open_input_file函数调用结构图（有些重复的函数调用就略掉了，大致是按流程往下的）。
函数大致说明：
AVFormatContext *avformat_alloc_context(void);
函数介绍：
Allocate an AVFormatContext.
avformat_free_context() can be used to free the context and everything
allocated by the framework within it.
说明：为AVFormatContext分配空间，并设置缺省值，申请的空间用avformat_free_context()释放。
int av_dict_set(AVDictionary **pm, const char *key, const char *value, int flags);
函数介绍：
Set the given entry in *pm, overwriting an existing entry.
*
* Note: If AV_DICT_DONT_STRDUP_KEY or AV_DICT_DONT_STRDUP_VAL is set,
* these arguments will be freed on error.
*
* @param pm pointer to a pointer to a dictionary struct. If *pm is NULL
* a dictionary struct is allocated and put in *pm.
* @param key entry key to add to *pm (will be av_strduped depending on flags)
* @param value entry value to add to *pm (will be av_strduped depending on flags).
* Passing a NULL value will cause an existing entry to be deleted.
* @return >= 0 on success otherwise an error code <0
*
* Note: If AV_DICT_DONT_STRDUP_KEY or AV_DICT_DONT_STRDUP_VAL is set,
* these arguments will be freed on error.
*
* @param pm pointer to a pointer to a dictionary struct. If *pm is NULL
* a dictionary struct is allocated and put in *pm.
* @param key entry key to add to *pm (will be av_strduped depending on flags)
* @param value entry value to add to *pm (will be av_strduped depending on flags).
* Passing a NULL value will cause an existing entry to be deleted.
* @return >= 0 on success otherwise an error code <0
说明：设置给定的AVDictionary数组，会覆盖已经存在的数据。
static AVCodec *find_codec_or_die(const char *name, enum AVMediaType type, int encoder)
函数说明：寻找编码器，找不到就退出程序。
AVDictionaryEntry *av_dict_get(const AVDictionary *m, const char *key, const AVDictionaryEntry *prev, int flags);
函数说明：根据key来查找AVDictionaryEntry。
int avformat_open_input(AVFormatContext **ps, const char *filename, AVInputFormat *fmt, AVDictionary **options);
函数介绍：
/**
* 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 filename Name 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.
*/
* 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 filename Name 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.
*/
说明：打开input流并读取头信息，打开的流必须用avformat_close_input()关闭。
static AVCodec *choose_decoder(OptionsContext *o, AVFormatContext *s, AVStream *st)
函数说明：先调用MATCH_PER_STREAM_OPT匹配出codec_names若成功匹配，则调用find_codec_or_die查找解码器，若不成功则调用avcodec_find_decoder查找解码器。
AVDictionary **setup_find_stream_info_opts(AVFormatContext *s, AVDictionary *codec_opts);
函数介绍：
/**
* Setup AVCodecContext options for avformat_find_stream_info().
*
* Create an array of dictionaries, one dictionary for each stream
* contained in s.
* Each dictionary will contain the options from codec_opts which can
* be applied to the corresponding stream codec context.
*
* @return pointer to the created array of dictionaries, NULL if it
* cannot be created
*/
* Setup AVCodecContext options for avformat_find_stream_info().
*
* Create an array of dictionaries, one dictionary for each stream
* contained in s.
* Each dictionary will contain the options from codec_opts which can
* be applied to the corresponding stream codec context.
*
* @return pointer to the created array of dictionaries, NULL if it
* cannot be created
*/
说明：为avformat_find_stream_info函数创建AVDictionary**。
int avformat_find_stream_info(AVFormatContext *ic, AVDictionary **options);
函数介绍：
/**
* 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.
*/
* 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.
*/
说明：读取media的packets来获取stram info，可用于读取没有头的MPEG文件信息，也可以计算MPEG-2 repeat frame mode的实际framerate。
static void add_input_streams(OptionsContext *o, AVFormatContext *ic)
函数介绍：
/* Add all the streams from the given input file to the global
* list of input streams. */
* list of input streams. */
说明：把给定的输入文件中的流加入全局输入流list里去。可以从函数调用图里看到，此函数用流的数目来做循环。
void av_dump_format(AVFormatContext *ic, int index, const char *url, int is_output);
函数介绍：
/**
* Print detailed information about the input or output format, such as
* duration, bitrate, streams, container, programs, metadata, side data,
* codec and time base.
*
* @param ic the context to analyze
* @param index index of the stream to dump information about
* @param url the URL to print, such as source or destination file
* @param is_output Select whether the specified context is an input(0) or output(1)
*/
* Print detailed information about the input or output format, such as
* duration, bitrate, streams, container, programs, metadata, side data,
* codec and time base.
*
* @param ic the context to analyze
* @param index index of the stream to dump information about
* @param url the URL to print, such as source or destination file
* @param is_output Select whether the specified context is an input(0) or output(1)
*/
说明：打印信息，如：duration, bitrate, streams, container, programs, metadata, side data, codec and time base。
static AVDictionary *strip_specifiers(AVDictionary *dict)
函数介绍：
/* return a copy of the input with the stream specifiers removed from the keys */
说明：拷贝dict。
http://blog.csdn.net/dancing_night/article/details/44962089