Last Updated on March 29, 2023 by Prepbytes
The volatile keyword in C is used to indicate to the compiler that a variable’s value may change unexpectedly. This is often the case when a variable is being accessed by multiple threads or when it represents hardware that is external to the computer. This article discusses what the volatile keyword in C is, how it works, and how to use volatile keywords in C.
What is a Volatile Keyword in C?
A volatile keyword in C is just a qualifier used by the programmer when declaring a variable in source code. It tells the compiler that the variable value can be modified at any point without needing any task from the source code. When we declare a variable, we normally use the word volatile. The volatile keyword is used mostly to prevent optimizations on objects in our source code. As a result, an object labeled as ‘volatile’ cannot be optimized because its value can be readily altered by the code.
Syntax of Volatile Keyword in C
The syntax for using the volatile keyword in C is as follows:
volatile data_type variable_name;This declares a volatile variable of the specified data type and assigns it the given variable name. The keyword volatile is placed before the variable name to indicate that the variable’s value may change unexpectedly and should not be optimized by the compiler.
How does Volatile Keyword in C Works?
When a variable is declared as volatile, the compiler generates code that ensures that every access to the variable reads or writes its actual value, rather than using any cached or optimized value. This guarantees that the program behaves as expected, even if the variable’s value changes unexpectedly.
Without the volatile keyword, the compiler may optimize the code by assuming that the value of the variable will not change, and it may reuse the previous value read from the variable instead of reading its current value. This optimization can cause incorrect behavior if the value of the variable changes unexpectedly.
For example, consider a program that reads data from a hardware device and stores it in a variable. Without the volatile keyword, the compiler may assume that the value of the variable will not change during the program’s execution, and it may optimize the code by reading the value of the variable only once. However, the value of the variable may change unexpectedly due to external factors, such as a change in the hardware device’s state. In this case, using the volatile keyword ensures that the program reads the current value of the variable every time it is accessed, rather than using any cached or optimized value.
Example of Volatile Keyword in C
Here is an example to demonstrate the working of a volatile keyword in C:
Code Implementation:
#include <stdio.h>
int main (void) {
const volatile int prepBuddy = 125;
int *ptr = ( int* ) &prepBuddy;
printf("The initial value of the prepBuddy is : %d \n ", prepBuddy);
*ptr = 251;
printf("The modified value of the prepBuddy is : %d \n ", prepBuddy);
return 0;
} Output:
The initial value of the local_value is: 125
The modified value of the local_value is: 251 Explanation: In the preceding code, you can see that we declared a constant volatile variable of an integer data type with the name local_value and assigned it the value 125. Then we declared an integer data type pointer in which we will store the address value of "local_value." Furthermore, we print the old value first, followed by the changed value on the screen. This change is only allowed because we use the volatile keyword in the variable’s declaration.
Advantages of Using Volatile Keyword in C
Here are some advantages of using the volatile keyword in C:
- Consistency in multi-threaded programming: When multiple threads access a shared variable, the value of the variable can change unexpectedly, leading to synchronization issues. The volatile keyword ensures that the variable’s value is always read and written as expected, even if it changes unexpectedly, thereby ensuring consistency in multi-threaded programming.
- Correct operation with hardware devices: In embedded programming, hardware devices generate interrupts that can change a program’s control flow. The volatile keyword ensures that the program reads the value of the hardware device’s register correctly and updates the program’s state accordingly, thereby enabling correct operation with hardware devices.
- Prevents compiler optimizations: The compiler can optimize code by assuming that a variable’s value will not change, leading to incorrect behavior if the value changes unexpectedly. The volatile keyword prevents compiler optimizations and ensures that the variable’s value is always read and written as expected, even if it changes unexpectedly.
Disadvantages of Using Volatile Keyword in C
Here are some disadvantages of using the volatile keyword in C:
- Reduced performance: The use of the volatile keyword can lead to reduced performance because the compiler cannot optimize the code as much as it would be able to without the keyword. This can result in slower code execution and increased memory usage.
- Potential for code complexity: The use of the volatile keyword can make the code more complex and difficult to read, especially in large programs with many volatile variables.
- Increased debugging difficulty: The use of the volatile keyword can make debugging more difficult, especially in cases where unexpected behavior occurs due to changes in the value of a volatile variable.
- Limited applicability: The volatile keyword is only useful in specific cases where there is a need for synchronization between hardware devices or multiple threads. In most cases, it is not necessary and can even hinder performance.
Conclusion
In conclusion, the volatile keyword in C is an important feature that is used to indicate that a variable may change unexpectedly due to external factors. The keyword ensures that the value of the variable is read and written to memory, rather than being optimized, thereby preventing potential issues in multi-threaded programming, embedded programming, and when working with hardware devices. While the use of the "volatile" keyword may result in slower code execution and can make debugging more challenging, it is an essential tool for certain programming scenarios.
FAQs
Here are some frequently asked questions on the volatile keyword in C.
Q1: When should I use the volatile keyword in my code?
Answer: The volatile keyword should be used when the value of a variable may change unexpectedly due to external factors, such as in multi-threaded programming or when working with hardware devices.
Q2: Can I use the volatile keyword in c with all types of variables?
Answer: Yes, the volatile keyword in c can be used with any type of variable.
Q3: How does the volatile keyword affect multi-threaded programming?
Answer: The volatile keyword ensures that the value of a shared variable is always read and written to memory as expected, even if it changes unexpectedly due to multiple threads accessing it.
Q4: Can the volatile keyword in c be used with const variables?
Answer: Yes, the volatile keyword in c can be used with const variables to indicate that the value may change unexpectedly, even if it is declared as const.
Q5: Does the volatile keyword in c guarantee thread safety?
Answer: No, the volatile keyword in c does not guarantee thread safety. It only ensures that the value of a shared variable is always read and written to memory as expected.
Q6: Can the volatile keyword in c be used with static variables?
Answer: Yes, the volatile keyword in c can be used with static variables to indicate that the value may change unexpectedly.