RE: asynMotorController exception safety

["Davidsaver, Michael" <mdavidsaver@bnl.gov>]

> -----Original Message-----
> From: tech-talk-bounces@aps.anl.gov [mailto:tech-talk-
> bounces@aps.anl.gov] On Behalf Of Mark Rivers
> Sent: Tuesday, September 13, 2011 1:27 PM
> To: Till Straumann; EPICS Techtalk
> Subject: RE: asynMotorController exception safety
> 
> ...
> If your constructor throws an exception, does that automatically
result
> in a call to the destructor for the base classes?

Absolutely yes.

If the constructor of an object (or sub-object) completes then its
destructor will be called.  Sub-objects may be base classes, or member
objects.  This behavior why container classes can effectively manage
resources.

I think the basic problem is that asyn ports can't be unregistered.
When the asynPortDriver instance is destroyed it frees its
asynStandardInterfaces member structure.  This leaves asynManager
holding invalid pointers.

@Till, When I face this situation I set a flag if the constructor fails
which prevents the port from ever connecting.  Something like:

class myport : asynPortDriver {
  bool zombie;
  ...
}

myport::myport()
  : zombie(false)
{
try {
  // normal initialization
}catch(...){
  zombie=true;
}
}

myport::connect(){
  if(zombie)
    return asynError;
}


A better solution would be to avoid freeing asynStandardInterfaces.

Michael


> I would not have
> thought so, but I'm not enough of a C++ expert to know the answer to
> that.
> 
> Mark
> 
> 
> -----Original Message-----
> From: Till Straumann [mailto:strauman@slac.stanford.edu]
> Sent: Tuesday, September 13, 2011 11:40 AM
> To: EPICS Techtalk; Mark Rivers
> Subject: asynMotorController exception safety
> 
> I wrote a driver class derived from asynMotorController
> (which BTW was easy, thanks for the great package).
> 
> I would like the driver to be robust in case e.g., of the
> absence of e.g., hardware or a serial communication channel.
> 
> Hence, my class does something like:
> 
> class MyDrv : public asynMotorController {
> public:
>      MyDrv() : asynMotorController(<parameters go here>)
>      {
>          status = detect_hardware(<parameters>);
>          if ( status ) {
>              throw MyException("HW detection failed\n");
>          }
>      }
> };
> 
> 
> In my main application I create a driver object
> 
> 
> try {
>     new MyDrv();
> } catch (MyException &e) {
>     printf("Unable to create driver: %s\n", e.what());
> }
> 
> However, if hardware detection fails and the exception
> is thrown and subsequently caught then the IOC application
> will eventually (later) segfault in asynPortDriver::callbackTask()
> (when trying to lock the mutex). It seems that the work
> of the superclass constructor(s)
> (asynMotorController/asynPortDriver/...) is
> not properly undone.
> 
> No segfault happens if I comment the entire try block
> (including the 'new' statement), i.e., if I create no
> driver at all (but still load the .db file etc.).
> 
> How am I supposed to handle failure in the constructor?
> 
> Thanks
> -- Till