Calloc providing a pointer that begins inside already allocated memory?

Question

I'm getting some kind of pointer collision,

Basically, in one function I do,

a = calloc(1,28); // gives me 0x100100d10

Then pretty soon in a subfunction I do,

b = calloc(1,16); // gives me 0x100100d20;

first address + 28 is 0x0..d2C, ie, extends over the pointer provided in the second calloc...

Whats going on here?

Pointer values are from printf, not gdb.

Can you boil this down to a *minimal*, *compilable* code snippet? — Oliver Charlesworth, May 12 '11 at 15:32
It's *extremely* unlikely that anything is wrong with calloc, so you probably need to show some more code. — James, May 12 '11 at 15:34
Exactly how are you printing the pointer values? Those addresses look suspect to me (and downright impossible if you're running on a 32-bit system). — Michael Burr, May 12 '11 at 17:20

score 4 · Answer 1 · answered May 12 '11 at 15:38

If this is really happening as you describe, then you have corrupted the heap by writing outside the bounds of an allocated block (or perhaps even by using an uninitialized pointer or pointer to already-freed memory), thus invoking undefined behavior. The tool valgrind can probably help you track the problem down, or if your program isn't too big, you can simply search by hand for invalid pointer usage.

Calloc providing a pointer that begins inside already allocated memory?

1 Answers1