Volatile Example: The Value of the Keyword ‘Volatile’ in C

The keyword “volatile” informs the compiler that the value of a variable can be changed from the outside. This can occur under the control of the operating system, hardware, or another stream. Because the value is changeable, the compiler loads it from memory each time.

A volatile integer variable can be declared as:

int volatile х;
volatile int х;

To declare a pointer to this variable, do the following:

volatile int *х;
int volatile *х;

A volatile pointer on a nonvolatile data is used rarely, but:

int *volatile х;

If you want to declare a volatile pointer to a volatile memory area, you should do the following:

int volatile *volatile х;

The volatile variables are not optimized and it can be useful. Imagine the following function:

int opt = 1;
void Fn(void) {
    start:
        if (opt == 1)
            goto start;
        else
            break;
}

At first glance, the program will loop. The compiler can optimize it as follows:

void Fn(void) {
    start:
    int opt = 1;
    if (true)
        goto start;
)

Now the cycle will definitely be endless. However, the external operation allows writing 0 to the variable opt and break the cycle.

To prevent such optimization, you can use the keyword “volatile”, for example, to announce that some external element of the system changes the variable:

volatile int opt = 1;
void Fn(void) {
    start:
    if (opt == 1)
        goto start;
    else
        break;
}

Volatile variables are used as global variables in multithreaded programs. Any thread can change common variables. We do not want to optimize these variables.

Thanks for your attention!

References

McDowell, Gayle Laakmann. Cracking the Coding Interview: 150 Programming Questions and Solutions.

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