When Should You Use the `volatile` Keyword in C ?

Understanding the Need for the Volatile Keyword

Despite its widespread use, the volatile keyword often raises questions about its purpose. To clarify, volatile is a keyword used to instruct the compiler to refrain from optimizing certain code sections. This becomes crucial when a variable's value can be modified from outside the current program, a fact unknown to the compiler.

Example of Optimization Gone Wrong

Consider the following code snippet:

int some_int = 100;

while (some_int == 100) {
    // Your code
}

In this example, the compiler may optimize the while loop into something equivalent to while(true), assuming its condition will never change. However, if some_int can be modified from an external source, this optimization would prevent the loop from terminating correctly.

The Role of Volatile

To prevent such unintended optimization, the volatile keyword can be used. It signals to the compiler that some_int's value could be changed externally, prohibiting the compiler from optimizing it away.

volatile int some_int = 100;

In this case, the compiler will treat some_int as volatile and avoid aggressive optimizations involving it, ensuring its actual value is used in the while loop.

Technical Explanation

According to the C Standard ($7.1.5.1/8), volatile qualifies an object as a "hint to the implementation to avoid aggressive optimization." This means the compiler is recommended to exercise caution when optimizing code that operates on volatile objects, as their values may be altered beyond the compiler's scope of awareness.

The above is the detailed content of When Should You Use the `volatile` Keyword in C ?. For more information, please follow other related articles on the PHP Chinese website!