So far, the examples have shown const to be conceptually simple. But what does volatile really mean? For the compiler, it means don't take any code generation shortcuts when accessing such an object. On the other hand, ISO C makes it the programmer's responsibility to declare volatile every object that has the appropriate special properties.
The usual four examples of volatile objects are:
An object that is a memory-mapped I/O port
An object that is shared between multiple concurrent processes
An object that is modified by an asynchronous signal handler
An automatic storage duration object declared in a function that calls setjmp, and whose value is changed between the call to setjmp and a corresponding call to longjmp
The first three examples are all instances of an object with a particular behavior: its value can be modified at any point during the execution of the program. Thus, the following seemingly infinite loop is valid as long as flag has a volatile qualified type.
flag = 1; while (flag);
Presumably, some asynchronous event sets flag to zero in the future. Otherwise, because the value of flag is unchanged within the body of the loop, the compilation system is free to change the above loop into a truly infinite loop that completely ignores the value of flag.
The fourth example, involving variables local to functions that call setjmp, is more involved. The details about the behavior of setjmp and longjmp indicates that the values for objects matching the fourth case are unpredictable. For the most desirable behavior, longjmp must examine every stack frame between the function calling setjmp and the function calling longjmp for saved register values. The possibility of asynchronously created stack frames makes this job even harder.
When an automatic object is declared with a volatile qualified type, the compiler must generate code that exactly matches what the programmer wrote. Therefore, the most recent value for such an automatic object is always in memory and not just in a register, and is guaranteed to be up-to-date when longjmp is called .