A better malloc() example
In this example, we want to have an array called values known in main(),
and we would like to use dynamic memory allocation to get the memory needed
to hold the elements of the array. Further, we would like to write a function
called GetMemory that will do the dynamic allocation to get that code out of
main().
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.
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.
<![CDATA[
/*********************************
** File malloc2.c
** Author: S. Bogar
** Date: 1/3/95
** Section: 101
** SSN: 123-45-6789
** 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 **array);
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
** 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
** there is no return value
********************************/
void GetMemory (int count, int **array)
{
/* malloc the memory and set the caller's pointer
** to point to it
*/
*array = (int *) malloc( count * sizeof(int) ) ;
if (*array == NULL)
{
printf("Oops, we're out of memory\n") ;
exit(-1);
}
}
]]>
output
<![CDATA[
linux1[89] % a.out
Enter number of elements in integer values: 10
values[0] = 1
values[1] = 2
values[2] = 4
values[3] = 8
values[4] = 16
values[5] = 32
values[6] = 64
values[7] = 128
values[8] = 256
values[9] = 512
linux1[90] % a.out
Enter number of elements in integer values: 20
values[0] = 1
values[1] = 2
values[2] = 4
values[3] = 8
values[4] = 16
values[5] = 32
values[6] = 64
values[7] = 128
values[8] = 256
values[9] = 512
values[10] = 1024
values[11] = 2048
values[12] = 4096
values[13] = 8192
values[14] = 16384
values[15] = 32768
values[16] = 65536
values[17] = 131072
values[18] = 262144
values[19] = 524288
linux1[91] % a.out
Enter number of elements in integer values: 88888888888888
Oops, we're out of memory
linux1[92] %
]]>
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