Pointers and functions in C

Passing pointers to functions in C

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

Following is 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 the 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 the following result −

Average value is: 214.40000

Return pointer from functions in C

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We have seen in the last chapter how C programming allows to return an array from a function. Similarly, C also allows to return a pointer from a function. To do so, you would have to declare a function returning a pointer as in the following example −

int * myFunction() {
   .
   .
   .
}

Second point to remember is that, it is not a good idea to return the address of a local variable outside the function, so you would have to define the local variable as static variable.

Now, consider the following function which will generate 10 random numbers and return them using an array name which represents a pointer, i.e., address of first array element.

When the above code is compiled together and executed, it produces the following result −#include <stdio.h>
#include <time.h>
 
/* function to generate and return random numbers. */
int * getRandom( ) {

   static int  r[10];
   int i;
 
   /* set the seed */
   srand( (unsigned)time( NULL ) );
	
   for ( i = 0; i < 10; ++i) {
      r[i] = rand();
      printf("%d\n", r[i] );
   }
 
   return r;
}
 
/* main function to call above defined function */
int main () {

   /* a pointer to an int */
   int *p;
   int i;

   p = getRandom();
	
   for ( i = 0; i < 10; i++ ) {
      printf("*(p + [%d]) : %d\n", i, *(p + i) );
   }
 
   return 0;
}

1523198053
1187214107
1108300978
430494959
1421301276
930971084
123250484
106932140
1604461820
149169022
*(p + [0]) : 1523198053
*(p + [1]) : 1187214107
*(p + [2]) : 1108300978
*(p + [3]) : 430494959
*(p + [4]) : 1421301276
*(p + [5]) : 930971084
*(p + [6]) : 123250484
*(p + [7]) : 106932140
*(p + [8]) : 1604461820
*(p + [9]) : 149169022