Access Violation using simple Indy server code - c++

Question

In the following code, I have a simple server that sends a message to all clients 2 times a second, and another message 8-10 times a minute.
The problem is I am getting an error at runtime:
access violation at 00479740 read address FFFFFFD0
But only in a few systems, and only 1 or 2 times a day. This software works about 10 hours a day.
I have tried to use similar code with the ICS library and is seems to work well.
What's wrong in this code? Is there a better way to code it?
void __fastcall TDataNet::DataModuleCreate(TObject *Sender)
{
listaClient= new TThreadList();
psTx= new TStringList();
psRx= new TStringList();
}
void __fastcall TDataNet::DataModuleDestroy(TObject *Sender)
{
IdTCPServer1->Active= false;
listaClient->Free();
delete psTx;
delete psRx;
}
void __fastcall TDataNet::Send( TStrings *ps, TIdContext *AContext)
{
TList *lista;
static int cntSend= 0;
try
{
lista= listaClient->LockList();
if( AContext != NULL )
{
AContext->Connection->IOHandler->Write( ps, true, TIdTextEncoding_UTF8);
}
else
{
for( int i=0; i < lista->Count; i++ )
((TDatiClient*)lista->Items[i])->pThread->Connection->IOHandler->Write( ps, true, TIdTextEncoding_UTF8);
}
}
__finally
{
listaClient->UnlockList();
}
}
void __fastcall TDataNet::SetCambioPilota( void)
{
unsigned short hh, mm, ss, ms, hh1, mm1, ss1, ms1;
unsigned short hh2, mm2, ss2, ms2, hh3, mm3, ss3, ms3;
unsigned short hh4, mm4, ss4, ms4, dd4;
unsigned short hh5, mm5, ss5, ms5, dd5;
TStrings *ps;
UnicodeString s;
try
{
ps= psTx;
ps->Clear();
s= "<CAMBIO_PILOTA>";
ps->Add( s);
for( int i=0; i < MAX_PILOTI; i++ )
{
s.sprintf( L"<Pilota%02x= I%x,\"A%s\",\"C%s\",\"F%s\",f%x>",
i+1, gara.pilota[i].idnome,
gara.pilota[i].nome.c_str(), gara.pilota[i].nick.c_str(),
gara.pilota[i].nomeTeam.c_str(), gara.pilota[i].idPilotaT );
ps->Add( s);
}
s= "<END_CAMBIO_PILOTA>";
ps->Add( s);
Send( ps );
}
catch(...){}
}
void __fastcall TDataNet::SetDatiGara( void)
{
TStrings *ps;
UnicodeString s;
try
{
ps= psTx;
ps->Clear();
s= "<DATI_GARA>";
ps->Add( s);
s.sprintf( L"<eve=%d,A%x,B%x,C%x,D%x,E%x,F%x,G%x,H%x,I%x,J%x,K%x>", DataB->GetEventoInCorso().idEvento,
DataB->GetEventoInCorso().numEvento, DataB->GetEventoInCorso().subEvento,
DataB->GetNextEvento().idEvento, DataB->GetNextEvento().numEvento, DataB->GetNextEvento().subEvento,
gara.tkTempo, gara.tkDurata - gara.tkTempo,
gara.laps, gara.gDurata > 0 ? (gara.gDurata - gara.laps):0, gara.flInCorso ? (gara.gDurata > 0 ? 2:1):0,
gara.flFineGara );
ps->Add( s);
s= "<END_DATI_GARA>";
ps->Add( s);
Send( ps );
}
catch(...){}
}
void __fastcall TDataNet::Timer1Timer(TObject *Sender)
{
Timer1->Enabled= false;
SetDatiGara();
Timer1->Enabled= true;
}
void __fastcall TDataNet::IdTCPServer1Connect(TIdContext *AContext)
{
TDatiClient* dati;
dati= new TDatiClient;
dati->pThread= AContext;
AContext->Connection->IOHandler->ReadTimeout= 200;
AContext->Data= (TObject*)dati;
try
{
TList* lista;
lista= listaClient->LockList();
lista->Add( dati);
connessioni= lista->Count;
if( FmainWnd )
PostMessage( FmainWnd, WM_EVENTO_TCP, ID_CONNESSO, lista->Count);
int idEvento= DataB->GetEventoInCorso().idEvento;
if( idEvento )
SetCambioStato( idEvento, STATO_EVENTO_START, AContext);
}
__finally
{
listaClient->UnlockList();
}
}
void __fastcall TDataNet::IdTCPServer1Disconnect(TIdContext *AContext)
{
TDatiClient* dati;
dati= (TDatiClient*)AContext->Data;
AContext->Data= NULL;
try
{
listaClient->Remove( dati);
TList* lista;
lista= listaClient->LockList();
connessioni= lista->Count;
if( FmainWnd )
PostMessage( FmainWnd, WM_EVENTO_TCP, ID_DISCONNESSO, lista->Count);
}
__finally
{
listaClient->UnlockList();
}
delete dati;
}
void __fastcall TDataNet::IdTCPServer1Execute(TIdContext *AContext)
