How to understand the pointer star * in C?

Take it this way:

int *i means the value to which i points is an integer.

char **p means that p is a pointer which is itself a pointer to a char.

int i; //i is an int.int *i; //i is a pointer to an intint **i;//i is a pointer to a pointer to an int.

Is the * sign interpreted differently in declarations and expressions?

Yes. They're completely different. in a declaration * is used to declare pointers. In an expression unary * is used to dereference a pointer (or as the binary multiplication operator)

Some examples:

int i = 10; //i is an int, it has allocated storage to store an int.int *k; // k is an uninitialized pointer to an int.         //It does not store an int, but a pointer to one.k = &i; // make k point to i. We take the address of i and store it in kint j = *k; //here we dereference the k pointer to get at the int value it points            //to. As it points to i, *k will get the value 10 and store it in j

The rule of declaration in c is, you declare it the way you use it.

char *p means you need *p to get the char,

char **p means you need **p to get the char.