What is the purpose of the “volatile” keyword in the C language?

In the C language, the volatile keyword is used to inform the compiler that the value of a variable may be changed unexpectedly at any time. It is primarily used for two situations:

1. Prevent compiler optimization: When the compiler optimizes, it tries to minimize read and write operations on variables by storing the value of variables in registers to improve program efficiency. However, for variables marked as volatile, the compiler must ensure that each access reads the latest value from memory instead of using an old value stored in the register.
2. Multithreaded access to shared variables: In multithreaded programming, multiple threads may access a shared variable at the same time. If this variable is not of type volatile, the compiler may optimize the code that accesses this variable, causing other threads to not immediately detect changes to this variable. By using volatile to modify shared variables, it ensures that each access retrieves the latest value from memory, thereby ensuring data consistency between threads.

It is important to note that the volatile keyword cannot guarantee thread safety, it can only ensure that each read is the most up-to-date value. If thread safety is necessary, other synchronization mechanisms such as mutex or atomic operations are also needed.