Question

char* p = "hello world";

programmers usually assigns the address of an variable to a pointer. But in the above case, where is the pointer pointing to?

and what in the world is

int x =42;
int* p= &x;
int ** r = &p;

Answer 1

It’s pointing to an area in the program’s read-only memory (usually in the program’s machine code itself) containing the ASCII sequence hello world. Compare this to:

char p[] = "hello world";

Which creates an array of 12 chars on the stack, which is modifiable just like any other variable, and to:

char *p = strdup("hello world");

Which creates an array of 12 chars on the heap, and sets p to be a pointer to this readable and writable space of heap memory.

As for your (totally unrelated) second question:

int** r = &p;

Is simpler than it looks, though it’s also bad. &p is the address of p. So if we did:

int x;
int *y = &x;

Then the pointer y points to the variable x, so assigning to *y changes x (and vice versa). We can do this for arbitrarily complex types:

int *x;
int **y = &x;

Now y is still a pointer that points to x, but x is also a pointer. So in your example, r is a pointer to a pointer to an int, and it’s value is the address of p (a pointer to a char). However, it’s a bad idea to do this because many platforms have alignment issues with casting from a char * type to a larger pointer type.