C++, an "impossible" behavior - c++

Question

If you've been programming for a while then you probably noticed something completely impossible occurs every now and then for which you are convinced there is no possible explanation ("IT'S A COMPILER BUG!!"). After you find out what it was caused by though you are like "oooohhh".
Well, it just happened to me :(
Here AuthDb is NOT NULL but a valid pointer:
SingleResult sr(AuthDb, format("SELECT Id, Access, Flags, SessionKey, RealmSplitPreference FROM accounts WHERE Name = '%s'") % Escaped(account_name));
Here it mysteriously becomes NULL:
struct SingleResult : public BaseResult
{
SingleResult(Database *db, const boost::format& query) { _ExecuteQuery(db, query.str()); }
}
Notice that it's the immediate next call. It can be explained much better with two screenshots:
http://img187.imageshack.us/img187/5757/ss1zm.png
http://img513.imageshack.us/img513/5610/ss2b.png
EDIT: AuthDb is a global variable. It itself keeps pointing to the right thing; but the copy of the ptr Database *db points at NULL.
ASM code (unfortunately I don't even know how to read it :/)
Of the first screenshot
01214E06 mov eax,dword ptr [ebp-328h]
01214E0C push eax
01214E0D push offset string "SELECT Id, Access, Flags, Sessio"... (13C6278h)
01214E12 lea ecx,[ebp-150h]
01214E18 call boost::basic_format<char,std::char_traits<char>,std::allocator<char> >::basic_format<char,std::char_traits<char>,std::allocator<char> > (11A3260h)
01214E1D mov dword ptr [ebp-32Ch],eax
01214E23 mov ecx,dword ptr [ebp-32Ch]
01214E29 mov dword ptr [ebp-330h],ecx
01214E2F mov byte ptr [ebp-4],2
01214E33 mov ecx,dword ptr [ebp-330h]
01214E39 call boost::basic_format<char,std::char_traits<char>,std::allocator<char> >::operator%<Snow::Escaped> (11A3E18h)
01214E3E push eax
01214E3F mov edx,dword ptr [__tls_index (144EC40h)]
01214E45 mov eax,dword ptr fs:[0000002Ch]
01214E4B mov ecx,dword ptr [eax+edx*4]
01214E4E mov edx,dword ptr [ecx+12A3Ch]
01214E54 push edx
01214E55 lea ecx,[sr]
01214E58 call Snow::SingleResult::SingleResult (11A27D4h)
01214E5D mov byte ptr [ebp-4],4 // VS GREEN ARROW IS HERE
01214E61 lea ecx,[ebp-150h]
01214E67 call boost::basic_format<char,std::char_traits<char>,std::allocator<char> >::~basic_format<char,std::char_traits<char>,std::allocator<char> > (11A1DBBh)
01214E6C mov byte ptr [ebp-4],5
01214E70 lea ecx,[ebp-170h]
01214E76 call Snow::Escaped::~Escaped (11A42D2h)
const bool account_found = !sr.Error();
01214E7B lea ecx,[sr]
01214E7E call Snow::BaseResult::Error (11A2964h)
01214E83 movzx eax,al
01214E86 test eax,eax
01214E88 sete cl
01214E8B mov byte ptr [account_found],cl
if (!account_found) {
01214E8E movzx edx,byte ptr [account_found]
01214E92 test edx,edx
01214E94 jne AuthSession+1C0h (1214F10h)
client.Kill(format("%s: Attempted to login with non existant account `%s'") % client % account_name, true);
Second screenshot
011A8E7D mov dword ptr [ebp-10h],ecx
011A8E80 mov ecx,dword ptr [this]
011A8E83 call Snow::BaseResult::BaseResult (11A31D9h)
011A8E88 mov dword ptr [ebp-4],0
011A8E8F lea eax,[ebp-30h]
011A8E92 push eax
011A8E93 mov ecx,dword ptr [query]
011A8E96 call boost::basic_format<char,std::char_traits<char>,std::allocator<char> >::str (11A1E01h)
011A8E9B mov dword ptr [ebp-34h],eax
011A8E9E mov ecx,dword ptr [ebp-34h]
011A8EA1 mov dword ptr [ebp-38h],ecx
011A8EA4 mov byte ptr [ebp-4],1
011A8EA8 mov edx,dword ptr [ebp-38h]
011A8EAB push edx
011A8EAC mov eax,dword ptr [db]
011A8EAF push eax
011A8EB0 mov ecx,dword ptr [this]
011A8EB3 call Snow::SingleResult::_ExecuteQuery (124F380h)
011A8EB8 mov byte ptr [ebp-4],0 // VS GREEN ARROW HERE
011A8EBC lea ecx,[ebp-30h]
011A8EBF call std::basic_string<char,std::char_traits<char>,std::allocator<char> >::~basic_string<char,std::char_traits<char>,std::allocator<char> > (11A2C02h)
011A8EC4 mov dword ptr [ebp-4],0FFFFFFFFh
011A8ECB mov eax,dword ptr [this]
011A8ECE mov ecx,dword ptr [ebp-0Ch]
011A8ED1 mov dword ptr fs:[0],ecx
011A8ED8 pop edi
011A8ED9 add esp,38h
011A8EDC cmp ebp,esp
011A8EDE call _RTC_CheckEsp (12B4450h)
011A8EE3 mov esp,ebp
011A8EE5 pop ebp
011A8EE6 ret 8
UPDATE
Following the suggestion of peterchen, I added ASSERT(AuthDb); here:
ASSERT(AuthDb);
SingleResult sr(AuthDb, format("SELECT Id, Access, Flags, SessionKey, RealmSplitPreference FROM accounts WHERE Name = '%s'") % Escaped(account_name));
And it failed O.o And yet the debugger keeps insisting that it's not NULL.. It's not shadowed by a local
UPDATE2*
cout << AuthDb; is 0 in there even if the debugger says it's not
FOUND THE PROBLEM
Database *AuthDb = NULL, *GameDb = NULL; in a .cpp
extern thread Database *AuthDb, *GameDb; in a .h
The variable was marked thread (TLS - Thread local storage) in the header, but not TLS in the definition...
Countless hours wasted on this super stupid mistake, no warnings or hints or anything from the compiler that I feel like killing right now. :( Oh well, like I said for each impossible behavior there is a solution that once known seems obvious :)
Thanks to everyone who helped, I was really desperate!

