Cracking the Coding Interview, 4 Edition - 150 Programming Interview Questions and Solutions

Solutions to Chapter 13 | C++
13.5 What is the significance of the keyword “volatile” in C?
pg 76
SOLUTION
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.
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