volatile

volatile is an modifier of data type in c.

volatile data_type var_name;

volatile int data;

A volatile variable is a variable that can change unexpectedly. The volatile keyword is intended to prevent the compiler from applying any optimizations on the variable.
A volatile variable value can be changed by the background routine/callback of system. the background routine can be interrupt status, timer, GPIO indications, etc.
Variables declared as volatile are not cached, so compiler should not reuse the value already loaded in a register, but access the memory again as the value in register is not guaranteed to be the same as the value stored in memory.
Volatile variable will take more execution time beacuse everytime we need to read it from memory.

Best practices:

We can decleare a variable as volatile when there are possibility that the value of variable can be changed by system.

Rules:

volatile modifier can not be used with void, enum and function.

Advanced reader:

Most of time, memory mapped variable will be declared as volatile.

C Programming