Answer 1

Is AuthDB a thread-local variable?
Maybe the debugger isn't handling it correctly. What if you ASSERT(AuthDB) before the constructor is called?
UPDATE: If it is thread-local, it simply hasn't been initialized in this thread.

Answer 2

One possibility is that in the second screenshot, you have the debugger stopped at the very beginning of the function, before the stack has been manipulated, and so the variable locations aren't correct. You might also be after the end of the function, where the stack has already been torn down. I've seen that sort of thing before.
Expand that function to be a multiline function, so that it looks like this:
struct SingleResult : public BaseResult
{
SingleResult(Database *db, const boost::format& query)
{
_ExecuteQuery(db, query.str());
}
}
... and see if it still shows db as null when you have the debugger stopped on the _ExecuteQuery line.

Answer 3

Do you have a local AuthDB that is null and hides your global one?
(I'd expect the debugger in this case to correctly show you the local one... but with VS quality decay, I'd not rely on that)
I'd change the code to:
_ASSERTE(AuthDb);
SingleResult sr(AuthDb, format(...));
....
struct SingleResult : public BaseResult
{
SingleResult(Database *db, const boost::format& query)
{
_ASSERTE(db);
_ExecuteQuery(db, query.str());
}
}
And follow the disassembly in the debugger.

Answer 4

Well I'm not sure what classes/functions you're using here, but from a quick glance, shouldn't it be:
SingleResult sr(AuthDb, format("SELECT Id, Access, Flags, SessionKey, RealmSplitPreference FROM accounts WHERE Name = '%s'", Escaped(account_name)));
instead of:
SingleResult sr(AuthDb, format("SELECT Id, Access, Flags, SessionKey, RealmSplitPreference FROM accounts WHERE Name = '%s'") % Escaped(account_name));
It seems to me you're putting a modulus of the of Escaped(account_name) rather than passing that as an argument to format. However, I could just be confused.
Mike

Answer 5

I have no guess what is going on, but if I were debugging this I would be looking at the assembly language to see what is happening. You may need to get a better understanding of your platforms calling convention (i.e. how are arguments passed, on the stack, in registers, etc.) to troubleshoot this.

Answer 6

There is likely a bug somewhere else in your program. I suggest you find the problem by looking elsewhere in your code.

Answer 7

Another possibility--you might be looking at a memory overwrite due to a wild pointer somewhere.
Assuming your debugger supports it when you hit the first line set a memory-write breakpoint on AuthDb.