Pointers are arguably the most difficult feature of C to understand. But, they are one of the features which make C an excellent language.

In this article, we will go from the very basics of pointers to their usage with arrays, functions, and structure.

So relax, grab a coffee, and get ready to learn all about pointers.

## A. Definition, Notation, Types and Arithmetic

### 1. What exactly are pointers?

Before we get to the definition of pointers, let us understand what happens when we write the following code:

``````int digit = 42;
``````

A block of memory is reserved by the compiler to hold an `int` value. The name of this block is `digit` and the value stored in this block is `42`.

Now, to remember the block, it is assigned with an address or a location number (say, 24650).

The value of the location number is not important for us, as it is a random value. But, we can access this address using the `&` (ampersand) or address of operator.

``````printf("The address of digit = %d.",&digit);
/* prints "The address of digit = 24650. */
``````

We can get the value of the variable `digit` from its address using another operator `*` (asterisk), called the indirection or dereferencing or value at address operator.

``````printf("The value of digit = %d.", *(&digit);
/* prints "The value of digit = 42. */
``````

### 2. Definition and Notation

The address of a variable can be stored in another variable known as a pointer variable. The syntax for storing a variable's address to a pointer is:

``````dataType *pointerVariableName = &variableName;
``````

For our `digit` variable, this can be written like this:

``````int *addressOfDigit = &digit;
``````

or like this:

``````int *addressOfDigit;
``````

This can be read as - A pointer to `int` (integer) `addressOfDigit` stores the `address of(&)` `digit` variable.

#### Few points to understand:

`dataType` – We need to tell the computer what the data type of the variable is whose address we are going to store. Here, `int` was the data type of `digit`.

It does not mean that `addressOfDigit` will store a value of type `int`. An integer pointer (like `addressOfDigit`) can only store the address of variables of integer type.

``````int variable1;
int variable2;
char variable3;
``````

`*` – A pointer variable is a special variable in the sense that it is used to store an address of another variable. To differentiate it from other variables that do not store an address, we use `*` as a symbol in the declaration.

Here, we can assign the address of `variable1` and `variable2` to the integer pointer `addressOfVariables` but not to `variable3` since it is of type `char`. We will need a character pointer variable to store its address.

We can use our `addressOfDigit` pointer variable to print the address and the value of `digit` as below:

``````printf("The address of digit = %d.", addressOfDigit);
/* prints "The address of digit = 24650." */
printf("The value of digit = %d.", *addressOfDigit);
/*prints "The value of digit = 42. */
``````

Here, `*addressOfDigit` can  be read as the value at the address stored in `addressOfDigit`.

Notice we used `%d` as the format identifier for `addressOfDigit`. Well, this is not completely correct. The correct identifier would be `%p`.

Using `%p`, the address is displayed as a hexadecimal value. But the memory address can be displayed in integers as well as octal values. Still, since it is not an entirely correct way, a warning is shown.

``````int num = 5;
int *p = &num;
``````

The output according to the compiler I'm using is the following:

``````Address using %p = 000000000061FE00
``````
This is the warning shown when you use  %d - " warning: format '%d' expects argument of type 'int', but argument 2 has type 'int *' ".

### 3. Some Special Pointers

#### The Wild Pointer

``````char *alphabetAddress; /* uninitialised or wild pointer */
char alphabet = "a";
alphabetAddress = &alphabet; /* now, not a wild pointer */
``````

When we defined our character pointer `alphabetAddress`, we did not initialize it.

Such pointers are known as wild pointers. They store a garbage value (that is, memory address) of a byte that we don't know is reserved or not (remember `int digit = 42;`, we reserved a memory address when we declared it).

Suppose we dereference a wild pointer and assign a value to the memory address it is pointing at. This will lead to unexpected behaviour since we will write data at a  memory block that may be free or reserved.

#### Null Pointer

To make sure that we do not have a wild pointer, we can initialize a pointer with a `NULL` value, making it a null pointer.

``````char *alphabetAddress = NULL /* Null pointer */
``````

A null pointer points at nothing, or at a memory address that users can not access.

#### Void Pointer

A void pointer can be used to point at a variable of any data type. It can be reused to point at any data type we want to. It is declared like this:

``````void *pointerVariableName = NULL;
``````

Since they are very general in nature, they are also known as generic pointers.

With their flexibility, void pointers also bring some constraints. Void pointers cannot be dereferenced as any other pointer. Appropriate typecasting is necessary.

``````void *pointer = NULL;
int number = 54;
char alphabet = "z";
pointer = &number;
printf("The value of number = ", *pointer); /* Compilation Error */
/* Correct Method */
printf("The value of number = ", *(int *)pointer); /* prints "The value at number = 54" */
pointer = &alphabet;
printf("The value of alphabet = ", *pointer); /* Compilation Error */
printf("The value of alphabet = ", *(char *)pointer); /* prints "The value at alphabet = z */
``````

Similarly, void pointers need to be typecasted for performing arithmetic operations.

