FFmpeg源代码：AVFormatContext相关接口

 与AVFormatContext结构体相关的接口函数声明位于libavformat\avformat.h中

avformat_alloc_context

/**
 * Allocate an AVFormatContext.
 * avformat_free_context() can be used to free the context and everything
 * allocated by the framework within it.
 */
AVFormatContext *avformat_alloc_context(void);

初始化结构体，为AVFormatContext结构体分配内存,并设置AVFormatContext的字段的默认值。

avformat_free_context

/**
 * Free an AVFormatContext and all its streams.
 * @param s context to free
 */
void avformat_free_context(AVFormatContext *s);

销毁AVFormatContext, 释放内存。

avformat_open_input

​​​​​​​
/**
* 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, ff_const59 AVInputFormat *fmt, AVDictionary **options);

该接口注释已经写的很详细了，中文简单概述下

ps：函数调用成功之后处理过的AVFormatContext结构体。
url：打开的视音频流的URL。
fmt：强制指定AVFormatContext中AVInputFormat的。这个参数一般情况下可以设置为NULL，这样FFmpeg可以自动检测AVInputFormat。
dictionay：附加的一些选项，一般情况下可以设置为NULL。

该接口的功能是打开一个输入媒体文件，读取文件头（如果有的话，MPEG没有format header）并探测视频封装格式，将媒体文件信息存储到ps中。 

需要注意的是，有些封装格式并没有header 或者header中并没有存储足够的信息，所以还需要调用avformat_find_stream_info函数来读取并解码一些帧来获取足够的信息。

avformat_open_input 不会打开codec.

 具体代码分析可以参见雷神的这篇文章，写的很详细了，不再赘述：FFmpeg源代码简单分析：avformat_open_input()

avformat_find_stream_info

/**
 * Read packets of a media file to get stream information. This
 * is useful for file formats with no headers such as MPEG. This
 * function also computes the real framerate in case of MPEG-2 repeat
 * frame mode.
 * The logical file position is not changed by this function;
 * examined packets may be buffered for later processing.
 *
 * @param ic media file handle
 * @param options  If non-NULL, an ic.nb_streams long array of pointers to
 *                 dictionaries, where i-th member contains options for
 *                 codec corresponding to i-th stream.
 *                 On return each dictionary will be filled with options that were not found.
 * @return >=0 if OK, AVERROR_xxx on error
 *
 * @note this function isn't guaranteed to open all the codecs, so
 *       options being non-empty at return is a perfectly normal behavior.
 *
 * @todo Let the user decide somehow what information is needed so that
 *       we do not waste time getting stuff the user does not need.
 */
int avformat_find_stream_info(AVFormatContext *ic, AVDictionary **options);

 该函数通过读取一部分音视频数据来获取流信息，同时能够获取真实的帧率。该函数主要用于给每个媒体流（音频/视频）的AVStream结构体赋值，获取真实的流信息。