Const : Constant, the size cannot be changed while the program is running, that is, it cannot become an lvalue. But the value stored in memory can be modified through a pointer
When const is on the left side of *, the data itself is a constant.
When const is on the right side of *, the pointer is a constant.
The formula is to count from the left and point to the right
const is used to modify functions, and is generally used to modify function return values that cannot be changed. That is
const int *func(); //在 * 号的左侧
The volatile
compiler warning directive warns the compiler to fetch the value of a variable from memory every time. It
mainly modifies variables that may be accessed by multiple threads, such as values that will be changed by hardware interrupts.