ctypes - unloading implicitly loaded dlls

pigmartian scottpig1 at comcast.net
Mon Jul 28 02:54:26 CEST 2008

(my apologies if this is a repost, but it sure seems like the first 
attempt disappeared into the ether...)

I'm writing a program that uses functionality from two different sets of 
cdlls which reside in two different directories, call them 'libA.dll' 
and 'libB.dll'.  Although I don't directly use it, both directories 
contain a dll with the same name, although they aren't in fact 
identical.  Call them, "libC.dll".  However, the c-functions I call from 
the clls I do use seem to implicitly use "libC.dll".  The problem that 
occurs after I load one dll and call functions in it, when I try to load 
the second dll I get windows errors because the second dll tries to call 
a function in its version of libC.dll, but it finds the version meant 
for libB.dll, which doesn't contain that function.

Oy, I hope some sample code makes it clearer:

def demo():

      A = ctypes.cdll.LoadLibrary('/path1/libA.dll')
      A.foo() # implicitly uses '/path1/libC.dll'


      # CRASH!
      B = ctypes.cdll.LoadLibrary('/path2/libB.dll')
      # "The procedure entry point some_func could not be located
      # in the dynamic link library libC.dll.":

      # libB.dll wants to use code from '/path2/libC.dll', but
      # instead it finds '/path1/libC.dll' already loaded
      # in memory, which doesn't
      # contain the function call it wants.

Assuming my understanding of things is correct, then I believe what I 
need to do is to remove /path1/libC.dll from memory before I try loading 
libB.dll, but I haven't found any way of doing that.  Can anyone offer 
my some suggestions?  Or, am I S.O.L.?

* the two sets of dlls are supplied by a vendor for working with its 
COTS packages; I don't have any control over the dll names used or the 
code therein.
* If I leave out the call to A.foo(), then I don't crash, but if I leave 
out the FreeLibrary call as well then I do crash.
* I've tried manipulating the PATH before loading the dlls, to no effect.
* I've tried del'ing A and running gc.collect() before loading B.



More information about the Python-list mailing list