On 10/3/06, Oliver Andrich <[EMAIL PROTECTED]> wrote: > 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? >
Not a ctypes expert but I can pretty much guaranteee that Python won't correctly release that memory by itself. Are you sure that description is actually a string object? By my recollection it should be a c_char_p object, and if you pass it to MagickRelinquishMemory ctypes should do the right thing. If it doesn't, then you can use a more literal translation of the C code, and use a c_void_p instead, using ctypes.string_at(description) to get a string out of the buffer. > Best regards, > Oliver > > -- > Oliver Andrich <[EMAIL PROTECTED]> --- http://roughbook.de/ > -- > http://mail.python.org/mailman/listinfo/python-list > -- http://mail.python.org/mailman/listinfo/python-list
