unsigned char alloc和free issue

I'm confused about one strange thing....I have an unsigned char array.... I allocate it using calloc and record some bytes data in it... but when I free this unsigned char and allocate it again, I see that it reserves the same address in memory which was allocated previous time. I understand why....But I cannot understand why the data that I'm trying to write there second time is not written...There is written the data that was written first time....Can anybody explain me this???????I'm confused about one strange thing....I have