a call to GetMessage causes a thread to stop - c++builder

Question

I am writing a multi-threaded application using Borland C++ (Delphi Forms). I have recently learned that I can use Windows' Messaging Service within these classes when I call the PostThreadMessage() function:
System = new STSystem(SystemName,1000,1,NULL);
while (PostThreadMessage(System->ThreadID,ST_MSG_SYSTEM_INIT,0,0) == 0)
{
Sleep(0);
};
The above seems to work just fine. The issue lies on the retrieval end of this process inside of the Thread Execution function:
void __fastcall STSystem::Execute()
{
ST_Message STMSG;
while(FStatus != Destroyed)
{
FHeartBeat++;
if(GetMessage(MSG,NULL,ST_MSG_SYSTEM_START,ST_MSG_SYSTEM_END))
{
STMSG.Value = MSG->wParam;
if((STMSG.dSYS + (8*STMSG.dSEC) + (64*STMSG.dDEP)) == FSystemID)
{
RXMessages[RxQueueIn++] = STMSG.MSG; // Message
RXMessages[RxQueueIn++] = MSG->lParam; // Data
}
}
if(TaskList->Count>0)
ProcessTask();
if(RxQueueIn!=RxQueueOut)
ProcessRxMessage();
if(TxQueueIn!=TxQueueOut)
ProcessTxMessage();
Sleep(0);
};
}
The above works for about two thread cycles and then stops; the thread stops, not the program. I have tried using the PeekMessage() function instead of the GetMessage() function in the IF clause following the FHeartbeat++ counter. This prevents the thread from stopping however, the INIT message sent in the first block of code is still not found.
I hope this example is not too specific. I have tried to leave in anything that was pertinent. Basically, this is a message pump for a class that has no window.

Answer 1

GetMessage() blocks the calling thread when there are no messages to retrieve. Like Luis said, you need to make sure the thread has a message queue before you start posting messages to it, and you need to check the return value of PostThreadMessage() for failures. A message queue is not created in a thread until any user32.dll function is called within the thread for the first time. For example:
System = new STSystem(SystemName,1000,1,NULL);
while (!System->Ready)
Sleep(100);
if (!PostThreadMessage(System->ThreadID,ST_MSG_SYSTEM_INIT,0,0))
{
DWORD err = GetLastError();
//...
}
void __fastcall STSystem::Execute()
{
// create a message queue
PeekMessage(MSG, NULL, 0, 0, PM_NOREMOVE);
Ready = true;
ST_Message STMSG;
while(FStatus != Destroyed)
{
FHeartBeat++;
if(GetMessage(MSG,NULL,ST_MSG_SYSTEM_START,ST_MSG_SYSTEM_END)) // or PeekMessage()
{
STMSG.Value = MSG->wParam;
if((STMSG.dSYS + (8*STMSG.dSEC) + (64*STMSG.dDEP)) == FSystemID)
{
RXMessages[RxQueueIn++] = STMSG.MSG; // Message
RXMessages[RxQueueIn++] = MSG->lParam; // Data
}
}
if(TaskList->Count>0)
ProcessTask();
if(RxQueueIn!=RxQueueOut)
ProcessRxMessage();
if(TxQueueIn!=TxQueueOut)
ProcessTxMessage();
Sleep(0);
};
}

Answer 2

To send messages to a tread it must have a message queue, in the remarks section of this link:
http://msdn.microsoft.com/en-us/library/windows/desktop/ms644946(v=vs.85).aspx
you can fine the steps reqired to create a message queue for a thread.
By the way, if PostThreadMessage returns 0 (FALSE), there is an error and you must check the value returned by GetLastError.