python Ctype : problem with callback in making python custom library

Question

The whole scenario is like this:

• there is a function setmessagelistener(a_structure,function_pointer) in my c library.

• i am writing a python library which is a wrapper on above mentioned c library using Ctypes. so what i did is something like this:

  def setlistener(a_structure,function_pointer)
  
  
   listenerDeclaration = CFUNCTYPE(c_void_p, c_void_p)
  
  
   listenerFunction = listenerDeclaration(function_pointer)
  
  
   setMsgListener = c_lib.setMessageListener
  
  
   setMsgListener.argtypes = [c_void_p, c_void_p]
  
  
   setMsgListener.restype = c_ubyte
  
  
   if (setMsgListener(a_structure, listenerFunction)==0):
  
  
  sys.exit("Error setting message listener.")
  
  
  

• Now,another python program uses above function from my python library to do the work,but the problem is:

when i run the program it gives me segmentation fault because the local object( listenerfunction) is already garbage collected(i think so) when control returned from the library function setListener() as it being a local object.

is there any workaround/solution to this problem?or am i missing something? please suggest... thanx in advance

Answer 1

As you suspect, you need to keep a reference to listenerFunction as long as it is needed. Perhaps wrap the function in a class, create an instance and set the listenerFunction as a member variable.

See the Python documentation for ctypes callbacks, especially the important note at the end of the section.