There is a very real difference between characters and strings, and strcat concatenates strings.

A character constant like '!' represents a single character. A string literal between double quotes usually represents multiple characters. A string literal like "!" seems to represent a single character, but it actually contains two: the ! you requested, and the which terminates all strings in C.

Characters in C are represented by small integers corresponding to their character set values Strings are represented by arrays of characters; you usually manipulate a pointer to the first character of the array. It is never correct to use one when the other is expected. To append a ! to a string, use

strcat(string, "!");