Passing pointers to functions in C

C programming language allows you to pass a pointer to a function. To do so, simply declare the function parameter as a pointer type.

Following a simple example where we pass an unsigned long pointer to a function and change the value inside the function which reflects back in the calling function:

#include <stdio.h>
#include <time.h>
 
void getSeconds(unsigned long *par);

int main ()
{
   unsigned long sec;


   getSeconds( &sec );

   /* print the actual value */
   printf("Number of seconds: %ld\n", sec );

   return 0;
}

void getSeconds(unsigned long *par)
{
   /* get the current number of seconds */
   *par = time( NULL );
   return;
}

When the above code is compiled and executed, it produces following result:

Number of seconds :1294450468

The function which can accept a pointer, can also accept an array as shown in the following example:

#include <stdio.h>
 
/* function declaration */
double getAverage(int *arr, int size);
 
int main ()
{
   /* an int array with 5 elements */
   int balance[5] = {1000, 2, 3, 17, 50};
   double avg;
 
   /* pass pointer to the array as an argument */
   avg = getAverage( balance, 5 ) ;
 
   /* output the returned value  */
   printf("Average value is: %f\n", avg );
    
   return 0;
}

double getAverage(int *arr, int size)
{
  int    i, sum = 0;       
  double avg;          
 
  for (i = 0; i < size; ++i)
  {
    sum += arr[i];
  }
 
  avg = (double)sum / size;
 
  return avg;
}

When the above code is compiled together and executed, it produces following result:

Average value is: 214.40000