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POINTERS

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Declaration a point about pointers

int j, k, l;

j, k, l are integer type. we can say that int type declaration distributes

over the list of names j, k, l

int *p, q, r; Here, only p is int pointer

q and r are ordinary int variables

The indirection operator (*) does not distribute toall variable names in a declaration. Each pointermust be declared with the * prefixed to the name.

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Pointing to nothing !

NULL is a symbolic constant defined in

with 0 (zero) A pointer with the value NULL points to

nothing.

The value 0 (NULL) is the only integer valuethat can be assigned directly to a pointervariable.

int *p = NULL;

*p = 5; /* this results in run-time error */

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Functions

A function can return only one value back to the

calling function.

If you want more values to be communicated

back to the calling function, then you have to useaddresses and pointers.

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Functions an examplevoid find_min_max_marks(float *, float *, float *, int);

main( )

{

float min, max, marks[40];

;

find_min_max_marks(marks, &min, &max, 40);

}

void find_min_max_marks(float *m, float *a, float *z, int size)

{

int j;

*z = m[0]; *a = m[0];

for (j=1; j *z) *z = m[j];

}

}

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Comparing two strings

strcmp(s, t) is a function available in the

standard library () that comparesthe character strings s and t.

It returns an integer which is 0 (zero) s and t are same Negative s is lexicographically less than t

Positive s is lexicographically larger than t

The return value is obtained by subtractingthe characters at the first position where s

and t disagree.

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strcmp array version

int strcmp(char *s, char *t)

{

int j;

for(j=0; s[j] == t[j]; j++)

if( s[j] == \0) return 0;

return s[j] t[j] ;}

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strcmp pointer version

int strcmp( char *s, char *t)

{

for ( ; *s == *t; s++, t++)

if(*s == \0) return 0;

return *s - *t;

}

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const with pointers

Passing addresses to functions could be

dangerous. By mistake that function canmodify the original variables value.

To overcome this kind of problems, one candeclare that the contents pointed by thepointer are constant.

One can also force to say that a pointer hasto point to the same location (whatever becontents of the location).

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const with pointers

const can be used in 4 ways with pointers

a non-constant pointer to non-constant data a constant pointer to non-constant data

a non-constant pointer to constant data

a constant pointer to constant data

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A non-constant pointer to non-

constant data int *p, j = 10, k = 20;

p = &j;*p = 15; /* contents pointed by p is modified */

p = &k; /* p is modified */

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A constant pointer to non-constant

data Pointer always points to the same memory

location. But contents at that memory canchange.

int x, y;

int * const ptr = &x;

*ptr = 7; /* OK same as x = 7*/

ptr = &y; /* error */

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A constant pointer to non-constant

data An array name is a constant pointer to the

beginning of the array.

All the data in the array can be accessed andchanged by using the array name and array

subscripting. But array name cannot point to some other

location.

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A non-constant pointer to constant

data pointer can be modified to point to some

other location. But the contents using the pointer (with

indirection operator *) can not be modified.

This is especially useful with functions.

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data const int * ptr;

int j = 10, k;ptr = &j; /* OK */

*ptr = 25; /* error */

j = 25; /* OK */

ptr = &k; /* OK */

*ptr = 100; /* error */k = 100; /* OK */

j = *ptr; /* OK */

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data void f( const int *);

int main( )

{

int y = 111;

f(&y);

}

void f( const int * ptr)

{

*ptr = 100; /* an error occurs */

}

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A constant pointer to

constant data int x = 5, y;

const int *const ptr = &x;y = *ptr; /* OK */

ptr = &y; /* error */

*ptr = 20; /* error */

If you want to pass an array whose contentsshould not be modified, then use this kind ofdeclarations.

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Pointer Expressions and

Pointer Arithmetic A limited set of arithmetic operations may be

performed with pointers A pointer may be incremented (++) or

decremented ( -- )

An integer may be added to a pointer (+ or += ) An integer may be subtracted from a pointer

( or = )

One pointer may be subtracted from another.

Pointer arithmetic is machine dependant.

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Increment ++ and decrement --

int *ptr, j = 5;

int a[20];ptr = a;

ptr ++; /* ptr now points to a[1] */

*ptr = 10; /* same as a[1] = 10 */

ptr = &j;

ptr ++;/* now ptr points to next integer after j */

Pointer arithmetic is meaningful only with arrays

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Subtracting a pointer from other

double d[20];

double *a, *b;int j;

a = &d[4];

b = &d[10];

j = b a; /* j gets value 6 */

Again, it would be meaningless if a and b arepointing to non-array elements.

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Assigning a pointer to another

Void pointer A pointer can be assigned to another pointer

if both pointers are of the same type. Otherwise, a type cast operator must be

used.

int *p; char *c;

p = (int *) c;

The exception to this rule is for pointer whosetype is void *

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void pointervoid pointer

void *ptr; int i;

ptr = &i; Here ptr can contain an address, but the type

of the value stored at that address is

unknown and hence, dereferencing using ptris disallowed.

*ptr = 5; /* error */

*((int *)ptr) = 5; /* Ok, type cast and thenassign*/

12 pointers

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