使用O_CREAT进行open（）调用的默认模式是什么，以及如何在打开/创建文件时正确设置它

I am trying to recreate the basic functionality of fopen() using the I/O system calls. I am somehow suppose to "set the default mode for open() calls with O_CREAT" however am unsure how to go about this. It's not perfect but this is what I've got so far. I am trying to recreate the basic functionality