Re: [Dev-C++] STL Map question

[Shane B. <ast...@te...>]

Actually, since I had about 20 responses on this topic, none of which
actually got close to the effect, nevermind the cause of the problem, I will
try and explain the situation in more detail.  However, to make the records
clear, I don't expect to find an answer now.  I have looked the code over a
number of times, and had another expert which is admittedly significantly
better with C++ and STL than myself, and even he cannot seem to find any
problems which relate.  Given old working code, and my new adjusted code,
many changes were made but I cannot determine what is causing the problem.

Now, the effect of the issue, is as follows. A network of 6,000-10,000
active users at any given time is regulated by a security service.  This
service runs and connects like a user, with special priviledges.  Anytime a
user connects to the network, the services is informed, and the User is
added to a global UserList.  The network also has smaller units, known as
Channels.  Each Channel contains 3 UserList's as well.  First, depicts all
users in the Channel, the second depicts all the operators of the channel,
and the third depicts the priviledged users who are not operators.  99% of
the time, the code work fine.  Users constantly added and removed from the
list.  Periodically, what seems like an issue without a pattern, the code
will crash with a segmentation fault while trying to call
"std::map<>::find()", with a valid key.  Whether the key is in the map, is
not definate, but the key passed in valid and should not have a problem
being searched.

This said, I have narrowed the issue down to one of 2 causes. 1) Either
User's are being added to lists, and not being removed, and the UserLists
are eventually gathering enough garbage that out of memory is being
encountered. The application has a signal hook for out of memory, and it is
not being raised.  So I am leaning more towards option 2) The User class is
being deleted, without being removed from the UserList.  However, I can find
no indication of this in the code.  The code is many thousands of lines, it
is not a small project by any means.  Providing the source for the issue,
would require the entire project as the issue is not specifically a syntax
issue, but rather a situational issue.  I do believe something has
invalidated a User class, which a UserList has a pointer to, and when trying
to use it, is causing a segmentation fault.  I'm sure most of the experts
here will agree with my conclusion and realize tracing this problem is no
easy task.

I've considered some commercial products like Purify, or some other memory
tracking products, but I have no experience with any, since I've never had
an issue I couldn't resolve purely with GDB.  Since the problem traces back
most likely to an invalid pointer reference, does anyone have suggestions on
how I could possibly detect where the pointer is being deleted?  I've
considered the idea of using smart pointers to track the issue down, but
again I'm not entirely certain of how to implement some code to track where
the User is being deleted without being removed from the channels they are
in... It could also potentially be an issue with overwritting memory, but
with how often it is caused in exactly the same way, it's unlikely to be
memory overbounding.
I noticed a location where the key being used was being changed (a
nickname), and in those cases, where the old code would just rename the
nickname, I have now remove from the map and then readd to the map. When it
tries to remove from the map is when it crashes.  I can only assume that the
problem is an invalid pointer, thus the map::find function is crashing
because of invalid memory.  The key was based on a const char *, but the
other expert I consulted suggested using std::string to help track the
problem until it is found... Changing to use std::string is how I determined
that the key was still valid (because it had a copy of the nick instead of a
pointer to the nick which may be invalidated at some point), and the key was
still intact as far as I can remember, the pointer however caused GDB itself
to indicate that the program crashed again from a call by GDB, when trying
to access a member of the pointer.


Okay, hopefully these details will clarify for all the people who tried to
help... The problem itself is not something so simple as was suggested so
far.  However, Hossein did make one point that I should have realized
myself.  Not that it matters, but I should use empty() instead of size() ==
0, since it's possible empty() can execute faster than size().  But speaking
from the ANSI compliance, size() == 0 is identical to empty() == true in
result, just not exection.  That said, it will not affect the problem.

    -Shane
----- Original Message ----- 
From: "Hossein Haeri" <ult...@em...>
To: <dev...@li...>
Sent: Friday, July 11, 2003 5:19 AM
Subject: RE: [Dev-C++] STL Map question


> I'm not sure if it helps, but just try (!map<>::empty()) instead of
> (map<>::size() > 0) to see what happens!
>
> Sorry if it seems a little off-topic. But that's because I still
> couldn't realise what had caused the bug. I'm trying to check if there
> is any bug in your C++ implementation's std::map<>. Maybe,
> map<>::size() does something causing pointer invalidation!
>
> Let me know the result!
>
> Regards,
> --Hossein
> -- 
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