{
Sleep( 100);
try
{
AContext->Connection->IOHandler->ReadStrings( psRx, -1);
if( psRx->Count >= 2 && psRx->Strings[0] == "<LAST_MINUTE>" && psRx->Strings[psRx->Count-1] == "<END_LAST_MINUTE>" )
{
psRx->Delete(0);
psRx->Delete(psRx->Count-1);
if( FmainWnd )
SendMessage( FmainWnd, WM_EVENTO_TCP, ID_LAST_MINUTE, (unsigned int)psRx);
}
psRx->Clear();
}
catch( ...) {}
AContext->Connection->CheckForGracefulDisconnect();
}

Answer 1

The error message means you are accessing something that is -48 bytes offset from a NULL pointer. I see all kinds of problems with this code, the least of which is that you are accessing things in a thread-unsafe manner, so you have race conditions that could corrupt memory, amongst other possible issues. For example, your OnExecute event handler is not protecting the psRx object from concurrent access, so multiple clients could populate it with data at the exact same time, corrupting its contents.
TIdTCPServer is a multi-threaded component, its events are fired in the context of worker threads, not the main UI thread, so your event handlers MUST use thread-safe coding.
Besides, what you are doing is not the safest way to handle asynchronous communications with TIdTCPServer anyway. I would suggest something more like the following instead:
class TDatiClient : public TIdServerContext
{
public:
TIdThreadSafeObjectList *Queue;
bool QueueHasObjects;
__fastcall TDatiClient(TIdTCPConnection *AConnection, TIdYarn *AYarn, TThreadList* AList = NULL)
: TIdServerContext(AConnection, AYarn, AList)
{
Queue = new TIdThreadSafeObjectList;
}
__fastcall ~TDatiClient()
{
delete Queue;
}
void __fastcall Send(TStrings *ps)
{
TStringList *toSend = new TStringList;
try
{
toSend->Assign(ps);
TList *lista = Queue->LockList();
try
{
lista->Add(toSend);
QueueHasObjects = true;
}
__finally
{
Queue->UnlockList();
}
}
catch (const Exception &)
{
delete toSend;
}
}
};
void __fastcall TDataNet::TDataNet(TComponent *Owner)
: TDataModule(Owner)
{
// this must be set before you activate the server...
IdTCPServer1->ContextClass = __classid(TDatiClient);
// do this at runtime instead of design-time so
// ContextClass can be set first...
IdTCPServer1->Active = true;
}
void __fastcall TDataNet::~TDataNet()
{
IdTCPServer1->Active = false;
}
void __fastcall TDataNet::Send(TStrings *ps, TIdContext *AContext)
{
static int cntSend = 0;
TList *lista = IdTCPServer1->Contexts->LockList();
try
{
if (AContext)
{
// make sure the client is still in the list...
if (lista->IndexOf(AContext) != -1)
static_cast<TDatiClient*>(AContext)->Send(ps);
}
else
{
for (int i = 0; i < lista->Count; ++i)
static_cast<TDatiClient*>(static_cast<TIdContext*>(lista->Items[i]))->Send(ps);
}
}
__finally
{
IdTCPServer1->Contexts->UnlockList();
}
}
void __fastcall TDataNet::SetCambioPilota()
{
UnicodeString s;
try
{
TStringList *ps = new TStringList;
try
{
s = _D("<CAMBIO_PILOTA>");
ps->Add(s);
for (int i = 0; i < MAX_PILOTI; ++i)
{
s.sprintf( _D("<Pilota%02x= I%x,\"A%s\",\"C%s\",\"F%s\",f%x>),
// TODO: if SetCambioPilota() is ever called in a worker thread,
// make sure these values are accessed in a thread-safe manner!
i+1, gara.pilota[i].idnome,
gara.pilota[i].nome.c_str(), gara.pilota[i].nick.c_str(),
gara.pilota[i].nomeTeam.c_str(), gara.pilota[i].idPilotaT);
ps->Add(s);
}
s = _D("<END_CAMBIO_PILOTA>");
ps->Add(s);
Send(ps);
}
__finally
{
delete ps;
}
}
catch (const Exception &)
{
}
}
void __fastcall TDataNet::SetDatiGara()
{
UnicodeString s;
try
{
TStringList *ps = new TStringList;
try
{
s = _D("<DATI_GARA>");
ps->Add(s);
s.sprintf( _D("<eve=%d,A%x,B%x,C%x,D%x,E%x,F%x,G%x,H%x,I%x,J%x,K%x>"),
// TODO: if SetDatiGara() is ever called in a worker thread,
// make sure these values are accessed in a thread-safe manner!
