How to Use the Volatile Keyword in C?

Last Updated : 23 Jul, 2025

In C, the volatile keyword is used to inform the compiler that the value of a variable may change at any time, without any action being taken by the code in the program. It is particularly useful in embedded systems, signal handlers, and multi-threaded applications. In this article, we will learn how to use the volatile keyword in C.

Volatile Keyword in C

We can use the volatile keyword for different purposes like declaring some global variables, signal handlers, variables across shared threads, etc. When a variable is declared as volatile, it tells the compiler:

  • The compiler must not assume the value of the variable remains constant between accesses, so it should read the value from memory every time it is used.
  • The compiler must not reorder instructions in a way that changes the access order of the volatile variable.

Syntax to Use Volatile Qualifier in C

volatile dataType varName;

C Program to Demonstrate the Use of Volatile Keyword

The below program demonstrates the use of volatile keyword in C.

C 
// C Program to Show how to use the Volatile Keyword

#include <pthread.h>
#include <stdio.h>

// Volatile variable to be accessed by multiple threads
volatile int volVar = 0;

// Mutex for synchronization
pthread_mutex_t mtx = PTHREAD_MUTEX_INITIALIZER;

// Function to increment the volatile variable
void* incValue(void* arg)
{
    for (int i = 0; i < 10; i++) {
        // Lock the mutex before accessing volVar
        pthread_mutex_lock(&mtx);
        volVar++;
        // Unlock the mutex after modifying volVar
        pthread_mutex_unlock(&mtx);
    }
    return NULL;
}

int main()
{
    pthread_t t1, t2;

    // Create two threads to increment volVar
    pthread_create(&t1, NULL, incValue, NULL);
    pthread_create(&t2, NULL, incValue, NULL);

    // Wait for both threads to finish
    pthread_join(t1, NULL);
    pthread_join(t2, NULL);

    // Output the final value of volVar
    printf("Final value of volVar: %d\n", volVar);

    return 0;
}


Output

Final value of volVar: 20
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