Creating C++ Blocking Function for Existing Asynchronous code - c++

Question

I have a C++ plugin I have written for a proprietary software which makes an asynchronous call to a function called OpenLibrary. To know when the library load has completed, I must register for a specific event. Then, when that event is triggered, an OnEvent routine is called. This logic is somewhat sterilized for proprietary reasons, but the asynchronous call and onEvent trigger works correctly. Unfortunately, since the OpenLibrary call is asynchronous, the loop is not blocked and continues without waiting for the EVENT_LIBRARY_LOADED event. I need to process the files serially.
...
void MyApp::main()
{
for(int i=0; i<total; ++i) {
pData->RegisterEvent( EVENT_LIBRARY_LOADED, this );
pData->OpenLibrary("c:/path/file.dat"); // asynchronous call
}
}
...
void MyApp::OnEvent( ID eventType )
{
if (eventType == EVENT_LIBRARY_LOADED) {
qDebug() << "Library load has completed";
}
}
...
The plugin requires VS2008 and also takes advantage of the Qt library.
I would like to create a function called waitForEvent, where subsequent code is blocked until the event has occurred then waitForEvent can return control back to the calling routines loop. This way, I can stay inside my main routines loop and simply wait for the event before continuing. Any suggestions appreciated.
UPDATE: I have tried both excellent suggestions below by Tas, but in either case, I get the same result. The WaitForSingleObject OR the condition_variable.wait BOTH prevent the EVENT_LIBRARY_LOADED event from triggering the OnEvent function from being called, which freezes the loop.
Any more suggestions appreciated.

Answer 1

If boost libraries are an option, use boost::condition_variable
You've already made it clear C++11 isn't an option (otherwise you could use std::condition_variable). boost::condition_variable will accomplish what you need to do, and it's very simple to use. You only need to call wait and notify_one:
void MyApp::main()
{
for(int i=0; i<total; ++i) {
pData->RegisterEvent( EVENT_LIBRARY_LOADED, this );
pData->OpenLibrary("c:/path/file.dat"); // asynchronous call
condition_variable.wait(); // wait until we've been signaled
}
}
void MyApp::OnEvent( ID eventType )
{
if (eventType == EVENT_LIBRARY_LOADED) {
qDebug() << "Library load has completed";
// signal completion:
condition_variable.notify_one();
}
}
Otherwise you could use Windows Event objects
These work very similar to the above but are a little more complicated to use (and also OS specific).
HANDLE hEvent = ::CreateEvent(NULL, TRUE, FALSE, NULL);
void MyApp::main()
{
for(int i=0; i<total; ++i) {
// Prepare signal (otherwise if the signal has been Set already, Wait will return instantly)
::ResetEvent(hEvent);
pData->RegisterEvent( EVENT_LIBRARY_LOADED, this );
pData->OpenLibrary("c:/path/file.dat"); // asynchronous call
// wait for event to signal:
::WaitForSingleObject(hEvent, INFINITE);
}
}
void MyApp::OnEvent( ID eventType )
{
if (eventType == EVENT_LIBRARY_LOADED) {
qDebug() << "Library load has completed";
// Signal event:
::SetEvent(hEvent);
}
}