C Pointers

A pointer is a variable that stores the memory address of another variable. Pointers are one of the most powerful parts of C.

Memory Addresses

Every variable lives somewhere in memory, and that place has an address. Use & to get it:

Example

#include <stdio.h>

int main(void) {
  int age = 21;
  printf("Value: %d\n", age);
  printf("Address: %p\n", (void *) &age);
  return 0;
}

Output

Value: 21
Address: 0x7ffd5e3c9a4c

The address is a hexadecimal number. It will be different on your computer, and it changes each time you run the program.

Creating a Pointer

Put * after the type to make a pointer variable:

Example

#include <stdio.h>

int main(void) {
  int price = 500;
  int *ptr = &price;     // ptr holds the address of price

  printf("%d\n", *ptr);  // read the value through the pointer
  *ptr = 650;            // change the value through the pointer
  printf("%d\n", price);
  return 0;
}

Output

500
650

Symbol	In a declaration	In an expression
`*`	`int *p` makes a pointer	`*p` gives the value at the address (dereference)
`&`		`&x` gives the address of x

Think of a pointer as a house address written on a paper. & writes the address down, and * goes to the house to see what is inside.

Pointers and Functions

Because C passes copies, a function cannot change your variables. Pass a pointer and it can:

Example

#include <stdio.h>

void addBonus(int *salary, int bonus) {
  *salary = *salary + bonus;
}

int main(void) {
  int salary = 40000;
  addBonus(&salary, 5000);
  printf("New salary: %d\n", salary);
  return 0;
}

Output

New salary: 45000

This is exactly why scanf("%d", &x) needs &: scanf must know where to put the value.

Swapping Two Values

Example

#include <stdio.h>

void swap(int *a, int *b) {
  int temp = *a;
  *a = *b;
  *b = temp;
}

int main(void) {
  int x = 3, y = 8;
  swap(&x, &y);
  printf("x = %d, y = %d\n", x, y);
  return 0;
}

Output

x = 8, y = 3

Pointers and Arrays

The name of an array works like a pointer to its first item. Adding 1 to the pointer moves to the next item:

Example

#include <stdio.h>

int main(void) {
  int nums[3] = {10, 20, 30};
  int *p = nums;

  printf("%d\n", *p);
  printf("%d\n", *(p + 1));
  printf("%d\n", *(p + 2));
  return 0;
}

Output

10
20
30

*(p + 1) and nums[1] mean the same thing.

NULL Pointers

A pointer that points to nothing should be set to NULL. Always check before using it:

Example

int *data = NULL;
if (data != NULL) {
  printf("%d\n", *data);
}

Using a pointer that is NULL or not set up crashes the program, often with a "segmentation fault".

Exercise

If int *p = &x; which expression gives the value stored in x?

p&p*p&x