DataB->GetEventoInCorso().idEvento,
DataB->GetEventoInCorso().numEvento, DataB->GetEventoInCorso().subEvento,
DataB->GetNextEvento().idEvento, DataB->GetNextEvento().numEvento, DataB->GetNextEvento().subEvento,
gara.tkTempo, gara.tkDurata - gara.tkTempo, gara.laps,
(gara.gDurata > 0) ? (gara.gDurata - gara.laps) : 0,
gara.flInCorso ? ((gara.gDurata > 0) ? 2 : 1) : 0,
gara.flFineGara);
ps->Add(s);
s = _D("<END_DATI_GARA>");
ps->Add(s);
Send(ps);
}
__finally
{
delete ps;
}
}
catch (const Exception &)
{
}
}
void __fastcall TDataNet::Timer1Timer(TObject *Sender)
{
Timer1->Enabled = false;
SetDatiGara();
Timer1->Enabled = true;
}
void __fastcall TDataNet::IdTCPServer1Connect(TIdContext *AContext)
{
TDatiClient* dati = static_cast<TDatiClient*>(AContext);
AContext->Connection->IOHandler->DefStringEncoding = TIdTextEncoding_UTF8;
TList* lista = IdTCPServer1->Contexts->LockList();
try
{
// TODO: this event is fired in a worker thread, so make sure
// that connessioni, DataB, and SetCambioStato() are all being
// accessed in a thread-safe manner!
int connessioni = lista->Count;
if (FmainWnd)
PostMessage(FmainWnd, WM_EVENTO_TCP, ID_CONNESSO, connessioni);
int idEvento = DataB->GetEventoInCorso().idEvento;
if (idEvento)
SetCambioStato(idEvento, STATO_EVENTO_START, AContext);
}
__finally
{
IdTCPServer1->Contexts->UnlockList();
}
}
void __fastcall TDataNet::IdTCPServer1Disconnect(TIdContext *AContext)
{
TDatiClient* dati = static_cast<TDatiClient*>(AContext);
TList* lista = IdTCPServer1->Contexts->LockList();
try
{
int connessioni = lista->Count - 1;
if (FmainWnd)
PostMessage(FmainWnd, WM_EVENTO_TCP, ID_DISCONNESSO, connessioni);
}
__finally
{
IdTCPServer1->Contexts->UnlockList();
}
}
void __fastcall TDataNet::IdTCPServer1Execute(TIdContext *AContext)
{
TDatiClient* dati = static_cast<TDatiClient*>(AContext);
TStringList *ps;
if (dati->QueueHasObjects)
{
TObjectList *objs = new TObjectList(false);
try
{
TList *lista = dati->Queue->LockList();
try
{
objs->Assign(lista);
lista->Clear();
objs->OwnsObjects = true;
}
__finally
{
dati->QueueHasObjects = (lista->Count > 0);
dati->Queue->UnlockList();
}
for (int i = 0; i < objs->Count; ++i)
{
ps = static_cast<TStringList*>(objs->Items[i]);
AContext->Connection->IOHandler->Write(ps, true);
}
}
__finally
{
delete objs;
}
}
if (AContext->Connection->IOHandler->InputBufferIsEmpty())
{
AContext->Connection->IOHandler->CheckForDataOnSource(200);
if (AContext->Connection->IOHandler->InputBufferIsEmpty())
{
AContext->Connection->IOHandler->CheckForDisconnect();
return;
}
}
ps = new TStringList;
try
{
AContext->Connection->IOHandler->ReadStrings(ps, -1);
if ((ps->Count >= 2) && (ps->Strings[0] == _D("<LAST_MINUTE>")) && (ps->Strings[ps->Count-1] == _D("<END_LAST_MINUTE>")))
{
ps->Delete(0);
ps->Delete(ps->Count-1);
if (FmainWnd)
SendMessage(FmainWnd, WM_EVENTO_TCP, ID_LAST_MINUTE, reinterpret_cast<LPARAM>(ps));
}
}
__finally
{
delete ps;
}
}