail;
    s->probe_score = ret;

    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\'\n", s->format_whitelist);
        ret = AVERROR(EINVAL);
        goto fail;
    }

    avio_skip(s->pb, s->skip_initial_bytes);

    /* 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;

    /* Allocate private data. */
    if (s->iformat->priv_data_size > 0) {//输入格式有私有数据时，私有数据保存在s->priv_data，并将输入格式的AVClass赋值给指向私有数据的指针（暗含私有数据的第一个成员是指向AVClass对象的指针，可通过av_opt_set*函数设置私有对象成员的数据）
        if (!(s->priv_data = av_mallocz(s->iformat->priv_data_size))) {
            ret = AVERROR(ENOMEM);
            goto fail;
        }
        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 */
    if (s->pb)
        ff_id3v2_read_dict(s->pb, &s->internal->id3v2_meta, ID3v2_DEFAULT_MAGIC, &id3v2_extra_meta);

    if (!(s->flags&AVFMT_FLAG_PRIV_OPT) && s->iformat->read_header)
        if ((ret = s->iformat->read_header(s)) < 0) //调用read_header函数创建AVStream,赋值到s->streams[]指针数组中，如h264的输入格式（AVInputFormat)的read_header函数ff_raw_video_read_header，FF_DEF_RAWVIDEO_DEMUXER(h264, "raw H.264 video", h264_probe, "h26l,h264,264,avc", AV_CODEC_ID_H264)

            goto fail;

    if (!s->metadata) {
        s->metadata = s->internal->id3v2_meta;
        s->internal->id3v2_meta = NULL;
    } else if (s->internal->id3v2_meta) {
        int level = AV_LOG_WARNING;
        if (s->error_recognition & AV_EF_COMPLIANT)
            level = AV_LOG_ERROR;
        av_log(s, level, "Discarding ID3 tags because more suitable tags were found.\n");
        av_dict_free(&s->internal->id3v2_meta);
        if (s->error_recognition & AV_EF_EXPLODE)
            return AVERROR_INVALIDDATA;
    }

    if (id3v2_extra_meta) {
        if (!strcmp(s->iformat->name, "mp3") || !strcmp(s->iformat->name, "aac") ||
            !strcmp(s->iformat->name, "tta")) {
            if ((ret = ff_id3v2_parse_apic(s, &id3v2_extra_meta)) < 0)
                goto fail;
            if ((ret = ff_id3v2_parse_chapters(s, &id3v2_extra_meta)) < 0)
                goto fail;
            if ((ret = ff_id3v2_parse_priv(s, &id3v2_extra_meta)) < 0)
                goto fail;
        } else
            av_log(s, AV_LOG_DEBUG, "demuxer does not support additional id3 data, skipping\n");
    }
    ff_id3v2_free_extra_meta(&id3v2_extra_meta);

    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);

    for (i = 0; i < s->nb_streams; i++)
        s->streams[i]->internal->orig_codec_id = s->streams[i]->codecpar->codec_id;

    if (options) {
        av_dict_free(options);
        *options = tmp;
    }
    *ps = s;
    return 0;

fail:
    ff_id3v2_free_extra_meta(&id3v2_extra_meta);
    av_dict_free(&tmp);
    if (s->pb && !(s->flags & AVFMT_FLAG_CUSTOM_IO))
        avio_closep(&s->pb);
    avformat_free_context(s);
    *ps = NULL;
    return ret;
}