C dynamic arrays

simple.c

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

struct S {
	int a;
};

int main()
{
	struct S *arr=(struct S *) malloc(2*sizeof(struct S));
	unsigned int s_size=0;

	arr[0].a=10;
	arr[1].a=20;

	printf("array size:%d\n",s_size);
	printf("arr[0]=%d\n",arr[0].a);
	printf("arr[1]=%d\n",arr[1].a);

	free(arr);

	return 0;
}

with-constructor.c

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

struct S {
	int a;
};

void add_one_element(struct S **arr, int *count, const int value)
{
	struct S *temp_val;

	if (*count==0) {
		temp_val = malloc(sizeof(struct S));
	} else {
		temp_val = (struct S *) realloc(*arr,(1+*count) * sizeof(struct S *));
	}

	if (temp_val==NULL) {
		perror("malloc");
		abort;
	}
	(*(temp_val+*count)).a = value;
	*arr = temp_val;

	(*count)++;
}

int main()
{
	struct S *arr;
	unsigned int arr_size=0;

	printf("size of struct S   :%d\n",sizeof(struct S));
	printf("size of struct S * :%d\n",sizeof(struct S *));

	assert(arr_size==0);
	add_one_element(&arr,&arr_size, 123);
	assert(arr_size==1);
	add_one_element(&arr,&arr_size, 456);
	assert(arr_size==2);
	add_one_element(&arr,&arr_size, 789);
	assert(arr_size==3);
	add_one_element(&arr,&arr_size, 1010);
	assert(arr_size==4);

	assert(arr[0].a==123);
	assert(arr[1].a==456);
	assert(arr[2].a==789);
	assert(arr[3].a==1010);

	free(arr);

	return 0;
}