Thread: Define the size of an integer type dynamic array

Hi guys,
I would like to define the count of an integer array but I got stuck in that. I increased the size of the array dynamically using the possibilities of 'realloc' method.

May I mess up the number of the array with these reallocation ? Or is it an incorrect usage of 'realloc' ?
Any help would be appreciated

You can see my simplified code:

Code:

#include<stdio.h>
int main()
{
    char myString[16]="cows_cats_chicks"; 
    int c_LetterCounter = 0;     
    int* c_LetterPosArray = (int*)malloc(0);  // the array which stores the positions of the 'c' letters
    
    int index=0;
    for (index=0;index<strlen(myString); index++)
    {
        if (myString[index] == 'c')
        {
            c_LetterCounter++;
            c_LetterPosArray = realloc(c_LetterPosArray, c_LetterCounter*sizeof(int));
            c_LetterPosArray[c_LetterCounter-1] = index;
            printf("The current index of the 'c' letter: %d\n",  c_LetterPosArray[c_LetterCounter-1]);
        }
    }
    
    // just an example, the array isn't empty
    printf("The 2nd position of 'c' letter in the 'cows_cats_chicks' string: %d\n", c_LetterPosArray[1]);
    
    int len = sizeof(c_LetterPosArray) / sizeof(int);
    printf("The size of the integer array: %d\n", len);
    
    int len2 = sizeof(c_LetterPosArray) / sizeof(c_LetterPosArray[0]);
    printf("The size of the integer array with different calculation: %d\n", len2);
    
    getchar();
}

But that simple array size definition work very well in static arrays:

Code:

int arr[] ={1,2,3,4,3,3,3};
int len = sizeof(arr) / sizeof(int);

Very thanks for your help!

One tip abour realloc... Never do things like this:

Code:

c_LetterPosArray = realloc(c_LetterPosArray, c_LetterCounter*sizeof(int));

if realloc fails it will return NULL, but the original buffer won't be deallocated and you'll get a memory leakage. The secure approach should be something like this:

Code:

p = realloc( c_LettterPosArray, c_LetterCounter * sizeof(int) );
if ( p == NULL )
{
  // error handler...
  free( c_LetterPosArray );
  exit( 1 ); 
}
c_LetterPosArray = p;

Hi guys,
thank you very much for these useful information! Sizeof method doesn't work on pointers. Besides of that thanks for the correction with regards to the safe using of the memory reallocation. I included your suggestions in my code, and i will use that error handling in the future.
Thanks again!

Code:

#include<stdio.h>
int main()
{
    char myString[16]="cows_cats_chicks"; 
    int c_LetterCounter = 0;     
    int* c_LetterPosArray = NULL;  // the array which stores the positions of the 'c' letters
    int* p = NULL; // temporary pointer ensuring the security
     
    int index=0;
    for (index=0;index<strlen(myString); index++)
    {
        if (myString[index] == 'c')
        {
            c_LetterCounter++;
            p = realloc(c_LetterPosArray, c_LetterCounter*sizeof(int));
            if (p== NULL)
            {
            	// simple error handling
				printf("Memory allocation fails!\n");
				free( c_LetterPosArray );
  				return 0;
			}
			c_LetterPosArray = p;						
            c_LetterPosArray[c_LetterCounter-1] = index;
            printf("The current index of the 'c' letter: %d\n",  c_LetterPosArray[c_LetterCounter-1]);
        }
    }     
    getchar();
}

Dear laserlight,
very thanks for the correction.
The importance of the usage of the terminating zero ( '\0' character), at the char array declaration, is engraving in my mind.