a C program that simulates a different version of Unix?s diff command

Part II- D

? Write a C program that simulates a different version of Unix?s diff command (to be called diffnew.c). For example, given two text files file1.txt and file2.txt:
diffnew file1.txt file2.txt flag
returns:
o Zero (0) if the two files are exactly identical
o -1 if either one of the two files is missing or cannot be opened, or an illegal flag value is used. Provide an explanation of the encountered error.
o A positive integer indicating:
the number of characters that are different in the two files (if 1 flag is used)
the number of words that are different in the two files (if 2 flag is used)

Example output would look like:

Files contain 15 different characters.