avformat_open_input  

打开媒体函数，先上全部源码。然后逐语句分析。

int avformat_open_input(AVFormatContext **ps, const char *filename,
                        ff_const59 AVInputFormat *fmt, AVDictionary **options)
{
    //第一部分解析
    AVFormatContext *s = *ps;
    int i, ret = 0;
    AVDictionary *tmp = NULL;
    ID3v2ExtraMeta *id3v2_extra_meta = NULL;
	//第二部分解析 
    if (!s && !(s = avformat_alloc_context()))
        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);
    }
    //第三部分解析 
    //如果用户指定了输入格式，直接使用它  
    if (fmt)
        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;

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

    if (!(s->url = av_strdup(filename ? filename : ""))) {
        ret = AVERROR(ENOMEM);
        goto fail;
    }

#if FF_API_FORMAT_FILENAME
FF_DISABLE_DEPRECATION_WARNINGS
    av_strlcpy(s->filename, filename ? filename : "", sizeof(s->filename));
FF_ENABLE_DEPRECATION_WARNINGS
#endif
    if ((ret = init_input(s, filename, &tmp)) < 0)
        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);

    
    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;

    
    if (s->iformat->priv_data_size > 0) {
        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;
        }
    }

    
    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)
            goto fail;

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

    if (id3v2_extra_meta) {
        if (!strcmp(s->iformat->name, "mp3") || !strcmp(s->iformat->name, "aac") ||
            !strcmp(s->iformat->name, "tta") || !strcmp(s->iformat->name, "wav")) {
            if ((ret = ff_id3v2_parse_apic(s, id3v2_extra_meta)) < 0)
                goto close;
            if ((ret = ff_id3v2_parse_chapters(s, id3v2_extra_meta)) < 0)
                goto close;
            if ((ret = ff_id3v2_parse_priv(s, id3v2_extra_meta)) < 0)
                goto close;
        } 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 close;

    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;

close:
    if (s->iformat->read_close)
        s->iformat->read_close(s);
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;
}

第一部分:以下是定义了一些必备的变量。

    AVFormatContext *s = *ps;
    int i, ret = 0;
    AVDictionary *tmp = NULL;
    ID3v2ExtraMeta *id3v2_extra_meta = NULL;

第二部分:创建上下文结构,其作用为开辟相关空间并且进行一些初始化赋值。返回上下文结构的指针。

    if (!s && !(s = avformat_alloc_context()))
        return AVERROR(ENOMEM);

avformat_alloc_context 函数详细解析：

AVFormatContext *avformat_alloc_context(void)
{
	//定义封装上下文结构体
    AVFormatContext *ic;
    AVFormatInternal *internal;
	//使用sizeof计算结构体大小并且分配空间
    ic = av_malloc(sizeof(AVFormatContext));
    if (!ic) return ic;

    internal = av_mallocz(sizeof(*internal));
    if (!internal) {
        av_free(ic);
        return NULL;
    }
    avformat_get_context_defaults(ic);
    ic->internal = internal;
    ic->internal->offset = AV_NOPTS_VALUE;
    ic->internal->raw_packet_buffer_remaining_size = RAW_PACKET_BUFFER_SIZE;
    ic->internal->shortest_end = AV_NOPTS_VALUE;

    return ic;
}

其中av_malloc封装了c的malloc内存分配函数

void *av_malloc(size_t size)
{
    void *ptr = NULL;

	//max_aloc_size 为 2147483647 
    if (size > max_alloc_size)
        return NULL;

#if HAVE_POSIX_MEMALIGN
    if (size) //OS X on SDK 10.6 has a broken posix_memalign implementation
    if (posix_memalign(&ptr, ALIGN, size))
        ptr = NULL;
#elif HAVE_ALIGNED_MALLOC
    ptr = _aligned_malloc(size, ALIGN);
#elif HAVE_MEMALIGN
#ifndef __DJGPP__
    ptr = memalign(ALIGN, size);
#else
    ptr = memalign(size, ALIGN);
#endif
    
    
    
#else
    ptr = malloc(size);
#endif
    if(!ptr && !size) {
        size = 1;
        ptr= av_malloc(1);
    }
#if CONFIG_MEMORY_POISONING
    if (ptr)
        memset(ptr, FF_MEMORY_POISON, size);
#endif
    return ptr;
}

第三部分：

如果已经事先定义好了AVInputFormat那么就把定义好的直接赋予在这里。具体结构可以参考下面

typedef struct AVInputFormat {
    const char *name; // 输入格式的短名称
    const char *long_name; // 格式的长名称（相对于短名称而言，更易于阅读）
    
    int flags;
    const char *extensions; // 如果定义了扩展，就不会进行格式探测。但因为该功能目前支持不够，不推荐使用
    const struct AVCodecTag * const *codec_tag; // 见名知意
    const AVClass *priv_class; ///< AVClass for the private context
    const char *mime_type; // mime类型，它用于在探测时检查匹配的mime类型。
		
    struct AVInputFormat *next; // 用于链接下一个AVInputFormat
    int raw_codec_id; // 原始demuxers将它们的解码器id保存在这里。
    int priv_data_size; // 私有数据大小，可以用于确定需要分配多大的内存来容纳下这些数据。
		
    int (*read_probe)(AVProbeData *);
    
    int (*read_header)(struct AVFormatContext *);
    
    int (*read_packet)(struct AVFormatContext *, AVPacket *pkt);
     // 关闭流，AVFormatContext和AVStreams并不会被这个函数释放。
    int (*read_close)(struct AVFormatContext *);
    
    int (*read_seek)(struct AVFormatContext *,
                     int stream_index, int64_t timestamp, int flags);
    // 获取stream[stream_index]的下一个时间戳，如果发生异常返回AV_NOPTS_VALUE
    int64_t (*read_timestamp)(struct AVFormatContext *s, int stream_index,
                              int64_t *pos, int64_t pos_limit);
    // 开始或者恢复播放，只有在播放rtsp格式的网络格式才有意义。
    int (*read_play)(struct AVFormatContext *);
    int (*read_pause)(struct AVFormatContext *);// 暂停播放，只有在播放rtsp格式的网络格式才有意义。
    
    int (*read_seek2)(struct AVFormatContext *s, int stream_index, int64_t min_ts, int64_t ts, int64_t max_ts, int flags);
    // 返回设备列表和其属性
    int (*get_device_list)(struct AVFormatContext *s, struct AVDeviceInfoList *device_list);
    // 初始化设备能力子模块
    int (*create_device_capabilities)(struct AVFormatContext *s, struct AVDeviceCapabilitiesQuery *caps);
    // 释放设备能力子模块
    int (*free_device_capabilities)(struct AVFormatContext *s, struct AVDeviceCapabilitiesQuery *caps);
} AVInputFormat;