09 1 Pointers

Pointers| SCP1103 Programming Technique C | Jumail, FSKSM, UTM, 2005 | Last Updated: September 2006 Slide 1

Pointers

byJumail Bin Taliba

Faculty of Computer Science & Information System


Today’s Topics

• Concept of Variables• Concept of Pointers• Declaring Pointer Variables• Manipulating Pointers


Variables• Variable is a space in

memory that is used to hold a value.

• Each variable has a name, content and address

• Variable name is a logical reference, used by the programmer

• Address is a physical reference or actual location, and is used by the computer


Variables• A char variable uses one byte

Print the addresses

Output:

142300 142301


Variables• An integer variable uses four bytes.• The address of an integer variable is the first byte.


Pointer Variables• What we have used so far are ordinary

variables or also called data variables.

• A data variable contains a value (e.g. integer number, a real number or a character) .

• A pointer variable is another type of variable that contains the address of other variable. It also known as address variable.


PointersExample:

a is a data variablep is a pointer variable that stores the address of a – that is &a.


Pointer Variable Declaration• A pointer variable is declared like the data variable. The

difference is, put an asterisk (*) in front of it.

Example:

int a=-123; // a is a data variable int *p; // p is pointer variable p = &a; // p then holds the address of variable a


Example: p and q point to the same variable.

int a=-123;

int *p;

int *q;

p=&a;

q=p;


The type of a pointer must be matched with the type of the variable that the pointer points to.

int n; int *p; double *q; p=&n; // this is OK q=&n; // this is wrong, type mismatch q=p; // this is also wrong


Multiple Declarations

• To declare multiple pointers in a statement, usethe asterisk for each pointer variableExample:

int *p1, p2;

Only p1 is a pointer variable; p2 is an ordinary variable

• You may also use typedef to make the declaration clear Example:

typedef int *IntPtr; IntPtr p1, p2;

Now, both p1 and p2 are pointer variables


Initialization of Pointer Variables

Pointer variables can also be declared and initialized in a single statement


Accessing Variables Through Pointer Variables

• There are two special operators for pointers: address-of operator (&) and indirection operator (*).

• Address-of operator, &– is used for getting the address of a variable– Example: &n

meaning: “give me the address of variable n”

• Indirection operator (also called dereferencing operator), *– is used for getting the content of a variable whose address is

stored in the pointer.– Example: *ptr

meaning: “give me the content of a variable whose address is in pointer ptr”

– Notice that, a pointer declaration also uses an asterisk (*) . Don’t get confused . Pointer declaration and indirection operator are different.

– Indirection operator must only be used with a pointer variable. If n is a data variable, the following would be an error. *n


The & and * Operatorsvoid main(void){ int a = 5; int *ptr = &a; printf("%d ",ptr); printf("%d ",&a); printf("%d ",&ptr); printf("%d ",*ptr); *ptr = *ptr + 5; printf("%d ",*ptr); printf("%d ",a);}

MemoryAddress Content

5

2000

2000

2004

a

ptr

Prints 2000

The addresses are specified by the computer. The addresses above are used only for examples.

Prints 2000

Prints 2004

Prints 5. This is how it works. ptr contains 2000. Go to the address 2000 and get its content => 5. Means that, the value of *ptr is 5.

This means, a = a + 5, because ptr holds the address of a. The new value of a is 10

Prints 10

Prints 10

2008

200920102011

printf("%d ", *a); // This would be an error, because // a is not a pointer variable


The & and * operators are canceling each other

Example: int x; int *p=&x;

*&x is equivalent to x and p.

However, the following would be an error because x is not a pointer. The * operator can only be used with a pointer variable.

&*x // Error! This means &(*x)


int x;int *p=&x;int *q=&x;


Caution! Pointer assignments


The malloc Function• Using pointers, variables can be manipulated

even if there is no identifier for them. This variable is called a dynamic variable (or sometimes nameless variable)

– To create a pointer to a new dynamic variable of type int, use the malloc function

Example:

int *p;

p = (int*) malloc( sizeof(int) );

– You have to include stdlib.h to use the malloc function

p

??

dynamic variable

pointer variable


The malloc Function– The new variable can only be accessed through p. The new variable is then referred to as *p1

– It can be used anyplace an integer variable can scanf(“%d”,&*p1 ); // or simply scanf(“%d”,p1);*p1 = *p1 + 7;

p

??

dynamic variable

pointer variable


• Dynamic variables are variables that are created and destroyed while the program is running.

• Static variables (sometimes called automatic variables) are variables that are automatically created and destroyed by the computer.

Dynamic and Static Variables

p

1010 dynamic variable

staticvariable

9999

static variable

p

n

Example:

int n=99;int *p;p = new int;*p = 10;


Basic Memory Management

• An area of memory called the freestore isreserved for dynamic variables– New dynamic variables use memory in the freestore– If all of the freestore is used, calls to new will fail

• Unneeded memory can be recycled– When variables are no longer needed, they can be

deleted and the memory they used is returned to the freestore


The free Function

• When dynamic variables are no longer needed, free them to return memory to the freestore

Example:

The value of p is now undefined and the memory used by the variable that p pointed to is back in the freestore

free(p);

??p

After:Before:p

??


Pointers to Pointers

Output: 58

Output: 58

Output: 58

Technology

09 1 Pointers