line argument is not present, it will read from stdin. If a file cannot be opened, your program must print an appropriate error message to stderr. Write a function double sum(FILE in) to compute the sum for each file. The function must compute the sum and return it. The main function will print the sum. The output should look like this: The file names and sums will be in fields of width 10 , separated by a single space. If the program is reading from stdin, use "stdin" for the file name and omit the total. The cat2.c example program illustrates how to open files given as command line arguments and print appropriate error messages