The __inline keyword tells the compiler that it can substitute the code within the function definition for every instance of a function call. Substitution occurs at the discretion of the compiler. You can specify that individual functions be __inline functions or you can use the /Ob2 command-line option to have the compiler optimize your code by generating as many functions inline as possible. The __inline keyword (and the inline keyword when compiling with /Ze) are new keywords for Microsoft C, version 7.0. See topic for more information.