memory exception using _aligned_malloc and _aligned_free

[m37h0d]

//i am allocating cache-aligned buffers using:
Matrix::Matrix(const unsigned int _rows, const unsigned int _columns):
//...
  data((double*)_aligned_malloc(sizeof(double)*(rows*columns+1),32))
//...
{}
//and freeing them like so:
Matrix::~Matrix()
{
    if(data)_aligned_free(data);
    data=0;
}

data is aligned on 0x20 boundaries, just like i wanted, but problem is, i'm getting a heap corruption exception inside _aligned_free at location 0x10 LESS than where my buffer starts.

what gives? am i doing something wrong or is this something evil about aligned malloc/free?

[Caligulaminus]

I'd say the error is somewhere else in your code.
Some out of bounds memory access probably.

[m37h0d]

don't think so. here's the stack trace:

>	msvcr100d.dll!_free_dbg_nolock(void * pUserData=0x031e6770, int nBlockUse=1)  Line 1376 + 0x3b bytes	C++
 	msvcr100d.dll!_free_dbg(void * pUserData=0x031e6770, int nBlockUse=1)  Line 1265 + 0xd bytes	C++
 	msvcr100d.dll!_aligned_free_dbg(void * memblock=0x031e6780)  Line 3129 + 0xd bytes	C++
 	msvcr100d.dll!_aligned_free(void * memblock=0x031e6780)  Line 3087 + 0x9 bytes	C++
 	Math.NET.dll!LS::Math::Native::Matrix::~Matrix()  Line 60 + 0x16 bytes	C++
 	Math.NET.dll!LS::Math::Native::Matrix::QRFactorize(LS::Math::Native::Matrix & Q={...}, LS::Math::Native::Matrix & R={...})  Line 535 + 0xf bytes	C++

[Caligulaminus]

Oh I do think so.
_aligned_free() (or rather _free_dbg_nolock()) only finds the problem and throws an exception. The actual memory corruption occurred somewhere else before.

[m37h0d]

yep you got it. i had ++ when i should have had --. odd the stack track took me to that location instead of where the out-of-bounds access was actually occurring...

thanks

[Caligulaminus]

Not odd.
If the memory right before/after your array belongs to your process you can write there unhindered(is that correct english?). My guess is the debug version of your runtime lib marks the area before/after your allocated chunk of memory with some magic number to achieve exactly what happened. Uncover hidden out of boundary writes.