int avio_open(AVIOContext **s, const char *url, int flags);
int avio_open2(AVIOContext **s, const char *url, int flags,
               const AVIOInterruptCB *int_cb, AVDictionary **options);

从源码中来看avio_open就是调用avio_open2()来实现的，只是把avio_open2()的最后两个参数置NULL，
s：会创建一个AVIOContext对象。
url：输入输出的地址，可以是文件地址，也可以是网络地址，比如udp://，rtmp://等。
flags：打开地址的方式，有三个宏。
AVIO_FLAG_READ：只读。
AVIO_FLAG_WRITE：只写。
AVIO_FLAG_READ_WRITE：读写。
int_cb
协议级的中断回调。
options
每个协议的私有选项，比如udp可控制包大小等。
函数会根据URL路径来识别是哪种协议，其实文件在ffmpeg中也是协议，是file://…，但是这不符合我们书写的习惯，如果识别出是文件的话，函数会自动帮我们在前面补充file://…，而不用我们自己写。

if (!(ofmt->flags & AVFMT_NOFILE)) {
    //  若不是自动打开文件的模式，则需要此手动操作
		AVDictionary*dic = NULL;
		av_dict_set(&dic, "pkt_size", "1316", 0);	//Maximum UDP packet size
		//av_dict_set(&dic, "fifo_size", "18800", 0);
		//av_dict_set(&dic, "buffer_size", "1000000", 0);
		//av_dict_set(&dic, "bitrate", "11000000", 0);
		//av_dict_set(&dic, "buffer_size", "1000000", 0);//1316
		av_dict_set(&dic, "reuse", "1", 0);
		ret = avio_open2(&ofmt_ctx->pb, out_filename, AVIO_FLAG_WRITE, NULL, &dic);
	}

libavdevice库是libavformat库的一个补充。它提供各类不一样的平台相关的复用器和解复用器，例如grabbing device(gdigrab Windows下录屏接口)，音频捕获和回放等，libavdevice中的复用器大部分是AVFMT_NOFILE类型，AVFMT_NOFILE类型的复用器，不须要调用avio_open和avio_close对I/O进行操做，AVFMT_NOFILE类型的复用器都有本身的I/O函数。
不单单libavdevice中的大部分是AVFMT_NOFILE类型，libavformat也存在很多的解复用器。

关于AVFMT_NOFILE，参考：[AVFMT_NOFILE解析](https://www.shangmayuan.com/a/027018e619644bbc8782f7e9.html)

/**

• Create and initialize a AVIOContext for accessing the
• resource indicated by url.
• @note When the resource indicated by url has been opened in
• read+write mode, the AVIOContext can be used only for writing.
• @param s Used to return the pointer to the created AVIOContext.
• In case of failure the pointed to value is set to NULL.
• @param url resource to access
• @param flags flags which control how the resource indicated by url
• is to be opened
• @return >= 0 in case of success, a negative value corresponding to an
• AVERROR code in case of failure
  */
  int avio_open(AVIOContext **s, const char *url, int flags);

/**

• Create and initialize a AVIOContext for accessing the
• resource indicated by url.
• @note When the resource indicated by url has been opened in
• read+write mode, the AVIOContext can be used only for writing.
• @param s Used to return the pointer to the created AVIOContext.
• In case of failure the pointed to value is set to NULL.
• @param url resource to access
• @param flags flags which control how the resource indicated by url
• is to be opened
• @param int_cb an interrupt callback to be used at the protocols level
• @param options A dictionary filled with protocol-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 in case of success, a negative value corresponding to an
• AVERROR code in case of failure
  */
  int avio_open2(AVIOContext **s, const char *url, int flags,
  const AVIOInterruptCB *int_cb, AVDictionary **options);