写一个文件并使其只读

I write a database for students in C under Linux. So after entering my data and after closing the program, the data will be saved automatically in a text-file. How can I make this file read only? I will later add a function that will read the data from the file, so only the program can process data. Any user who opens the file can only read data but he has no rights to change the data in the file. In other words how can I make my saved data secure? Here is some code:I write a database for students in C under Linu