What are the scenarios where volatile is used in the C language?

1. Hardware register access: In embedded systems, hardware registers may be frequently modified, so the volatile keyword is needed to inform the compiler not to optimize and to read data from the register every time.
2. Shared variables between threads: in a multi-threaded environment, if a variable is shared by multiple threads and these threads will access the variable simultaneously, using the volatile keyword is necessary to ensure the visibility of the variable, meaning each read will be the most up-to-date value.
3. Variables in the interrupt service routine: In an interrupt service routine, variables may be modified by the interrupt, so the volatile keyword is needed to ensure that the latest value is always read from memory.
4. Global variables in multitasking operating systems: In multitasking operating systems, global variables may be accessed by multiple tasks simultaneously, requiring the use of the volatile keyword to ensure visibility and consistency of the variables.

In conclusion, the “volatile” keyword is used in scenarios where variable visibility and prevention of compiler optimization are needed, typically in concurrent programming or embedded system development.