Void pointers are of great use in C. Library functions `malloc()` and `calloc()` which dynamically allocate memory return void pointers. `qsort()`, an inbuilt sorting function in C, has a function as its argument which itself takes void pointers as its argument.

#### Dangling Pointer

A dangling pointer points to a memory address which used to hold a variable. Since the address it points at is no longer reserved, using it will lead to unexpected results.

``````main(){
int *ptr;
ptr = (int *)malloc(sizeof(int));
*ptr = 1;
printf("%d",*ptr); /* prints 1 */
free(ptr); /* deallocation */
*ptr = 5;
printf("%d",*ptr); /* may or may not print 5 */
}
``````

Though the memory has been deallocated by `free(ptr)`, the pointer to integer `ptr` still points to that unreserved memory address.

### 4. Pointer Arithmetic

We know by now that pointers are not like any other variable. They do not store any value but the address of memory blocks.

So it should be quite clear that not all arithmetic operations would be valid with them. Would multiplying or dividing two pointers (having addresses) make sense?

#### Pointers have few but immensely useful valid operations:

1. You can assign the value of one pointer to another only if they are of the same type (unless they're typecasted or one of them is `void *`).
``````int ManU = 1;
``````

2.   You can only add or subtract integers to pointers.

``````int myArray = {3,6,9,12,15};
int *pointerToMyArray = &myArray[0];
pointerToMyArray += 3; /* Valid */
pointerToMyArray *= 3; /* Invalid */
``````

When you add (or subtract) an integer (say n) to a pointer, you are not actually adding (or subtracting) n bytes to the pointer value. You are actually adding (or subtracting) n-times the size of the data type of the variable being pointed bytes.

``````int number = 5;
/* Suppose the address of number is 100 */
int *ptr = &number;
int newAddress = ptr + 3;
/* Same as ptr + 3 * sizeof(int) */
``````

The value stored in `newAddress` will not be 103, rather `112`.

3.  Subtraction and comparison of pointers is valid only if both are members of the same array. The subtraction of pointers gives the number of elements separating them.

``````int myArray = {3,6,9,12,15};
int sixthMultiple = 18;
int *pointer1 = &myArray[0];
int *pointer2 = &myArray[1];
int *pointer6 = &sixthMuliple;
/* Valid Expressions */
if(pointer1 == pointer2)
pointer2 - pointer1;
/* Invalid Expressions
if(pointer1 == pointer6)
pointer2 - pointer6
``````

4.  You can assign or compare a pointer with `NULL`.

The only exception to the above rules is that the address of the first memory block after the last element of an array follows pointer arithmetic.

Pointer and arrays exist together. These valid manipulations of pointers are immensely useful with arrays, which will be discussed in the next section.

## B. Arrays and Strings

### 1. Why pointers and arrays?

In C, pointers and arrays have quite a strong relationship.

The reason they should be discussed together is because what you can achieve with array notation (`arrayName[index]`) can also be achieved with pointers, but generally faster.

### 2. 1-D Arrays

Let us look at what happens when we write `int myArray[5];`.

Five consecutive blocks of memory starting from `myArray[0]` to `myArray[4]` are created with garbage values in them. Each of the blocks is of size 4 bytes.

Thus, if the address of myArray[0] is `100` (say), the address of the rest of the blocks would be `104`, `108`, `112`, and `116`.

Have a look at the following code:

``````int prime[5] = {2,3,5,7,11};
printf("Result using &prime = %d\n",&prime);
printf("Result using prime = %d\n",prime);
printf("Result using &prime[0] = %d\n",&prime[0]);

/* Output */
Result using &prime = 6422016
Result using prime = 6422016
Result using &prime[0] = 6422016
``````

So, `&prime`, `prime`, and `&prime[0]` all give the same address, right? Well, wait and read because you are in for a surprise (and maybe some confusion).

Let's try to increment each of `&prime`, `prime`, and `&prime[0]` by 1.

``````printf("Result using &prime = %d\n",&prime + 1);
printf("Result using prime = %d\n",prime + 1);
printf("Result using &prime[0] = %d\n",&prime[0] + 1);

/* Output */
Result using &prime = 6422036
Result using prime = 6422020
Result using &prime[0] = 6422020
``````

Wait! How come `&prime + 1` results in something different than the other two? And why are `prime + 1` and `&prime[0] + 1` still equal? Let's answer these questions.

`prime` and `&prime[0]` both point to the 0th element of the array `prime`. Thus, the name of an array is itself a pointer to the 0th element of the array.

Here, both point to the first element of size 4 bytes. When you add 1 to them, they now point to the 1st element in the array. Therefore this results in an increase in the address by 4.

`&prime`, on the other hand, is a pointer to an `int` array of size 5. It stores the base address of the array `prime[5]`, which is equal to the address of the first element. However, an increase by 1 to it results in an address with an increase of 5 x 4 = 20 bytes.

In short, `arrayName` and `&arrayName[0]` point to the 0th element whereas `&arrayName` points to the whole array.

We can access the array elements using subscripted variables like this:

``````int prime[5] = {2,3,5,7,11};
for( int i = 0; i < 5; i++)
{
printf("index = %d, address = %d, value = %d\n", i, &prime[i], prime[i]);
}
``````

We can do the same using pointers which are always faster than using subscripts.

``````int prime[5] = {2,3,5,7,11};
for( int i = 0; i < 5; i++)
{
printf("index = %d, address = %d, value = %d\n", i, prime + i, *(prime + i));
}
``````

Both methods give the output:

``````index = 0, address = 6422016, value = 2
index = 1, address = 6422020, value = 3
index = 2, address = 6422024, value = 5
index = 3, address = 6422028, value = 7
index = 4, address = 6422032, value = 11
``````

Thus, `&arrayName[i]` and `arrayName[i]` are the same as `arrayName + i` and  `*(arrayName + i)`, respectively.

### 3. 2-D Arrays

Two-dimensional arrays are an array of arrays.

``````int marks[5][3] = { { 98, 76, 89},
{ 81, 96, 79},
{ 88, 86, 89},
{ 97, 94, 99},
{ 92, 81, 59}
};
``````

Here, `marks` can be thought of as an array of 5 elements, each of which is a one-dimensional array containing 3 integers. Let us work through a series of programs to understand different subscripted expressions.

``````printf("Address of whole 2-D array = %d\n", &marks);
printf("Addition of 1 results in %d\n", &marks +1);

/* Output */
Address of whole 2-D array = 6421984
Addition of 1 results in 6422044
``````

Like 1-D arrays, `&marks` points to the whole 2-D array, `marks[5][3]`. Thus, incrementing to it by 1 ( = 5 arrays X 3 integers each X 4 bytes = 60) results in an increment by 60 bytes.

``````printf("Address of 0th array = %d\n", marks);
printf("Addition of 1 results in %d\n", marks +1);
printf("Address of 0th array =%d\n", &marks[0]);
printf("Addition of 1 results in %d\n", &marks[0] + 1);

/* Output */
Address of 0th array = 6421984
Addition of 1 results in 6421996
Address of 0th array = 6421984
Addition of 1 results in 6421996
``````

If `marks` was a 1-D array, `marks` and `&marks[0]` would have pointed to the `0th` element. For a 2-D array, elements are now 1-D arrays. Hence, `marks` and `&marks[0]` point to the `0th` array (element), and the addition of 1 point to the `1st` array.

``````printf("Address of 0th element of 0th array = %d\n", marks[0]);
printf("Addition of 1 results in %d\n", marks[0] + 1);
printf("Address of 0th element of 1st array = %d\n", marks[1]);
printf("Addition of 1 results in %d\n", marks[1] + 1);

/* Output */
Address of 0th element of 0th array = 6421984
Addition of 1 results in 6421988
Address of 0th element of 1st array = 6421996
Addition of 1 results in 6422000
``````

And now comes the difference. For a 1-D array, `marks[0]` would give the value of the 0th element. An increment by 1 would increase the value by 1.

But, in a 2-D array, `marks[0]` points to the `0th` element of the `0th` array. Similarly, `marks[1]` points to the `0th` element of the `1st` array. An increment by 1 would point to the `1st` element in the `1st` array.

``````printf("Value of 0th element of 0th array = %d\n", marks[0][0]);
printf("Addition of 1 results in %d", marks[0][0] + 1);

/* Output */
Value of 0th element of 0th array = 98
Addition of 1 results in 99
``````

This is the new part. `marks[i][j]` gives the value of the `jth` element of the `ith` array. An increment to it changes the value stored at `marks[i][j]`. Now, let us try to write `marks[i][j]` in terms of pointers.

We know `marks[i] + j` would point to the `ith` element of the `jth` array from our previous discussion. Dereferencing it would mean the value at that address. Thus, `marks[i][j]` is the same as  `*(marks[i] + j)`.

From our discussion on 1-D arrays, `marks[i]` is the same as `*(marks + i)`. Thus, `marks[i][j]` can be written as `*(*(marks + i) + j)` in terms of pointers.

Here is a summary of notations comparing 1-D and 2-D arrays.

Expression 1-D Array 2-D Array
&arrayName points to the address of whole array
points to the address of whole array
arrayName points to the 0th element
points to the 0th element (array)
&arrayName[i] points to the the ith element
points to the ith element (array)
arrayName[i] gives the value of the ith element
adding 1 increases the value of the ith element
points to the 0th element of the ith array
adding 1 increases the address to 1st element of the ith array
arrayName[i][j] Nothing gives the value of the jth element of the ith array
adding 1 increases the value of the jth element of the ith array
Pointer Expression To Access The Elements *( arrayName + i) *( *( arrayName + i) + j)

### 4. Strings

A string is a one-dimensional array of characters terminated by a `null(\0)`. When we write `char name[] = "Srijan";`, each character occupies one byte of memory with the last one always being `\0`.

Similar to the arrays we have seen, `name` and `&name[0]` points to the `0th` character in the string, while `&name` points to the whole string. Also, `name[i]` can be written as `*(name + i)`.

``````/* String */
char champions[] = "Liverpool";

printf("Pointer to whole string = %d\n", &champions);
printf("Addition of 1 results in %d\n", &champions + 1);

/* Output */
Address of whole string = 6421974
Addition of 1 results in 6421984

printf("Pointer to 0th character = %d\n", &champions[0]);
printf("Addition of 1 results in %d\n", &champions[0] + 1);

/* Output */
Address of 0th character = 6421974
Addition of 1 results in a pointer to 1st character 6421975

printf("Pointer to 0th character = %d\n", champions);
printf("Addition of 1 results in a pointer to 1st character %d\n", champions + 1);

/* Output */
Address of 0th character = 6421974
Addition of 1 results in 6421975

printf("Value of 4th character = %c\n", champions[4]);
printf("Value of 4th character using pointers = %c\n", *(champions + 4));

/* Output */
Value of 4th character = r
Value of 4th character using pointers = r
``````

A two-dimensional array of characters or an array of strings can also be accessed and manipulated as discussed before.

``````/* Array of Strings */
char top[6][15] = {
"Liverpool",
"Man City",
"Man United",
"Chelsea",
"Leicester",
"Tottenham"
};

printf("Pointer to 2-D array = %d\n", &top);
printf("Addition of 1 results in %d\n", &top + 1);

/* Output */
Pointer to 2-D array = 6421952
Addition of 1 results in 6422042

printf("Pointer to 0th string = %d\n", &top[0]);
printf("Addition of 1 results in %d\n", &top[0] + 1);

/* Output */
Pointer to 0th string = 6421952
Addition of 1 results in 6421967

printf("Pointer to 0th string = %d\n", top);
printf("Addition of 1 results in %d\n", top + 1);

/* Output */
Pointer to 0th string = 6421952
Addition of 1 results in 6421967

printf("Pointer to 0th element of 4th string = %d\n", top[4]);
printf("Pointer to 1st element of 4th string = %c\n", top[4] + 1);

/* Output */
Pointer to 0th element of 4th string = 6422012
Pointer to 1st element of 4th string = 6422013

printf("Value of 1st character in 3rd string = %c\n", top[3][1]);
printf("Same using pointers = %c\n", *(*(top + 3) + 1));

/* Output */
Value of 1st character in 3rd string = h
Same using pointers = h
``````

### 5. Array of Pointers

Like an array of `int`s and an array of `char`s, there is an array of pointers as well. Such an array would simply be a collection of addresses. Those addresses could point to individual variables or another array as well.

The syntax for declaring a pointer array is the following:

``````dataType *variableName[size];

/* Examples */
int *example1[5];
char *example2[8];
``````

Following the operators precedence, the first example can be read as -  `example1` is an array(`[]`) of 5 pointers to `int`. Similarly, `example2` is an array of 8 pointers to `char`.

We can store the two-dimensional array to string `top` using a pointer array and save memory as well.

``````char *top[] = {
"Liverpool",
"Man City",
"Man United",
"Chelsea",
"Leicester",
"Tottenham"
};
``````

`top` will contain the base addresses of all the respective names. The base address of `"Liverpool"` will be stored in `top[0]`, `"Man City"` in `top[1]`, and so on.

In the earlier declaration, we required 90 bytes to store the names. Here, we only require ( 58 (sum of bytes of names) + 12 ( bytes required to store the address in the array) ) 70 bytes.

The manipulation of strings or integers becomes a lot easier when using an array of pointers.

If we try to put `"Leicester"` ahead of `"Chelsea"`, we just need to switch the values of `top[3]` and `top[4]` like below:

``````char *temporary;
temporary = top[3];
top[3] = top[4];
top[4] = temporary;
``````

Without pointers, we would have to exchange every character of the strings, which would have taken more time. That's why strings are generally declared using pointers.

### 6. Pointer to Array

Like "pointer to `int`" or "pointer to `char`", we have pointer to array as well. This pointer points to whole array rather than its elements.

Remember we discussed how `&arrayName` points to the whole array? Well, it is a pointer to array.

A pointer to array can be declared like this:

``````dataType (*variableName)[size];

/* Examples */
int (*ptr1)[5];
char (*ptr2)[15];
``````

Notice the parentheses. Without them, these would be an array of pointers. The first example can be read as - `ptr1` is a pointer to an array of 5 `int`(integers).

``````int goals[] = { 85,102,66,69,67};
int (*pointerToGoals)[5] = &goals;
printf("Address stored in pointerToGoals %d\n", pointerToGoals);
printf("Dereferncing it, we get %d\n",*pointerToGoals);

/* Output */
Dereferencing it, we get 6422016
``````

When we dereference a pointer, it gives the value at that address. Similarly, by dereferencing a pointer to array, we get the array and the name of the array points to the base address. We can confirm that `*pointerToGoals` gives the array `goals` if we find its size.

``````printf("Size of goals[5] = %d, *pointerToGoals);

/* Output */
Size of goals[5] = 20
``````

If we dereference it again, we will get the value stored in that address. We can print all the elements using `pointerToGoals`.

``````for(int i = 0; i < 5; i++)
printf("%d ", *(*pointerToGoals + i));

/* Output */
85 102 66 69 67
``````

Pointers and pointer to arrays are quite useful when paired up with functions. Coming up in the next section!

## C. Functions

### 1. Call by Value vs Call by Reference

Have a look at the program below:

``````#include <stdio.h>

int multiply(int x, int y){
int z;
z = x * y;
return z;
}

main(){
int x = 3, y = 5;
int product = multiply(x,y);
printf("Product = %d\n", product);
/* prints "Product = 15" */
}
``````

The function `multiply()` takes two `int` arguments and returns their product as `int`.

In the function call `multiply(x,y)`, we passed the value of `x` and `y` ( of `main()`), which are actual arguments, to `multiply()`.

The values of the actual arguments are passed or copied to the formal arguments `x` and `y` ( of `multiply()`). The `x` and `y` of `multiply()` are different from those of `main()`. This can be verified by printing their addresses.

``````#include <stdio.h>

int multiply(int x, int y){
printf("Address of x in multiply() = %d\n", &x);
printf("Address of y in multiply() = %d\n", &y);
int z;
z = x * y;
return z;
}

main(){
int x = 3, y = 5;
printf("Address of x in main() = %d\n", &x);
printf("Address of y in main() = %d\n", &y);
int product = multiply(x,y);
printf("Product = %d\n", product);
}

/* Output */
Address of x in main() = 6422040
Address of y in main() = 6422036
Address of x in multiply() = 6422000
Address of y in multiply() = 6422008
Product = 15
``````

Since we created stored values in a new location, it costs us memory. Wouldn't it be better if we could perform the same task without wasting space?

Call by reference helps us achieve this. We pass the address or reference of the variables to the function which does not create a copy. Using the dereferencing operator `*`, we can access the value stored at those addresses.

We can rewrite the above program using call by reference as well.

``````#include <stdio.h>

int multiply(int *x, int *y){
int z;
z = (*x) * (*y);
return z;
}

main(){
int x = 3, y = 5;
int product = multiply(&x,&y);
printf("Product = %d\n", product);
/* prints "Product = 15" */
}
``````

### 2. Pointers as Function Arguments

In this section, we will look at various programs where we give `int`, `char`, arrays and strings as arguments using pointers.

``````#include <stdio.h>

float c = *a + *b;
}

void subtract(float *a, float *b){
float c = *a - *b;
printf("Subtraction gives %.2f\n",c);
}

void multiply(float *a, float *b){
float c = *a * *b;
printf("Multiplication gives %.2f\n",c);
}

void divide(float *a, float *b){
float c = *a / *b;
printf("Division gives %.2f\n",c);
}

main(){
printf("Enter two numbers :\n");
float a,b;
scanf("%f %f",&a,&b);
printf("What do you want to do with the numbers?\nAdd : a\nSubtract : s\nMultiply : m\nDivide : d\n");
char operation = '0';
scanf(" %c",&operation);
printf("\nOperating...\n\n");
switch (operation) {
case 'a':
break;
case 's':
subtract(&a,&b);
break;
case 'm':
multiply(&a,&b);
break;
case 'd':
divide(&a,&b);
break;
default:
printf("Invalid input!!!\n");

}

}
``````

We created four functions, `add()`, `subtract()`, `multiply()` and `divide()` to perform arithmetic operations on the two numbers `a` and `b`.

The address of `a` and `b` was passed to the functions. Inside the function using `*` we accessed the values and printed the result.

Similarly, we can give arrays as arguments using a pointer to its first element.

``````#include <stdio.h>

void greatestOfAll( int *p){
int max = *p;
for(int i=0; i < 5; i++){
if(*(p+i) > max)
max = *(p+i);
}
printf("The largest element is %d\n",max);
}
main(){
int myNumbers[5] = { 34, 65, -456, 0, 3455};
greatestOfAll(myNumbers);
/* Prints :The largest element is 3455" */
}
``````

Since the name of an array itself is a pointer to the first element, we send that as an argument to the function `greatestOfAll()`. In the function, we traverse through the array using loop and pointer.

``````#include <stdio.h>
#include <string.h>

void wish(char *p){
printf("Have a nice day, %s",p);
}

main(){
char name[20];
gets(name);
wish(name);
}
``````

Here, we pass in the string `name` to `wish()` using a pointer and print the message.

### 3. Pointers as Function Return

``````#include <stdio.h>

int* multiply(int *a, int *b){
int c = *a * *b;
return &c;
}

main(){
int a= 3, b = 5;
int *c = multiply (&a,&b);
printf("Product = %d",*c);
}
``````

The function `multiply()` takes two pointers to `int`. It returns a pointer to `int` as well which stores the address where the product is stored.

It is very easy to think that the output would be 15. But it is not!

When `multiply()` is called, the execution of `main()` pauses and memory is now allocated for the execution of `multiply()`. After its execution is completed, the memory allocated to `multiply()` is deallocated.

Therefore, though `c` ( local to `main()`) stores the address of the product, the data there is not guaranteed since that memory has been deallocated.

So does that mean pointers cannot be returned by a function? No!

We can do two things. Either store the address in the heap or global section or declare the variable to be `static` so that their values persist.

Static variables can simply be created by using the keyword `static` before data type while declaring the variable.

To store addresses in heap, we can use library functions `malloc()` and `calloc()` which allocate memory dynamically.

The following programs will explain both the methods. Both methods return the output as 15.

``````#include <stdio.h>
#include <stdlib.h>

/* Using malloc() */

int* multiply(int *a, int *b){
int *c = malloc(sizeof(int));
*c = *a * *b;
return c;
}

main(){
int a= 3, b = 5;
int *c = multiply (&a,&b);
printf("Product = %d",*c);
}

/* Using static keyword */
#include <stdio.h>

int* multiply(int *a, int *b){
static int c;
c = *a * *b;
return &c;
}

main(){
int a= 3, b = 5;
int *c = multiply (&a,&b);
printf("Product = %d",*c);
}
``````

### 4. Pointer to Function

Like pointer to different data types, we also have a pointer to function as well.

A pointer to function or function pointer stores the address of the function. Though it doesn't point to any data. It points to the first instruction in the function.

The syntax for declaring a pointer to function is:

`````` /* Declaring a function */
returnType functionName(parameterType1, pparameterType2, ...);

/* Declaring a pointer to function */
returnType (*pointerName)(parameterType1, parameterType2, ...);
pointerName = &functionName; /* or pointerName = functionName; */
``````

The below example will make it clearer.

``````int* multiply(int *a, int *b)
{
int *c = malloc(sizeof(int));
*c = *a * *b;
return c;
}

main()
{
int a=3,b=5;
int* (*p)(int*, int*) = &multiply; /* or int* (*p)(int*, int*) = multiply; */
int *c = (*p)(&a,&b); /* or int *c = p(&a,&b); */
printf("Product = %d",*c);
}
``````

The declaration for the pointer `p` to function `multiply()` can be read as ( following operator precedence) - `p` is a pointer to function with two `int`eger pointers ( or two pointers to `int`) as parameters and returning a pointer to `int`.

Since the name of the function is also a pointer to the function, the use of `&` is not necessary. Also removing `*` from the function call doesn't affect the program.

### 5. Array of Pointers to Functions

We have already seen how to create an array of pointers to `int`, `char`, and so on. Similarly, we can create an array of pointers to function.

In this array, every element will store an address of a function, where all the functions are of the same type. That is, they have the same type and number of parameters and return types.

We will modify a program discussed earlier in this section. We will store the addresses of `add()`, `subtract()`, `multiply()` and `divide()` in an array make a function call through subscript.

``````#include <stdio.h>

float c = *a + *b;
}

void subtract(float *a, float *b){
float c = *a - *b;
printf("Subtraction gives %.2f\n",c);
}

void multiply(float *a, float *b){
float c = *a * *b;
printf("Multiplication gives %.2f\n",c);
}

void divide(float *a, float *b){
float c = *a / *b;
printf("Division gives %.2f\n",c);
}

main(){
printf("Enter two numbers :\n");
float a,b;
scanf("%f %f",&a,&b);
printf("What do you want to do with the numbers?\nAdd : a\nSubtract : s\nMultiply : m\nDivide : d\n");
char operation = '0';
scanf(" %c",&operation);
void (*p[])(float* , float*) = {add,subtract,multiply,divide};
printf("\nOperating...\n\n");
switch (operation) {
case 'a':
p[0](&a,&b);
break;
case 's':
p[1](&a,&b);
break;
case 'm':
p[2](&a,&b);
break;
case 'd':
p[3](&a,&b);
break;
default:
printf("Invalid input!!!\n");

}

}
``````

The declaration here can be read as - `p` is an array of pointer to functions with two `float` pointers as parameters and returning void.

### 6. Pointer to Function as an Argument

Like any other pointer, function pointers can also be passed to another function, therefore known as a callback function or called function. The function to which it is passed is known as a calling function.

A better way to understand would be to look at `qsort()`, which is an inbuilt function in C. It is used to sort an array of integers, strings, structures, and so on. The declaration for `qsort()` is:

``````void qsort(void *base, size_t nitems, size_t size, int (*compar)(const void *, const void *));
``````

`qsort()` takes four arguments:

1. a `void` pointer to the start of an array
2. number of elements
3. size of each element
4. a function pointer that takes in two `void` pointers as arguments and returns an `int`

The function pointer points to a comparison function that returns an integer that is greater than, equal to, or less than zero if the first argument is respectively greater than, equal to, or less than the second argument.

The following program showcases its usage:

``````#include <stdio.h>
#include <stdlib.h>

int compareIntegers(const void *a, const void *b)
{
const int *x = a;
const int *y = b;
return *x - *y;
}

main(){

int myArray[] = {97,59,2,83,19,97};
int numberOfElements = sizeof(myArray) / sizeof(int);

printf("Before sorting - \n");
for(int i = 0; i < numberOfElements; i++)
printf("%d ", *(myArray + i));

qsort(myArray, numberOfElements, sizeof(int), compareIntegers);

printf("\n\nAfter sorting - \n");
for(int i = 0; i < numberOfElements; i++)
printf("%d ", *(myArray + i));
}

/* Output */

Before sorting -
97 59 2 83 19 97

After sorting -
2 19 59 83 97 97
``````

Since a function name is itself a pointer, we can write `compareIntegers` as the fourth argument.

## D. Structure

### 1. Pointer to Structure

Like integer pointers, array pointers, and function pointers, we have pointer to structures or structure pointers as well.

``````struct records {
char name[20];
int roll;
int marks[5];
char gender;
};

struct records student = {"Alex", 43, {76, 98, 68, 87, 93}, 'M'};

struct records *ptrStudent = &student;
``````

Here, we have declared a pointer `ptrStudent` of type `struct records`. We have assigned the address of `student` to `ptrStudent`.

`ptrStudent` stores the base address of `student`, which is the base address of the first member of the structure. Incrementing by 1 would increase the address by `sizeof(student)` bytes.

``````printf("Address of structure = %d\n", ptrStudent);
printf("Adress of member `name` = %d\n", &student.name);
printf("Increment by 1 results in %d\n", ptrStudent + 1);

/* Output */
Adress of member `name` = 6421984
Increment by 1 results in 6422032
``````

We can access the members of `student` using `ptrStudent` in two ways. Using our old friend `*` or using `->` ( infix or arrow operator).

With `*`, we will continue to use the `.`( dot operator) whereas with `->` we won't need the dot operator.

``````printf("Name w.o using ptrStudent : %s\n", student.name);
printf("Name using ptrStudent and * : %s\n", ( *ptrStudent).name);
printf("Name using ptrStudent and -> : %s\n", ptrStudent->name);

/* Output */
Name without using ptrStudent: Alex
Name using ptrStudent and *: Alex
Name using ptrStudent and ->: Alex
``````

Similarly, we can access and modify other members as well. Note that the brackets are necessary while using `*` since the dot operator(`.`) has higher precedence over `*`.

