dynamic arrays in c

array.c

#include "array.h"
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>

struct ArrayData *initArray() {
	struct ArrayData *newArray = malloc(sizeof(struct ArrayData));
	newArray->pointer = calloc(1000, sizeof(int));
	newArray->size = 1000;
	newArray->counter = 0;
	return newArray;
}

void resizeArray(struct ArrayData* array) {
	int newSize = (array->size * sizeof(int)) * 2;
	array->pointer = realloc(array->pointer, newSize);
	fflush (stdout);
	array->size *= 2;  // This is the number of elements, don't multiply by sizeof
}

int addElement(struct ArrayData *array, int number) {
	if (array->counter >= array->size) {
		resizeArray(array);
	}

	*(array->pointer + array->counter) = number;  // Pointer arithmetic
	array->counter += 1;

	return 0;
}

int getElement(struct ArrayData *array, int index) {
	if (array->counter >= array->size) {
		return -1;
	}

	int *data = array->pointer + index;
	return *data;
}

array.h

#ifndef ARRAY_H_
#define ARRAY_H_

struct ArrayData *initArray();
int addElement(struct ArrayData *array, int number);
int getElement(struct ArrayData *array, int index);

struct ArrayData {
  int *pointer;
  int counter;
	int size;
};

#endif /* ARRAY_H_ */

main.c

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

int main() {
	struct ArrayData *array;
	array = initArray();

	int get;

	int i;
	for (i = 0; i < 5000; i++) {
		addElement(array, rand() % 50000);
		get = getElement(array, i);
  		printf("%d\n", get);
	}

	free(array->pointer);
	free(array);
	return 0;
}

It is not recommended to over the existing pointer with the return from realloc. Assign the return value of realloc to a temporary variable so that if realloc fails you still have the old pointer (and can free the memory already allocated).

In your code if realloc fails and returns NULL the program will never be able to free the memory already allocated.