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C Pointer to Pointer, Pointer to Functions, Array of Pointers Explained with Examples

In C programming language, the concept of pointers is the most powerful concept that makes C stand apart from other programming languages. In the part-I of this series we discussed the fundamental concepts around C pointers.

In this article, we will try to develop understanding of some of the relatively complex concepts. 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

1. 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 :

<type-of-pointer> *const <name-of-pointer>

For example :

#include<stdio.h> 

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 <type-of-pointer> *<name-of-pointer>;

For example :

#include<stdio.h> 

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.

2. 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<stdio.h> 

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]

3. 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 :

<type> *<name>[<number-of-elements];

For example :

char *ptr[3];

The above line declares an array of three character pointers.

Lets take a working example :

#include<stdio.h> 

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 = [Himanshu] 

 p2 = [Arora] 

 p3 = [India] 

 arr[0] = [Himanshu] 

 arr[1] = [Arora] 

 arr[2] = [India]

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

4. C Function Pointers

Just like pointer to characters, integers etc, we can have pointers to functions.

A function pointer can be declared as :

<return type of function> (*<name of pointer>) (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<stdio.h> 

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 we might cover in some future article.

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{ 92 comments… add one }

  • cee January 27, 2012, 3:43 am

    Hi,
    This article is so good, I will print it out for my apprentice (and myself).
    It would be nice if this blog had a ‘print this site with all pictures and the whole formatting’ option.

    Best Regards,
    cee

  • Himanshu January 27, 2012, 4:18 am

    @cee
    Thanks!!!

  • Makis January 27, 2012, 6:40 am

    Very nice examples. Good work! I believe it would also be nice to cover some dynamic memory fundamentals using pointers (ie dynamic single and two-dimensional arrays). Also consider the possibility of covering this dynamic memory tutorial not only for C but also for the C++ language.

  • apol January 27, 2012, 12:12 pm

    Yeah ! you’ve wrote it as you said before the end of january 😉 Thanks a lot for this article and your site in general.

  • Jay January 29, 2012, 4:20 am

    The article is very nice..and useful as well.
    If the uses mentioned then it would be more and more helpful.

  • Balakrishnan M February 6, 2012, 12:44 am

    Clearly explained.. Useful one !!

  • divya March 28, 2012, 8:35 am

    explanation and article both are very nice ……..:)

  • priya April 15, 2012, 2:45 am

    thnk u so much..ifor giving such difficult concept in a nice manner…

  • raj May 19, 2012, 12:27 pm

    good explanation with examples……………..!!!thanks

  • Dev June 3, 2012, 7:43 am

    Awesome article . hope will get advance article also ..
    Thanks a lot. Really appreciate ur effort :)

  • Max steel June 8, 2012, 12:48 pm

    Awesome work man…!!! I be gained my comfort level wid ptr by ur article…thanks..

  • Anonymous June 14, 2012, 4:53 pm

    Great article, it contained great detail and simple explanations of pointers.
    Thanks.

  • samadhan July 5, 2012, 7:57 am

    u have explain the concept in nicely in manner,
    easy to understand

  • zah July 24, 2012, 4:12 am

    Thanks for this article mayn!! really useful! I think I got what i needed for my interview!!

    Thanks,
    Best regards!!

  • sony July 26, 2012, 3:25 am

    hiiiiiiii
    this article is really good it helps us lot in understnading pointer and its working
    Thanks.

  • shari July 26, 2012, 5:37 am

    simple and clean!

  • BB July 30, 2012, 12:36 am

    Useful peace of info :)

  • Nagarajan August 1, 2012, 10:48 am

    I have learnt so much from geekstuff especially this made pointer concepts very clear!!
    Thank you :-)

  • Syed Shareef Ali August 10, 2012, 1:15 am

    Awesome!!!…Very good article with clear information.
    Like to know few more advanced topics.!!Please point the similar links.

  • Krishnakumar gupta August 17, 2012, 4:22 am

    Hi
    This Artical is very good and easy to understand
    i want to know more advance concepts of pointer
    if you have please send me on my emailid

  • Mayank August 22, 2012, 11:36 pm

    Can you please suggest some good book to go into more detail of understanding and working on pointers?

  • lala August 30, 2012, 12:12 am

    concept is describedn in very simple way i like it

  • Mihai bairac September 5, 2012, 5:20 am

    Great article! Thank you!

  • kalpana b September 14, 2012, 6:06 am

    This article is easy to understand and very helpful…
    Thanks a lot!!

  • Ranjith Kumar September 21, 2012, 7:33 am

    #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(“**ptr=[%c] \n”,**ptr);
    return(0);
    }

  • Ramesh September 22, 2012, 3:01 am

    Nice explanation…

  • raju September 22, 2012, 5:30 am

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

  • Niranjan September 26, 2012, 10:45 pm

    Hi everybody i want 5-10 questions and solutions in-depth on pointers please help me

  • Anonymous October 1, 2012, 2:28 pm

    “”
    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’.

