Question

What are the differences between const and volatile pointer in C?

Answer 1

The difference really comes down to the difference between const and volatile. The only things these two concepts have in common is syntax. const is compiler-enforced and says “the programmer can not change this.” volatile says “this data might be changed by someone else” and so the compiler will not make any assumptions about that data. Without volatile the compiler might say “I put this data from memory into a register, and since I haven’t done anything to that data, I’m sure it’s the same and I don’t need to read it into the register again.” When the data is marked as volatile the compiler won’t make such an assumption (because someone else might have changed the data) and so it will reread the data into the register.

Now, are you asking for the difference between

int *const p;

and

int *volatile q;

or the difference between

const int* p;

and

volatile int* q;

In the former case: p is a pointer to an int and where that pointer points can not be changed by the programmer whereas q is a pointer to an int and where that pointer points could be changed by someone other than the programmer so the compiler makes no assumptions about that pointer.

So:

int *const p = (int*)malloc(sizeof(int));
int *volatile q = (int*)malloc(sizeof(int));
*p = 17; // legal;
p = (int*)malloc(sizoef(int)); // not legal
*q = 17; // legal;
q = (int*)malloc(sizeof(int)); // legal

In the latter case: p is a pointer to an int and what p is pointing to can not be changed by the programmer whereas q is a pointer to an int and what q is pointing to could be changed by someone other than the programmer so the compiler makes no assumptions about that data.

int i = 17;
int j = 34;
const int *p = &i;
volatile int *q = &i;
*p = 51; // not legal
p = &j; // legal
*q = 51; // legal
q = &j; // legal