A better malloc() example

The Motivation

• We want to have an array called values known in main( )
• We want to use dynamic memory allocation to get the memory needed to hold the elements of the array.
• We want to write a function called GetMemory( ) that will do the dynamic allocation to get that code out of main( ).

The Details

• Notice that we declare a int* called values in main( ), but it holds garbage originally.
• We would like the function GetMemory( ) to modify the variable, values, to hold the beginning address of the block of memory that is being dynamically allocated within the function.
• Since we would like the function to modify this variable, we will have to pass the address of values to the function, so that the function can modify it by derefencing that pointer.

Modification of a variable by a function requires that it be passed by reference
Remember that when we wanted to modify an integer variable, we passed the address of that variable into a function and the type of the function's parameter that accepts it is int*, or a pointer to an int.

Similarly if we want to modify a variable which is a pointer to an int, we will pass the address of the variable into the function and the type of the function's parameter that accepts it will be int**, or a pointer to a pointer to int.

The Code Example

/*********************************
 ** File malloc2.c
 ** Author: S. Bogar
 ** Date: 1/3/95
 ** Modified: 9/26/05
 ** Section: 101
 ** E-Mail: bogar@cs.umbc.edu
 **
 ** This file demonstrates passing a pointer
 ** by reference to a function
 ** A pointer to a pointer
*******************************/

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

/* function prototype */
void GetMemory (int count, int **arrayPtr);

int main()
{
   int i, *values, nrElements ;
   int powerOf2 ;


   printf("Enter number of elements in integer values: ") ;
   scanf("%d", &nrElements) ;

   /* get a block of memory for 'nrElements' integers
   ** since 'values' is a pointer to int
   ** '&values' is the address of (pointer to) a pointer
   ** to int so that 'values' can be modified in the 
   ** function
   */
   GetMemory (nrElements, &values);

   /* Lets use the memory as an arrray */
   powerOf2 = 1 ;
   for (i = 0; i < nrElements; i++)
   {
      values[i] = powerOf2 ;
      powerOf2 = 2 * powerOf2 ;
   }

   /* Print out contents of the array */
   for (i = 0; i < nrElements; i++)
   {
      printf("values[%d] = %d\n", i, values[i]) ;
   }

   /* Give up use of the memory block */
   free(values) ;

   return 0;
}

/***************
 ** Function: GetMemory
 **
 ** GetMemory() gets the memory space needed and modifies the pointer
 ** that will point to it in the calling function using call by reference
 **
 ** Inputs: Number of integers for which to allocate 
 **         space
 **         Pointer to the pointer that will point to 
 **         the memory
 ** Output: The pointer to the memory is set by this function
 **        there is no return value
 ********************************/

void GetMemory (int count, int **arrayPtr)
{
   /* malloc the memory and set the caller's pointer
   ** to point to it
   */
   *arrayPtr = (int *) malloc( count * sizeof(int) ) ;
   if (*arrayPtr == NULL)
   {
      fprintf(stderr, "Oops, we're out of memory\n") ;
      exit(-1);
   }
}   

output