Memory to be deallocated

Hi All,
I have written the following code.

Code:

#include <stdio.h> #include <stdlib.h> int main() { int *a, *b; a = (int*) malloc (sizeof(int) * 10); printf ("%d\n", a[-1]); b = (int*) malloc (sizeof(int) * 15); printf ("%d\n", b[-1]); return 0; }

I have written many mallocs and printed the '-1' Index of the each allocated memory. The output for the previous one comes as follows:

Code:

49 65

I have observed that it actually shows how much memory it has allocated.

Code:

For the 10(even) allocations, It allocates space for 10 Integers (4 * 10 = 40 bytes) + One 4 bytes space for -1 Index Itself + 1 (4 bytes) after the array = 40 + 4 + 4 = 48

And It adds 1 extra byte. (I don't know why?) Hence It prints = 49.

Code:

Similarly, for 15(odd) allocations, It allocates space for 15 Integers(4*15 = 60 bytes) + One 4 bytes space for -1 Index Itself = 60 + 4 = 64 bytes

And It adds 1 extra byte. (I don't know why?) Hence It prints = 65.

Why does It add an extra byte? This is my question.

Thanks and Best Regards,
Aakash Johari