> Pointers > Pointer declaration

Pointer declaration

Pointer

As any other variable, you need to declare a pointer. Furthermore, you have to initialize a pointer before you can use it.

You declare a pointer just like any other variable:

Syntax

type *name;

type can be any C type (such as char, int, or float for example).
name is the name of the pointer variable.
The * makes the variable a pointer.
Pointers are usually described as “a pointer to type”

Code example

int *myPointer        // declares a pointer named myPointer as an integer type

float *myOtherPointer // declares a pointer named myOtherPointer as a float type

myPointer = &myIntVar;        // assigns the memory address of myIntVar to myPointer

myOtherPointer = &myFloatVar; // assigns the memory address of myFloatVar to myOtherPointer

Creating pointer types with typedef

Syntax

typedef type *typeName;

Note

A pointer variable can now be declared as type typeName which is a synonym for type.
The * is no longer needed since typeName explicitly identifies the variable as a pointer to type.

Code example

typedef int *intPointer; // Create a pointer to int type

intPointer p;            // Create pointer to int (without *)
                         // Equivalent to int *p;

Initialization

To set a pointer to point to another variable, we use the & operator (address of), and the pointer variable is used without the dereference operator *:

p = &n;

This assigns the address of the variable n to the pointer p (p now points to n).

Note

p must be declared to point to the type of n (e.g. int n; int *p;).

Usage

When accessing the variable pointed to by a pointer, we use the pointer with the dereference operator *:

m = *p;

This assigns to the variable m, the value of what p is pointing to (n from the last example). Using *p, is the same as using the variable it points to (e.g. n).

In other words

Code example

int n, *p; // Create an integer variable and a pointer to an integer variable

p = &n;    // Assign the address of n to p
*p = 3;    // Equals to n = 3;

In the pointer world:

&n is a constant pointer which represents the address of n. This address of n will never change.
p is a variable pointer to int which can be assigned the address of any int any time.
*p represents the data pointed to by p and may be used anywhere you would use n.
* is the dereference operator, also called the indirection operator.
When declaring a pointer, * is used to indicate that the variable is a pointer rather than an ordinary variable.

Pointers at work

Consider the following code example. How is this working in an actual design?

Code example

int n, m;    // create two integer variables
int *p;      // create a pointer to an integer variable

 n = 0x1234; // assign 0x1234 to n
 m = 0xABCD; // assign 0xABCD to m
 p = &n;     // assign the address of n to p
*p = 0x4321; // assign 0x4321 to n using *p
 p = &m;     // assign the address of m to p
*p = 0xDCBA; // assign 0xDCBA to m using *p

Zero-initialized RAM at program start: Initialization with ZI RAM

Assign 0x1234 to n:

Assign 0xABCD to m:

Assign the address of n to p:

Assign 0x4321 to n using *p:

Assign the address of m to p:

Assign 0xDCBA to m using *p: