Passing Pointers to Functions in C

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A pointer In C is a variable that stores the address of another variable. It acts as a reference to the original variable. A pointer can be passed to a function, just like any other argument is passed.

A function in C can be called in two ways −

• Call by Value
• Call by Reference

To call a function by reference, you need to define it to receive the pointer to a variable in the calling function. Here is the syntax that you would use to call a function by reference −

type function_name(type *var1, type *var2, ...)

When a function is called by reference, the pointers of the actual argument variables are passed, instead of their values.

Advantages of Passing Pointers to Functions

Passing a pointer to a function has two advantages −

• It overcomes the limitation of pass by value. Changes to the value inside the called function are done directly at the address stored in the pointer. Hence, we can manipulate the variables in one scope from another.
• It also overcomes the limitation of a function that it can return only one expression. By passing pointers, the effect of processing of a function takes place directly at the address. Secondly, more than one values can be returned if we return a pointer of an array or struct variable.

In this chapter, we shall see how to −

• Pass pointers to int variables
• Pass pointers to array
• Pass pointer to structure

Example of Passing Pointers to Functions

Let us define a function add() that receives the references of two variables. When such a function is called, we pass the address of the actual argument. Let us call the add() function by reference from inside the main() function.

#include <stdio.h>

/* function declaration */
int add(int *, int *);

int main(){

   int a = 10, b = 20;
   int c = add(&a, &b);
   printf("Addition: %d", c);
}

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

Output

Addition: 30

Swap Values by Passing Pointers

One of the most cited applications of passing a pointer to a function is how we can swap the values of two variables.

The following function receives the reference of two variables whose values are to be swapped −

/* function definition to swap the values */
int swap(int *x, int *y){
   int z;
   z = *x;    /* save the value at address x */
   *x = *y;   /* put y into x */
   *y = z;    /* put z into y */

   return 0;
}

Example

The main() function has two variables a and b; their addresses are passed as arguments to the swap() function.

#include <stdio.h>

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

int main (){

   /* local variable definition */
   int a = 10;
   int b = 20;
   printf("Before swap, value of a: %d\n", a);
   printf("Before swap, value of b: %d\n", b);
   
   /* calling a function to swap the values */
   swap(&a, &b);
   printf("After swap, value of a: %d\n", a);
   printf("After swap, value of b: %d\n", b);

   return 0;
}

Output

When you execute this code, it will produce the following output −

Before swap, value of a: 10
Before swap, value of b: 20
After swap, value of a: 20
After swap, value of b: 10

Passing an Array Pointer to a Function

In C programming, the name of an array acts the address of the first element of the array; in other words, it becomes a pointer to the array.

Example

In this example, we declare an uninitialized array in main() and pass its pointer to a function along with an integer.

Inside the function, the array is filled with the square, cube and square root of the passed integer. The function returns the pointer of this array, using which the values are accessed and printed in the main() function.

#include <stdio.h>
#include <math.h>

int arrfunction(int, float *);

int main(){

   int x = 100;
   float arr[3];

   arrfunction(x, arr);

   printf("Square of %d: %f\n", x, arr[0]);
   printf("Cube of %d: %f\n", x, arr[1]);
   printf("Square root of %d: %f\n", x, arr[2]);
   
   return 0;
}

int arrfunction(int x, float *arr){
   arr[0] = pow(x,2);
   arr[1] = pow(x, 3);
   arr[2] = pow(x, 0.5);
}

Output

When you run this code, it will produce the following output −

Square of 100: 10000.000000
cube of 100: 1000000.000000
Square root of 100: 10.000000

Passing String Pointers to a Function

Let us have a look at another example, where we will pass string pointers to a function.

Example

In this program, two strings are passed to the compare() function. A string in C is an array of char data type. We use the strlen() function to find the length of the string.

#include <stdio.h>

int compare(char *, char *);

int main(){

   char str1[] = "BAT";
   char str2[] = "BALL";
   int ret = compare(str1, str2);
   
   return 0;
}

int compare (char *x, char *y){

   int val;

   if (strlen(x) > strlen(y)){
      printf("Length of Str1 is greater than or equal to the length of Str2");
   }
   else{
      printf("Length of Str1 is less than the length of Str2");
   }
}

Output

When you run this code, it will produce the following output −

Length of Str1 is less than the length of Str2

Passing Struct Pointer to a Function

In C programming, a structure is a heterogenous data type containing elements of different data types. Let's see how we can pass a struct pointer to a function.

Example

In this example, a struct variable rectangle is declared in main() and its address is passed to a user-defined function called area(). When called, the area() function is able to use the elements of the variable with the indirection operator "→". It computes the result and assigns it to the area element r→area.

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

struct rectangle{
   float len, brd;
   double area;
};

int area(struct rectangle *);

int main(){

   struct rectangle s;
   printf("Input length and breadth of a rectangle");
   scanf("%f %f", &s.len, &s.brd);
   area(&s);

   return 0;

}

int area(struct rectangle *r){

   r->area = (double)(r->len*r->brd);
   printf("Length: %f \n Breadth: %f \n Area: %lf\n", r->len, r->brd, r->area);
   
   return 0;
}

Output

Run the code and check its output −

Input length and breadth of a rectangle
10.5 20.5
Length: 10.500000 
Breadth: 20.500000 
Area: 215.250000

The logical extension of the concept of passing a pointer to a function leads to passing a Union pointer, i.e., the pointer of a multi-dimensional array, passing the pointer of a self-referential structure, etc., all these have important uses in different application areas such as complex data structures, hardware control programming, etc.