[Qt-interest] SegFaults I cannot interpret

[Thiago Macieira]

Andrea Franceschini wrote:
>Clearly these are something that go far beyond my ability of
>inspection (and patience, since I'm working on strict time constraint
>and a single step-debug session would take an entire day and even
>wouldn't give any warranty of producing the crash at all) so if
>anybody of you guys can give me some insight, I'm totally lost.

When you're out of other options, you can always resort to reducing your 
code until you can no longer reproduce the problem. When you do that, 
you'll see which change exactly causes the problem.

You'll also have a much smaller codebase which you can send to mailing 
lists and ask for help.

Looking at your backtraces, I can only make guesses at what the problem 
is. I need to see more of your code and how it happens to be of more help.

The first backtrace has this=0x0, which indicates that the QList's d-
pointer is 0. That can NEVER happen with QList, since d is initialised at 
the constructor to a value different from 0 and never set to 0. That means 
you're accessing an object that hasn't been constructed yet.

The second backtrace has a valid pointer address (properly aligned, in the 
heap region on 32-bit Linux), but it's crashing trying to dereference the 
d-pointer. Again, the d-pointer on QList is never invalid -- it does 
replacements atomically, so at any given point in a QList's lifetime, the 
d-pointers are valid.

That assumes that you're not accessing the same QList from multiple 
threads: QList is reentrant, not thread-safe.

So, my guesses are:
1) Doodle::Painter::paint is accessing a QList object before it is 
initialised or after it was deleted (dangling pointer). Note 
Painter.cpp:95 is copying a list and it's the source that has a problem.

2) you've got a memory overrun somewhere and you're writing garbage to 
QList::d

I don't think this is a threading issue because of the this=0x0 case: 
since QList never does that, it can only be 0x0 for external reasons.

-- 
Thiago Macieira - thiago.macieira (AT) nokia.com
  Senior Product Manager - Nokia, Qt Software
      Sandakerveien 116, NO-0402 Oslo, Norway
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