AudioParameterFloat assignment in plugin crashing host


I’m experiencing a strange behaviour in a certain host on Windows for the plugin I’m developing. When opening a project for this host which uses my plugin it makes the host non-responsive. This only happens when opening the project file from the Windows file explorer - it doesn’t happen if I open the host first, then go File->Open to open the file (which works perfectly).

The line of code that is triggering the crash is the assignment operator for the AudioParameterFloat instance.

BensynthpAudioProcessor::processBlock (AudioSampleBuffer& buffer, MidiBuffer& midiMessages){
// Sync macro param values of processor interface with synth
for (size_t m = 0; m < MACRO_COUNT; m++){
if (d->macroData[m].lastInternalValue != d->synth->getMacroParam((unsigned int)m)->getValuePercent()){
// Internal value changed. Update external value
*d->macroData[m].param = d->synth->getMacroParam((unsigned int)m)->getValuePercent(); // THIS LINE ALONE CAUSES THE ISSUE
d->macroData[m].lastExternalValue = d->macroData[m].param->get();
// Record change
d->macroData[m].lastInternalValue = d->synth->getMacroParam((unsigned int)m)->getValuePercent();
}else if (d->macroData[m].lastExternalValue != d->macroData[m].param->get()){
// External value changed. Update internal value
d->synth->getMacroParam((unsigned int)m)->setValuePercent(d->macroData[m].param->get());
d->macroData[m].lastInternalValue = d->synth->getMacroParam((unsigned int)m)->getValuePercent();
// Record change
d->macroData[m].lastExternalValue = d->macroData[m].param->get();

The following is unconditionally run in my AudioProcessor-derived constructor. It’s worth noting the crash never happens if my plugin state is saved with these parameters in their default value.

// Expose macro params to host
constexpr float MACRO_DEFAULT_VALUE = 0.5f;
for (size_t i = 0; i < MACRO_COUNT; i++) {
addParameter(d->macroData[i].param = new AudioParameterFloat(String(d->synth->getMacroParam((unsigned int)i)->getName()), String(“M”) + String(d->synth->getMacroParam((unsigned int)i)->getName()), 0.0f, 1.0f, MACRO_DEFAULT_VALUE)); // default value
d->synth->getMacroParam((unsigned int)i)->setValuePercent(MACRO_DEFAULT_VALUE);
d->macroData[i].lastInternalValue = MACRO_DEFAULT_VALUE;
d->macroData[i].lastExternalValue = MACRO_DEFAULT_VALUE;

Other than this it works properly in at least 4 different hosts, both on Windows and Linux. I strongly suspect I may have done something silly, but I also think it’s a possibility that the host might be doing something naughty. In the AudioParameter float assignment operator implementation has the following:

if (value != newValue)
setValueNotifyingHost (range.convertTo0to1 (newValue));

I’m guessing the ‘if’ statement is why save files don’t crash this host if they’re loading the default value, since the host is never notified. It seems possible the host is unprepared to deal with the notification being sent in processBlock.

If anyone has any ideas on what’s going wrong here I’d be very grateful.


Please let us know which Windows host and which plug-in format you’re using.


Whoops, I should have done that in my original post. Windows 7 64bit, VST2 and a 64bit build of Fl Studio. I’ll try a 32bit build and see if that makes a difference.



From the file manager it was launching a 32-bit version of FL Studio, which for some reason was attempting to load the 64-bit version of my VST (even though both 32-bit and 64-bit FL Studio versions appear to recognise that the VST was 64-bit only from their plugin scanner).

Having a 32-bit version of my VST installed fixed this as FL Studio will apparently always choose the correct version to load if it’s available. I suppose I should bring this to their attention; an error message would have been preferable to the program mysteriously hanging.

Sorry for wasting your time :confused:


Don’t worry - thank you for contributing the solution! If you’ve run into that problem there will be many more who have done the same.