FFmpeg .avformat_open_input接口

/**
 * 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);

该函数用于打开一个输入流并读取它的头部。

参数说明：

AVFormatContext **ps：格式化的上下文。要注意，如果传入的是一个AVFormatContext*的指针，则该空间须自己手动清理，若传入的指针为空，则FFmpeg会内部自己创建。

const char *filename：要打开的流的文件名，传入的地址。支持http,RTSP,以及普通的本地文件。地址最终会存入到AVFormatContext结构体当中。

AVInputFormat *fmt：强制指定输入的封装格式。一般传NULL，由FFmpeg自行探测。

AVDictionary **options：其它参数设置。它是一个字典，用于参数传递，不传则写NULL。参见：libavformat/options_table.h,其中包含了它支持的参数设置。

FFmpeg .avformat_open_input接口

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