What does volatile stand for in C language?

volatile in C language means that the variable may be changed by external factors and the compiler cannot optimize it. Functions include: preventing compiler optimization, indicating external modifications, and ensuring memory visibility. Commonly used in hardware register access, multi-thread programming, interrupt handling, and embedded systems. For example, volatile int shared_variable; prevents the compiler from caching the value of shared_variable into a register, ensuring that thread 2 can always get the latest value.

The meaning of volatile in C language

volatile is a keyword used to modify variables in C language , indicating that the variable may be changed by external factors and the compiler cannot optimize it.

Function

volatile keyword mainly has the following functions:

• Prevent compiler optimization:When a variable is When declared volatile, the compiler cannot optimize it, such as constant folding, register allocation, etc.
• Indicates external modification: It indicates to the compiler that the variable may be modified by external code, hardware, or other threads.
• Ensure memory visibility: Using volatile ensures that the latest value of a variable is visible across all threads and processors.

Usage scenarios

volatile keyword is often used in the following scenarios:

• Hardware register access: Variables accessing hardware registers should be declared volatile.
• Multi-threaded programming: In a multi-threaded environment, shared data should be declared volatile to avoid data competition.
• Interrupt handling: Variables accessed in the interrupt handler should be declared volatile.
• Embedded system: In embedded systems, variables that directly access hardware devices need to be declared volatile.

Example

The following is an example of using the volatile keyword:

volatile int shared_variable;

void thread_1() {
    shared_variable++;
}

void thread_2() {
    int local_copy = shared_variable;
    // ...
}

In this case, shared_variable is declared volatile, To prevent the compiler from caching the value of shared_variable into a register. This way, thread 2 can always get the latest value of shared_variable.

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