    “”
    You have a typo in this paragraph.
    its p1 with is pointer to pointer to character ch not p2

  • pari October 3, 2012, 2:41 pm

    Brief
    And useful ,thanks alot!

  • sibusiso October 10, 2012, 5:42 am

    Brief , to the point and very useful. thanks a Million.

  • SURBHI JAIN October 16, 2012, 3:00 am

    Easy to understand pointers through this article.

  • Pranoti October 21, 2012, 2:43 am

    Thank you very much for giving concepts of pointer that to in easy way. I like the way to put the example to under each concept of pointer,pointer to array,function pointers.
    Thanks…:)

  • sarita October 21, 2012, 9:22 am

    thanks!

  • Deepika October 30, 2012, 1:53 am

    Example and explanation both are goood…

  • ashutosh November 1, 2012, 9:28 am

    if int variable holds 2 bytes than what should pointer int occupy?????????? i.e int a=5 means a occupy 2 bytes and int *a where a=&a; than what should *a occupy in memory???????????

  • Neeti November 6, 2012, 9:03 am

    Thanks

  • pavithra November 6, 2012, 10:54 am

    Thank you…a very neat explanation
    what does this statement mean
    int (*ptr)[3] = &a[0];

  • Harsha P M November 13, 2012, 5:07 am

    can u tell about pointer to an array i.e int (*p)[10];.and also performance between
    array of pointers n pointer to an array

  • A. Das November 19, 2012, 9:23 am

    This article really helped. Thank You.

  • Kush Sahni November 23, 2012, 8:09 am

    Nice and easy way to explain pointers..!! good job

  • Alok November 28, 2012, 11:19 am

    Very nicely explained. This really helped.

  • Anonymous November 29, 2012, 1:15 am

    int (*p)[10]; can u tell about pointer to an array in the same site. ur doing a good job…hatsoff…

  • Sandip pawar December 1, 2012, 9:17 pm

    Ok,i can ustand

  • Anoop December 13, 2012, 11:04 pm

    Can you please implement the Inheritance concept in C using function pointer?

  • Ujjwal December 29, 2012, 5:40 am

    Above deceleration of every definition is very easy to understand……
    nice job buddy……….

  • zubair January 28, 2013, 12:21 pm

    this is very gud can you tell me the other example more

  • Bharath January 31, 2013, 11:54 am

    above described ex are nice.but i advice you to provide much more examples.

  • sajan palakkad February 24, 2013, 8:33 am

    yes it’s realy hlpful
    thnks

  • jignesh March 5, 2013, 1:26 am

    realy it’s very nice………….

  • Manikandan March 21, 2013, 11:07 pm

    Good to begin with

  • raj March 31, 2013, 8:21 am

    simple and very easy to understand

  • vijay chauhan April 6, 2013, 11:32 am

    superb!!! Easy to understand and very helpful as well..keep it up!

  • Rohit April 29, 2013, 6:00 am

    Thank you so much. Finally got a hang of the difference between the *ptr and the **ptr and the ***ptr.

    Thank you very much

  • karan patel May 4, 2013, 6:52 am

    #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;
    }

    you explain this concept . i am not understand what is the need of pointer to this function when func(2,3) does whatever we want to do ……….
    can you give such example which explain in what setutation we should use pointer to function?????

  • prasad May 27, 2013, 12:14 am

    it is good

  • sara June 13, 2013, 2:53 pm

    aWsm…gr8…i got the idea abt pointer in just 15 mins…:)

  • amit June 19, 2013, 8:35 pm

    nice explanation…of key points in pointers……

  • satyajit July 12, 2013, 1:07 am

    Thanks for such awesome explanation…

  • gaurav bansal July 16, 2013, 11:09 pm

    very nice example pointer programs .

  • sandeep July 30, 2013, 4:21 am

    nicee…very easy to understand

  • yogesh August 11, 2013, 4:52 am

    when i run function pointer example on my system it taks atleast 15s to execute,

    why so and please give me link of your further articles on pointer

  • surendra August 21, 2013, 10:10 pm

    please make me understand about the output of program.

    #include
    void main()
    {
    static int a[3]={0,1,2};
    int * p[]={a,a+1,a+2};
    printf(“%u\n%u\n%d”,p,*p,*(*p));
    getch();
    }

  • payal September 4, 2013, 8:22 am

    vry good explanation about pointer to pointet

  • Bidyut. September 6, 2013, 8:44 am

    Good explaination .

