Re: variable placement in structures
omnia neo <neo.omnia7@gmail.com> wrote:
For now I solved the issue this way(i still dont know the problem):
I replaced the placement of structure A in B as folows:
previous:
struct A
{
<other structures..its a huge list>
}
strcut B
{
struct A;
<other structures .. its a huge list again>
}
modified to:
struct A
{
<other structures..its a huge list>
}
strcut B
{
<other structures .. its a huge list again>
struct A;
}
Its more of a hack .. so im still looking for a permenant solution.
Again, the evidence points to a memory overwrite. An explanation may be
in order:
In memory an object of type B used to look like this:
|-------- struct B ---------|
| struct A part | rest of B |
You were having "strange behavior" in your program because some part of
the program was incorrectly writing to the beginning of a struct B
object and changing the struct A part then some other part of the
program was reading the struct A part of that B object and failing
because of the incorrect data inside the A part of the B object. By
moving the A part to the end of the B struct, you did not fix your
problem, you merely changed the which part of your B object that is
being stomped on.
Try this, it might help:
struct B {
char buffer[32];
struct A a;
// other structures in your huge list
};
The above will probably also seem to "fix" your problem, but it doesn't
really. Now, everywhere a B object is constructed, fill the buffer with
some value, like this:
memset(buffer, 0xCD, 32);
Everywhere that a pointer to a B object is dereferenced, check that the
buffer is unchanged, like this:
assert(find_if(buffer, buffer + 32, ¬CD) == buffer + 32);
where notCD is defined as:
bool notCD(char c) {
return c != 0xCD;
}
You will find that at some point, the buffer was changed even though
nothing in your program uses it. Then all you have to do is figure out
what changed that buffer. To do that check if your debugger can set up
watch points that will break if a particular block of memory changes,
and hope that the code added by the debugger to do this, doesn't move
the bug.