Constant and Volatile Qualifiers

const

• const is used with a datatype declaration or definition to specify an unchanging value
  • Examples:

    	const int five = 5;
    	const double pi = 3.141593;
    

• const objects may not be changed
  • The following are illegal:

    	const int five = 5;
    	const double pi = 3.141593;
    
    	pi = 3.2;
    	five = 6;
    

volatile

• volatile specifies a variable whose value may be changed by processes outside the current program
• One example of a volatile object might be a buffer used to exchange data with an external device:

  int
  check_iobuf(void)
  {
  	volatile int iobuf;
  	int val;
  
  	while (iobuf == 0) {
  	}
  	val = iobuf;
  	iobuf = 0;
  	return(val);
  }
  

  if iobuf had not been declared volatile, the compiler would notice that nothing happens inside the loop and thus eliminate the loop
• const and volatile can be used together
  • An input-only buffer for an external device could be declared as const volatile (or volatile const, order is not important) to make sure the compiler knows that the variable should not be changed (because it is input-only) and that its value may be altered by processes other than the current program