Because the file can contain "blank spaces" (i.e. spaces, tabs, returns, etc.) you will need to input the elements on a character by character basis, as opposed to trying to read all of the file at one time.
After you have read the contents of the file into an array, write the
content of the array back to the file, this time with the characters in
the reverse order that they were in before.
Your project should be divided into several different files. At a minimum, you should have one file that contains the main program, one file that contains auxiliary functions, and one header file.