Malloc and Realloc array of structs

Question

I am defining a type of structures

typedef struct structs{
int freeSpace;
} structs;   

I am creating a pointer to a structure

structs* arrayOfStructs;

I am allocating memory for array of size 5 of that structure

arrayOfStructs = malloc(5 * sizeof (arrayOfStructs));
arrayOfStructs[3].freeSpace = 99;
printf("\n%d", arrayOfStructs[3].freeSpace);

At some point I am reallocating that memory to array of size 10

arrayOfStructs = realloc(arrayOfStructs, 10 * sizeof (arrayOfStructs));
arrayOfStructs[8].freeSpace = 9;
printf("\n%d", arrayOfStructs[8].freeSpace);

And here I am setting freespace 17 at position 13 of the array which I expect to have only 10 positions.

arrayOfStructs[13].freeSpace = 17;
printf("\n%d", arrayOfStructs[13].freeSpace);
free(arrayOfStructs);

Why is this working ? What am I doing wrong ?

Answer 1

The behaviour of the program is undefined .

You are accessing memory that doesn't belong to you.

(What could well be happening is that realloc is obtaining more memory from the operating system than is actually required).