Advance Learning of Pointer and use with Array, Constant, Function, Pointer

As in my last post we talked all about fundamentals of Pointers in C. In this we will some more complex things related to Pointers so you need to more concentrate while you read the post. So in this part we will develop more complex ideas about Pointers.

The following are explained in this article with examples:

  1. Constant pointer and pointer to constant.
  2. Pointer to pointer with an example
  3. Array of pointers with an example
  4. Pointer to functions with an example

C Constant Pointer and Pointer to Constant

As a developer, you should understand the difference between constant pointer and pointer to constant.

C Constant pointer
A pointer is said to be constant pointer when the address its pointing to cannot be changed.

Lets take an example :

char ch, c;
char *ptr = &ch
ptr = &c

In the above example we defined two characters (ch’ and c’) and a character pointer ptr’. First, the pointer ptr’ contained the address of ch’ and in the next line it contained the address of c’. In other words, we can say that Initially ptr’ pointed to ch’ and then it pointed to c’.

But in case of a constant pointer, once a pointer holds an address, it cannot change it. This means a constant pointer, if already pointing to an address, cannot point to a new address.

If we see the example above, then if ptr’ would have been a constant pointer, then the third line would have not been valid.

A constant pointer is declared as :

*const
For example :

#include

int main(void)
{
char ch = 'c';
char c = 'a';

char *const ptr = &ch; // A constant pointer
ptr = &c; // Trying to assign new address to a constant pointer. WRONG!!!!

return 0;
}

When the code above is compiled, compiler gives the following error :

$ gcc -Wall constptr.c -o constptr
constptr.c: In function main’:
constptr.c:9: error: assignment of read-only variable ptr’

So we see that, as expected, compiler throws an error since we tried to change the address held by constant pointer.

Now, we should be clear with this concept. Lets move on.

C Pointer to Constant

This concept is easy to understand as the name simplifies the concept. Yes, as the name itself suggests, this type of pointer cannot change the value at the address pointed by it.

Lets understand this through an example :

char ch = 'c';
char *ptr = &ch
*ptr = 'a';

In the above example, we used a character pointer ptr’ that points to character ch’. In the last line, we change the value at address pointer by ptr’. But if this would have been a pointer to a constant, then the last line would have been invalid because a pointer to a constant cannot change the value at the address its pointing to.

A pointer to a constant is declared as :

const *;
For example :

#include

int main(void)
{
char ch = 'c';
const char *ptr = &ch; // A constant pointer 'ptr' pointing to 'ch'
*ptr = 'a';// WRONG!!! Cannot change the value at address pointed by 'ptr'.

return 0;
}

When the above code was compiled, compiler gave the following error :

$ gcc -Wall ptr2const.c -o ptr2const
ptr2const.c: In function main’:
ptr2const.c:7: error: assignment of read-only location *ptr’

So now we know the reason behind the error above ie we cannot change the value pointed to by a constant pointer.

C Pointer to Pointer

Till now we have used or learned pointer to a data type like character, integer etc. But in this section we will learn about pointers pointing to pointers.

As the definition of pointer says that its a special variable that can store the address of an other variable. Then the other variable can very well be a pointer. This means that its perfectly legal for a pointer to be pointing to another pointer.

Lets suppose we have a pointer p1′ that points to yet another pointer p2′ that points to a character ch’. In memory, the three variables can be visualized as :

So we can see that in memory, pointer p1 holds the address of pointer p2. Pointer p2 holds the address of character ch’.

So p2′ is pointer to character ch’, while p1′ is pointer to p2′ or we can also say that p2′ is a pointer to pointer to character ch’.

Now, in code p2′ can be declared as :

char *p2 = &ch;

But p1′ is declared as :

char **p1 = &p2;

So we see that p1′ is a double pointer (ie pointer to a pointer to a character) and hence the two *s in declaration.

Now,

p1′ is the address of p2′ ie 5000
*p1′ is the value held by p2′ ie 8000
**p1′ is the value at 8000 ie c’
I think that should pretty much clear the concept, lets take a small example :

#include

int main(void)
{
char **ptr = NULL;

char *p = NULL;

char c = 'd';

p = &c;
ptr = &p;

printf("\n c = [%c]\n",c);
printf("\n *p = [%c]\n",*p);
printf("\n **ptr = [%c]\n",**ptr);

return 0;
}

Here is the output :

$ ./doubleptr

c = [d]

*p = [d]

**ptr = [d]

C Array of Pointers

Just like array of integers or characters, there can be array of pointers too.

An array of pointers can be declared as :

*[<number-of-elements];

For example :

char *ptr[3];

The above line declares an array of three character pointers.

Lets take a working example :

#include

int main(void)
{
char *p1 = "Himanshu";
char *p2 = "Arora";
char *p3 = "India";

char *arr[3];

arr[0] = p1;
arr[1] = p2;
arr[2] = p3;

printf("\n p1 = [%s] \n",p1);
printf("\n p2 = [%s] \n",p2);
printf("\n p3 = [%s] \n",p3);

printf("\n arr[0] = [%s] \n",arr[0]);
printf("\n arr[1] = [%s] \n",arr[1]);
printf("\n arr[2] = [%s] \n",arr[2]);

return 0;
}

In the above code, we took three pointers pointing to three strings. Then we declared an array that can contain three pointers. We assigned the pointers p1′, p2′ and p3′ to the 0,1 and 2 index of array. Let’s see the output :

$ ./arrayofptr

p1 = [Pixie]

p2 = [Auror]

p3 = [Nepal]

arr[0] = [Pixie]

arr[1] = [Auror]

arr[2] = [Nepal]

So we see that array now holds the address of strings.

C Function Pointers

Just like pointer to characters, integers etc, we can have pointers to functions in C programming Languages.

A function pointer can be declared as :

(*) (type of function arguments)
For example :

int (*fptr)(int, int)

The above line declares a function pointer fptr’ that can point to a function whose return type is int’ and takes two integers as arguments.

Lets take a working example :

#include

int func (int a, int b)
{
printf("\n a = %d\n",a);
printf("\n b = %d\n",b);

return 0;
}

int main(void)
{
int(*fptr)(int,int); // Function pointer

fptr = func; // Assign address to function pointer

func(2,3);
fptr(2,3);

return 0;
}

In the above example, we defined a function func’ that takes two integers as inputs and returns an integer. In the main() function, we declare a function pointer fptr’ and then assign value to it. Note that, name of the function can be treated as starting address of the function so we can assign the address of function to function pointer using function’s name. Lets see the output :

$ ./fptr

a = 2

b = 3

a = 2

b = 3

So from the output we see that calling the function through function pointer produces the same output as calling the function from its name.

To conclude, in this article we touched some of the advanced concepts related to pointers. There can be some interesting problems related to pointers, which i might cover in some future article.