Cracking the Coding Interview - Fooo

Solutions to Chapter 13 | C++
13 5 What is the significance of the keyword “volatile” in C?
SOLUTION
2 1 9
Cracking the Coding Interview | Knowledge Based
pg 76
Volatile informs the compiler that the value of the variable can change from the outside,
without any update done by the code
Declaring a simple volatile variable:
volatile int x;
int volatile x;
Declaring a pointer variable for a volatile memory (only the pointer address is volatile):
volatile int * x;
int volatile * x;
Declaring a volatile pointer variable for a non-volatile memory (only memory contained is
volatile):
int * volatile x;
Declaring a volatile variable pointer for a volatile memory (both pointer address and memory
contained are volatile):
volatile int * volatile x;
int volatile * volatile x;
Volatile variables are not optimized, but this can actually be useful Imagine this function:
1 int opt = 1;
2 void Fn(void) {
3 start:
4 if (opt == 1) goto start;
5 else break;
6 }
At first glance, our code appears to loop infinitely The compiler will try to optimize it to:
1 void Fn(void) {
2 start:
3 int opt = 1;
4 if (true)
5 goto start;
6 }
This becomes an infinite loop However, an external program might write ‘0’ to the location
of variable opt Volatile variables are also useful when multi-threaded programs have global
variables and any thread can modify these shared variables Of course, we don’t want optimization
on them