C++ Builder Button Click Function Reentry - c++builder

Question

I have a question that whether the C++ builder VCL Button Click function executes in main UI thread or a temporary new created thread? I wrote a test code that Button Click function exited when user close some dialog, and I found that the Button Click function is entered again even if the previous invoked dialog is presented.
The test code looks like below. My question is that Button can be clicked again and Click function is re-entered due to Application->ProcessMessages() in DoPollingFunc()?
void SomeForm::DoPollingFunc( void )
{
while( 1 )
{
Application->ProcessMessages();
if( polling some flag )
{
break;
}
if( timeout condition )
{
break;
}
}
}
void __fastcall SomeForm::ButtonClick(TObject* Sender)
{
DoPollingFunc();
}

Answer 1

I have a question that whether the C++ builder VCL Button Click function executes in main UI thread or a temporary new created thread?
It runs in whatever thread actually creates the button, which is typically the main UI thread, unless you write your own multi-threaded code to do something different.
I wrote a test code that Button Click function exited when user close some dialog, and I found that the Button Click function is entered again even if the previous invoked dialog is presented.
The OnClick event is triggered when a click message is received by the button control.
If the dialog is modal, it blocks your Form from receiving user input while the dialog is visible, so the OnClick can only be triggered if the dialog's internal message loop is receiving and dispatching a click message for that particular button. Which means the button click must be getting simulated by some piece of code while the dialog is visible.
If the dialog is not modal (such as TFindDialog), it does not block your Form from receiving user input while the dialog is visible, and the dialog operates within the main message loop, so the user can still interact with the button directly while the dialog is visible.

Answer 2

The code executes in the main VCL thread, unless you start, from the OnClick, some other thread to handle the response (and then sync back somehow).
It's not clear what you did with your test case. It is possible to have the OnClick execute multiple times if the button was clicked multiple times, but they will be sequential, not parallel.

Answer 3

If the call to Application->ProcessMessages(); picks up a button click message, then yes, SomeForm::ButtonClick will be called , which in turn will call DoPollingFunc. So your call stack will look like (most recent first):
TApplication::ProcessMessages
SomeForm::DoPollingFunc
SomeForm::ButtonClick
TApplication::ProcessMessages
SomeForm::DoPollingFunc
...
To avoid this happening you might want to use a static variable to detect re-entry. For example:
struct locker
{
bool &b;
locker(bool &b): b(b) {}
bool lock() { return b++; }
~locker() { b = false; }
};
void SomeForm::DoPollingFunc( void )
{
static bool reentry_flag = false;
locker reentry_lock(reentry_flag);
if ( reentry_lock.lock() )
return; // Already in this function at an earlier level
// ... main code here ...
}