Oliver Andrich schrieb: > Hi everybody, > > I am currently playing around with ctypes and a C library (libWand > from ImageMagick), and as I want to easily deploy it on Mac, Linux and > Windows, I prefer a ctypes solution over a C module. At least on > windows, I would have resource problems to compile the C module. So, > ctypes is the perfect choice for me. > > But I am currently encountering a pattern inside the C library, that > has to be used to free memory. Coding in C I have to do the following > > char *description; > long severity; > > description = MagickGetException(wand, &severity); > /* > do something with the description: print it, log it, ... > */ > description = (char *) MagickRelinquishMemory(description); > exit(-1); /* or something else what I want to after an exception occured > */ > > So, this looks easy and is sensible from C's point of view. Now I try > to translate this to Python and ctypes. > > dll.MagickGetException.argtypes = [c_long, POINTER(c_long)] > dll.MagickGetException.restype = c_char_p > > severity = c_long() > description = dll.MagickGetException(byref(severity)) > > # So far so good. The above works like a charm, but the problem follows > now > # ctypes already converted the char * for description to a String object. > # That means description has arrived in an area under Python's control. > > # these definitions are the easy part > dll.MagickRelinquishMemory.argtypes = [c_void_p] > dll.MagickRelinquishMemory.restype = c_char_p > > # but this obviously must cause problems and causes problems > dll.MagickRelinquishMemory(description) > > So, my question is, how do I deal with this situation? Can I ignore > the call to MagickRelinquishMemory, cause Python takes care of the > resources already? Or is it impossible to use it at all, and I have to > think about a different solution?
ctypes only releases memory that it has allocated itself. So, if you don't call your MagickRelinquish function a memory leak will occur. Chris Mellon has already pointed out a possible solution, but there is also a different way. You could use a subclass of c_char_p as the restype attribute: class String(c_char_p): def __del__(self): MagickRelinquishMemory(self) The difference is that you will receive an instance of this class when you call the MagickGetException funcion. To access the 'char *' value, you can access the .value attribute (which is inherited from c_char_p), and to free the memory call MagickRelinquishMemory in the __del__ method. Thomas -- http://mail.python.org/mailman/listinfo/python-list