### 2. Array Of Structure

We can create an array of type `struct records` and use a pointer to access the elements and their members.

``````struct records students[10];

/* Pointer to the first element ( structure) of the array */
struct records *ptrStudents1 = &students;

/* Pointer to an array of 10 struct records */
struct records (*ptrStudents2)[10] = &students;
``````

Note that `ptrStudent1` is a pointer to `student[0]` whereas `ptrStudent2` is a pointer to the whole array of  10 `struct records`. Adding 1 to `ptrStudent1` would point to `student[1]`.

We can use `ptrStudent1` with a loop to traverse through the elements and their members.

``````
for( int i = 0; i <  10; i++)
printf("%s, %d\n", ( ptrStudents1 + i)->name, ( ptrStudents1 + i)->roll);
``````

### 3. Pointer to Structure as an Argument

We can also pass the address of a structure variable to a function.

``````#include <stdio.h>

struct records {
char name[20];
int roll;
int marks[5];
char gender;
};

main(){
struct records students = {"Alex", 43, {76, 98, 68, 87, 93},
'M'};
printRecords(&students);
}

void printRecords( struct records *ptr){
printf("Name: %s\n", ptr->name);
printf("Roll: %d\n", ptr->roll);
printf("Gender: %c\n", ptr->gender);
for( int i = 0; i < 5; i++)
printf("Marks in %dth subject: %d\n", i, ptr->marks[i]);
}

/* Output */
Name: Alex
Roll: 43
Gender: M
Marks in 0th subject: 76
Marks in 1th subject: 98
Marks in 2th subject: 68
Marks in 3th subject: 87
Marks in 4th subject: 93
``````

Note that the structure `struct records` is declared outside `main()`. This is to ensure that it is available globally and `printRecords()` can use it.

If the structure is defined inside `main()`, its scope will be limited to `main()`. Also structure must be declared before the function declaration as well.

Like structures, we can have pointers to unions and can access members using the arrow operator (`->`).

## E. Pointer to Pointer

So far we have looked at pointer to various primitive data types, arrays, strings, functions, structures, and unions.

The automatic question that comes to the mind is – what about pointer to pointer?

Well, good news for you! They too exist.

``````int var = 6;
int *ptr_var = &var;

printf("Address of var = %d\n", ptr_var);
printf("Address of ptr_var = %d\n", &ptr_var);

/* Output */
``````

To store the address of `int` variable `var`, we have the pointer to `int` `ptr_var`. We would need another pointer to store the address of `ptr_var`.

Since `ptr_var` is of type `int *`, to store its address we would have to create a pointer to `int *`. The code below shows how this can be done.

``````int * *ptr_ptrvar = &ptr_var; /* or int* *ppvar or int **ppvar */
``````

We can use `ptr_ptrvar` to access the address of `ptr_var` and use double dereferencing to access var.

``````printf("Address of ptr_var = %d\n", ptr_ptrvar);
printf("Address of var = %d\n", *ptr_ptrvar);
printf("Value at var = %d\n", *(*ptr_ptrvar));

/* Output */
Value at var = 6
``````

It is not required to use brackets when dereferencing `ptr_ptrvar`. But it is a good practice to use them. We can create another pointer `ptr_ptrptrvar`, which will store the address of `ptr_ptrvar`.

Since `ptr_ptrvar` is of type `int**`, the declaration for `ptr_ptrptrvar` will be

``````int** *ptr_ptrptrvar = &ptr_ptrvar;
``````

We can again access `ptr_ptrvar`, `ptr_var` and `var` using `ptr_ptrptrvar`.

``````printf("Address of ptr_ptrvar = %d\n", ptr_ptrptrvar);
printf("Value at ptr_ptrvar = %d\n",*ptr_ptrptrvar);
printf("Address of ptr_var = %d\n", *ptr_ptrptrvar);
printf("Value at ptr_var = %d\n", *(*ptr_ptrptrvar));
printf("Address of var = %d\n", *(*ptr_ptrptrvar));
printf("Value at var = %d\n", *(*(*ptr_ptrptrvar)));

/* Output */
Value at ptr_ptrvar = 6422024
Value at ptr_var = 6422036
Value at var = 6
``````

If we change the value at any of the pointer(s) using `ptr_ptrptrvar` or `ptr_ptrvar`, the pointer(s) will stop pointing to the variable.

## Conclusion

Phew! Yeah, we're finished. We started from pointers and ended with pointers (in a way). Don't they say that the curve of learning is a circle!

Try to recap all the sub-topics that you read. If you can recollect them, well done! Read the ones you can't remember again.

This article is done, but you shouldn't be done with pointers. Play with them. Next, you can look into Dynamic Memory Allocation to get to know pointers better.

Stay home, stay safe.