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POC++, an "impossible" behavior
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  1. POC++, an "impossible" behavior
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    copied!<p>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".</p> <p>Well, it just happened to me :(</p> <p>Here AuthDb is <strong>NOT</strong> NULL but a valid pointer:</p> <pre><code>SingleResult sr(AuthDb, format("SELECT Id, Access, Flags, SessionKey, RealmSplitPreference FROM accounts WHERE Name = '%s'") % Escaped(account_name)); </code></pre> <p>Here it mysteriously becomes NULL:</p> <pre><code>struct SingleResult : public BaseResult { SingleResult(Database *db, const boost::format&amp; query) { _ExecuteQuery(db, query.str()); } } </code></pre> <p>Notice that it's the immediate next call. It can be explained much better with two screenshots: <a href="http://img187.imageshack.us/img187/5757/ss1zm.png" rel="nofollow noreferrer">http://img187.imageshack.us/img187/5757/ss1zm.png</a><br> <a href="http://img513.imageshack.us/img513/5610/ss2b.png" rel="nofollow noreferrer">http://img513.imageshack.us/img513/5610/ss2b.png</a> </p> <p>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.</p> <hr> <p>ASM code (unfortunately I don't even know how to read it :/)</p> <p><strong>Of the first screenshot</strong></p> <pre><code>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&lt;char,std::char_traits&lt;char&gt;,std::allocator&lt;char&gt; &gt;::basic_format&lt;char,std::char_traits&lt;char&gt;,std::allocator&lt;char&gt; &gt; (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&lt;char,std::char_traits&lt;char&gt;,std::allocator&lt;char&gt; &gt;::operator%&lt;Snow::Escaped&gt; (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&lt;char,std::char_traits&lt;char&gt;,std::allocator&lt;char&gt; &gt;::~basic_format&lt;char,std::char_traits&lt;char&gt;,std::allocator&lt;char&gt; &gt; (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); </code></pre> <p><strong>Second screenshot</strong></p> <pre><code>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&lt;char,std::char_traits&lt;char&gt;,std::allocator&lt;char&gt; &gt;::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&lt;char,std::char_traits&lt;char&gt;,std::allocator&lt;char&gt; &gt;::~basic_string&lt;char,std::char_traits&lt;char&gt;,std::allocator&lt;char&gt; &gt; (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 </code></pre> <hr> <p><strong>UPDATE</strong> Following the suggestion of peterchen, I added <code>ASSERT(AuthDb);</code> here:</p> <pre><code>ASSERT(AuthDb); SingleResult sr(AuthDb, format("SELECT Id, Access, Flags, SessionKey, RealmSplitPreference FROM accounts WHERE Name = '%s'") % Escaped(account_name)); </code></pre> <p>And it failed O.o And yet the debugger keeps insisting that it's not NULL.. It's not shadowed by a local</p> <p><strong>UPDATE2*</strong> <code>cout &lt;&lt; AuthDb;</code> is 0 in there even if the debugger says it's not</p> <hr> <p><strong>FOUND THE PROBLEM</strong></p> <p><code>Database *AuthDb = NULL, *GameDb = NULL;</code> in a .cpp</p> <p><code>extern thread Database *AuthDb, *GameDb;</code> in a .h</p> <p>The variable was marked thread (TLS - Thread local storage) in the header, but not TLS in the definition...</p> <p>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 :)</p> <p>Thanks to everyone who helped, I was really desperate!</p>
 

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