Re: Access violation with heap memory

From:

"Scott McPhillips [MVP]" <org-dot-mvps-at-scottmcp>

Newsgroups:

microsoft.public.vc.mfc

Date:

Fri, 23 Jun 2006 11:18:38 -0400

Message-ID:

<e5fz0htlGHA.1208@TK2MSFTNGP02.phx.gbl>

Adeel wrote:

Hi,

I'm getting a runtime access violation using heap memory that was
allocated in another function. The details follow...

I have a setup in which I pass a pointer to a secondary function (in
another dll) which allocates a buffer dynamically, populates it and
returns. Now, when I try to access the buffer in the main function, I
get a runtime access violation message. The code below will help
clarify this...

<CODE>
void fillBuf (char *p)
{
    p = new char [10];
    //fill the buffer
}

void useBuf ()
{
    char *x = NULL;

    fillBuf(x);

    // access populated buffer
    // e.g. char z = x[3]; //<< ACCESS VIOLATION

    delete x;
}
</CODE>

I understand that this is probably bad design... 'new' and 'delete'
should happen in the same scope but the problem is I don't know the
size of the buffer beforehand, so can't allocate it before the calling
the secondary function.

Now you might say I break the call down to two steps... use the first
to fetch the size, use the size to allocate the buffer, then issue the
second call to get it populated. But is there a way to do this in one
call?

Or alternatively, how would you design a solution to such a scenario?

I can think of passing a reference to a CArray object accross to the
secondary function... but for some reason, I'm not entirely
comfortable with that... because later, there might be threads
involved in my program and I don't want to run into complications...
better stick with primitive types...

This problem is typically caused by static linking the C runtime
library. That gives you two memory managers, one in the exe and another
in the DLL, and allocating from one and deleting in the other won't
work. But if all modules link to the DLL version of the C runtime they
share a single memory manger and the problem is solved. The other
option is to add a function in the DLL that does the delete.

The same situation will occur if you pass a dynamic object such as a
CArray. But the CArray is no more or less prone to multithreading
issues: You have to take the same precautions with primitive types or
smart types.

--
Scott McPhillips [VC++ MVP]