本文简单分析FFmpeg中1个经常使用的函数：avformat_open_input()。该函数用于打开多媒体数据并且取得1些相干的信息。它的声明位于libavformatavformat.h，以下所示。
/**
* 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.
*/
int avformat_open_input(AVFormatContext **ps, const char *filename, AVInputFormat *fmt, AVDictionary **options);
* 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.
*/
int avformat_open_input(AVFormatContext **ps, const char *filename, AVInputFormat *fmt, AVDictionary **options);
代码中的英文注释写的已比较详细了，在这里拿中文简单叙述1下。
ps：函数调用成功以后处理过的AVFormatContext结构体。
file：打开的视音频流的URL。
fmt：强迫指定AVFormatContext中AVInputFormat的。这个参数1般情况下可以设置为NULL，这样FFmpeg可以自动检测AVInputFormat。dictionay：附加的1些选项，1般情况下可以设置为NULL。
函数履行成功的话，其返回值大于等于0。
该函数最典型的例子可以参考： 最简单的基于FFMPEG+SDL的视频播放器 ver2 （采取SDL2.0）
此前已粗略写了1篇关于avformat_open_input()的文章《 图解FFMPEG打开媒体的函数avformat_open_input》，还转载了1篇注释比较详细的文章《FFMPEG源码分析：avformat_open_input()（媒体打开函数）》。但是个人感觉这个函数确切太重要了，可以算作FFmpeg的“灵魂”，所以打算再写1篇文章分析1下它的结构。
函数调用关系图
函数调用结构图以下所示。
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波比源码 » FFmpeg源代码简单分析：avformat_open_input()
波比源码 » FFmpeg源代码简单分析：avformat_open_input()
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