函数原型
/**
* Open an input stream and read the header. The codecs are not opened.
* The stream must be closed with avformat_close_input().
*
* @param ps Pointer to user-supplied AVFormatContext (allocated by avformat_alloc_context).
* May be a pointer to NULL, in which case an AVFormatContext is allocated by this
* function and written into ps.
* Note that a user-supplied AVFormatContext will be freed on failure.
* @param url URL of the stream to open.
* @param fmt If non-NULL, this parameter forces a specific input format.
* Otherwise the format is autodetected.
* @param options A dictionary filled with AVFormatContext and demuxer-private options.
* On return this parameter will be destroyed and replaced with a dict containing
* options that were not found. May be NULL.
*
* @return 0 on success, a negative AVERROR on failure.
*
* @note If you want to use custom IO, preallocate the format context and set its pb field.
*/
int avformat_open_input(AVFormatContext **ps, const char *url, AVInputFormat *fmt, AVDictionary **options);
参数
AVFormatContext** ps:格式化上下文
动态链接库的空间只能在动态链接库里面清理，因此这里初始化 指向一个空的指针，然后，将指针地址传入动态链接库，在里面初始化，指针的地址的值就会发生变化，因此可以用 动态链接库close清理 ，这里分配的空间在程序退出时通过avformat_close_input函数释放。
const char *url:音视频流的url
可以是文件名，也可以是网络流url，会存储AVFormatContext 里面，以便rtsp等断开重连
AVInputFormat *fmt:音视频的封装格式
可以传NULL，让ffmpeg自己去探测
AVDictionary **options:选项参数
一组key value。 可以传NULL递。具体定义在源文件libavformat/options_table.h 里说明。
返回值
0 成功 负值 失败
