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Lesson 11: Pointers

In this lesson you will learn:

  • how to use pointers
  • what pointers are
  • how to declare pointers
  • the meaning of the ampersand& symbol
  • the meaning of the asterisk * symbol

Q: What is the difference between a blonde and an inflatable doll?
A: About 2 cans of hair spray

The C language takes a lot of flack for its ability to peek and poke directly into memory. This gives great flexibility and power to the language, but it also makes it one of the great hurdles that the beginner must have in using the language. . With that said, let's get started.

All variables obviously have to be stored into memory, but where are they stored? Imagine memory as this big long street with houses on it. Each variable is a house on a street. Each house can hold a number of people (value of variable). But how do you find out how many people (what value is stored) are in a particular house. You have to have some kind of address. Then with the address, you can go to the house and then ask it: How many are there? Then you can get the value of a variable. This is the basic concept behind pointers. This should seem very logical, if not please reread this tutorial and all will make sense.

Let us use this new knowledge to examine a couple of statements.

int var_x;
int* ptrX;
var_x = 6;
ptrX = &var_x;
*ptrX = 12;
printf("value of x : %d", var_x);

The first line causes the compiler to reserve a space in memory for a integer. The second line tells the compiler to reserve space to store a pointer. As you can notice the way you declare a pointer is simply to add a "*" asterisk to the end of the data type. A pointer is a storage location for an address. The third line should remind you of the scanf statements. The address "&" operator tells C to go to the place it stored var_x, and then give the address of the storage location to ptrX.
The fourth line is a bit more complex. An asterisk * in front of a variable tells C to de reference the pointer, and go to memory. Then you can make assignments to variable stored at that location. Since ptrX points to var_x, line 4 is equivalent to this command: var_x = 12; Pretty cool, eh? You can reference a variable and access its data through a pointer. Windows and all sorts of programs do this all the time. They hand pointers of data to each other, and allow other applications to access the memory they have.

Now you may be wondering, why are pointers so complex, or I've heard the using pointers can cause problems. It can, and for those who aren't careful misuse of pointer can do a lot of damage. Suppose that we forget to type in line 3 ptrX = &var_x; when we entered the program. What would happen if we executed it, who knows? Without this line ptrX is never assigned an address. Basically it points to some random data anywhere in memory. If you executed line 4 without line 3. You could get very weird results. Since ptrX hasn't been pointed to our var_x, maybe its points to system memory, and then you assign a value someplace you shouldn't and your computer crashes. This may not always happen, but it is certainly a possibility, so be very careful when using pointers. Make sure they are assigned to something before you use them.

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