The Basics of Programming

Pointers

In C Programming, you will have to use a new type of variable called a pointer. All a pointer does is store the address of another variable. It is basically like a shortcut or an alias to another part of memory. You can use pointers for many variable types including integers, floats, strings and structures. Structures are used a lot in some operating systems including the Amiga. So, if a structure is defined elsewhere in memory then you need to have a pointer to it.

So, to get the address of something, you use the ampersand (&) character e.g. &mywindow.

To define a pointer you use the asterick (*) character e.g. *mypointer.

A simple integer pointer can be created like this:

int num;      /* Integer variable called num */
int *num_ptr; /* A pointer of type int, called num_ptr */

num = 150;      /* Assign value of 150 to num */
num_ptr = # /* Get the address of num and store it in num_ptr */

printf("%d\n", *num_ptr); /* Print value of *num_ptr and you will get 150! */

For example, if you have structure called vehicle:

struct vehicle {
 char make[25];
 char model[25];
 char vehicletype[25];
 float enginesize;
 int cylinders;
 bool turbocharge;
 bool supercharge;
}

I can then create an instance of that vehicle and specify a name of that vehicle:

struct vehicle aston_martin;

aston_martin.make = "Aston Martin";
aston_martin.model = "DB9";
aston_martin.vehicletype = "Car";
aston_martin.enginesize = 6.2;
aston_martin.cylinders = 12;
aston_martin.turbocharge = false;
aston_martin.supercharge = false;

Now, I can create a pointer to my structure as follows, note that thee asterick is not required on the left side of the assignment statement.

struct vehicle *myvehicle;  /* Create a pointer of type vehicle called myvehicle */

myvehicle = &aston_martin; /* Assign the address of aston_martin to the pointer myvehicle */

Now, to access individual items of the structure, I cannot use the name dot item format anymore, I have to use a special structure pointer, -> (dash, right arrow):

float engine;
engine = *myvehicle->enginesize; /* Get the value of the enginesize from the vehicle pointed to by myvehicle. Since myvehicle is pointing the aston_martin, it will be 6.2 */

This is the equivalent of the statement:

float engine;
engine = aston_martin.enginesize; /* Get the engine size from the vehicle structure instance called aston_martin, ie 6.2 */

String variables are naturally pointers so if string variable called name is used, its address can be access by either &name or just name (no ampersand).

Introduction to Amiga C

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