The storage-class-specifier can be one of the following:
Serves as a type-specifier for another type-specifier. This does not create a variable—it is merely for convenience.
Indicates that the variable is defined outside of the current file. This brings the variables scope into the current scope. No variable is actually created by this declaration.
Causes a variable that is defined within a function to be preserved in subsequent calls to the function.
Causes a local variable to have a local lifetime (default).
Requests that the variable be accessed as quickly as possible. This request is not guaranteed. Normally, the variable's value is kept within a CPU register for maximum speed.