  • tarun sharma October 1, 2013, 10:56 am

    nice explanation,………..you reali made it look easy…..
    Thank you :)

  • Anonymous October 12, 2013, 2:54 am

    very good explanation about pointers

  • Shoaib October 22, 2013, 12:30 am

    Examples are good….
    need more example which returns pointers…..
    Please

  • allex mclen November 5, 2013, 6:20 am

    Thanks a lot man!!!
    these were some of the concepts which never get cleared…by my boring course book

  • muhammad arif November 11, 2013, 6:56 am

    i need more help on pointer . can you please advise me any good books on pointer
    thanks a lot….

  • Varun November 27, 2013, 6:08 pm

    int rows = 2, col = 45;
    ptr = (char **)malloc(sizeof (char) * rows);
    int i;
    for (i = 0; i < rows; i++)
    {
    ptr[i] = (char *)malloc(col* sizeof (char));
    }
    for (i = 0; i < rows; i++)
    {
    printf("Address of row-%d is %p\n", i, ptr+i);
    }

    for (i = 0; i < rows; i++)
    {
    printf("Enter Names\n");
    scanf("%s", ptr[i]);
    }

    for (i = 0; i < rows; i++)
    {
    printf("Entered Names: %s\n", ptr[i]);
    printf("Address(ptr[i]): %p\n", ptr[i]);
    printf("Address(ptr+i): %p\n", ptr+i);

    }
    printf("ptr[1] – ptr[0] = %p\n", ptr[1] – ptr[0]);

    Output:
    Address of row-0 is 0x9132008
    Address of row-1 is 0x913200c
    Enter Names
    sad
    Enter Names
    c
    Entered Names: sad
    Address(ptr[i]): 0x9132018
    Address(ptr+i): 0x9132008
    Entered Names: c
    Address(ptr[i]): 0x9132050
    Address(ptr+i): 0x913200c
    ptr[1] – ptr[0] = 0x38 //Here expecting 0x2d (Col = 45) but why 0x38??

    Not getting please let me know.

    Thanks in advance

  • KIRAN SOLANKI December 20, 2013, 3:07 am

    Your answer is good. I need example of pointer how is work and how point to one or more….

  • TiTo December 26, 2013, 9:52 pm

    You forgot pointer to array.. Rest are very good explained…

  • John Walker January 4, 2014, 5:50 am

    Great. After struggling with the details arrays of functions I sorted
    it out in a couple of hours,
    All the best
    Thankyou
    John Walker

  • B.MATHANGI January 17, 2014, 9:35 am

    exclusive and crispy informations about c it’s fabulus!!!

  • Darren B January 30, 2014, 9:31 am

    Excellent article. One of the best and clearest explanations of pointers I have seen.
    However, I may be mistaken but I think there is a typo in section “2. C Pointer to Pinter”.

    I think the comment

    “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’.”

    should be

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

    Thanks

  • tp February 20, 2014, 7:25 am

    Eminent article.
    It clears my doubts about pointer to pointer

    but will u describe what’s the difference between
    const int *p and int const *p

    Thanks

  • Sony March 11, 2014, 9:52 am

    It is a helpful article to clear the basic concept on pointer..thanks

  • DefinitionOfScum May 2, 2014, 2:30 pm

    This article and the one before it on pointer was amazing. Thank you very much, cleared up everything I ever wanted to know about pointers.

  • peddaraju May 5, 2014, 7:33 pm

    its good explanation…. can you provide me where this pointer to functions are used?

  • Aditi July 26, 2014, 12:31 am

    awesome explanation of pointers
    thankew!!!!

  • Azam khan September 7, 2014, 6:20 am

    Hi
    Explanation with ex is good

  • Vikash October 15, 2014, 12:17 pm

    Thank You. Very much helpful article.

  • Greg Lafrance October 31, 2014, 11:03 am

    Good article that brings greater clarity on the topic of pointers. Thanks very much.

  • sreeekanth December 12, 2014, 8:40 am

    i want the answer to both array of pointer to a function

  • saranya January 12, 2015, 9:41 pm

    tanx a lot,,,, nice explanation

  • bhaskar February 7, 2015, 5:58 am

    nice example

  • NITIN DAWALE February 7, 2015, 1:01 pm

    very nice

  • malhar February 19, 2015, 3:47 am

    Dont you think the output of the third type (array of pointers) is wrong ?
    You said that the array contains the address of the strings but your output does not shhow that

  • Kinjal March 2, 2015, 5:50 am

    nice explaination 😉 …….. Thænk Ü

  • Ashish Vishwakarma April 18, 2015, 10:57 pm

    Very good explain
    But i wont to know that
    What is pointer to function ?
    How can we explain it?

  • sivaji May 18, 2015, 8:53 pm

    you have given very clear explanation.thanks for giving

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