Addresses and pointers

Where a variable lives

Every variable sits somewhere in memory, at an address. &x means "the address of x".

int *p = &x; makes p a pointer to an int holding x's address. *p means "the int that p points at": reading *p reads x, and *p = 9; changes x.

Two uses of *, two meanings: in a declaration it says "p is a pointer"; in an expression it follows the pointer.

Changing x through p

#include <stdio.h>

int main() {
    int x = 4;
    int *p = &x;
    *p = *p * 10;
    printf("%d %d\n", x, *p);
    return 0;
}

1. Line 4: x holds 4, at some address in memory.
2. Line 5: p holds x's address: p points at x.
3. Line 6: *p reads x (4), times 10 is 40, and *p = stores it back into x.
4. Line 7: x and *p are the same box, so both print 40.

Which one moved?

#include <stdio.h>

int main() {
    int a = 1, b = 2;
    int *p = &a;
    *p = 5;
    p = &b;
    *p = *p + 10;
    printf("%d %d\n", a, b);
    return 0;
}

What does this print?

• 5 12
• 15 2
• 1 12

Exercise: Double through a pointer

Read a number into n. Without writing n again after the scanf line, double it using the pointer p and print it with p.