C/C++ :: Wrapping Alphabet Using ASCII

I'm currently working on a project that utilitzes a Caesar Cipher. Basically, it reads input from a file and then, depending upon the input file, requires encrypting or decrypting. The cipher used to encrypt/decrypt simply adds/subtracts an integer from each character in the input file. All input is lowercase alphabetic characters (ASCII 97-122). My problem is trying to figure out how to wrap the ciphered characters, e.g., if the cipher require a shift of +5 and the character read was 'x', a shift of +5 to the right would produce 'c' (beginning with 'x' and counting up 5 letters: 'y', 'z', 'a', 'b', 'c').

I've written some crude and cumbersome loops that will work, but there has to be a better way. It is my understanding that it can be done using the modulus operation, %, but I cannot figure out how. I spent several hours trying to figure out the modulus approach, but no luck.