I'm trying to determine the current working directory of a process given its PID. The command-line utility lsof does something similar. Here's the source to the python script: import ctypes from ctypes import util import sys PROC_PIDVNODEPATHINFO = 9 proc = ctypes.cdll.LoadLibrary(util.find_library("libproc")) print(proc.proc_pidinfo) class vnode_info(ctypes.Structure): _fields_ = [('data', ctypes.c_ubyte * 152)] class vnode_info_path(ctypes.Structure): _fields_ = [('vip_vi', vnode_info), ('vip_path', ctypes.c_char * 1024)] class proc_vnodepathinfo(ctypes.Structure): _fields_ = [('pvi_cdir', vnode_info_path), ('pvi_rdir', vnode_info_path)] inst = proc_vnodepathinfo() pid = int(sys.argv[1]) ret = proc.proc_pidinfo( pid, PROC_PIDVNODEPATHINFO, 0, ctypes.byref(inst), ctypes.sizeof(inst) ) print(ret, inst.pvi_cdir.vip_path) However, even though this script behaves as expected on Python 2.6, it does not work in Python 2.5: host:dir user$ sudo /usr/bin/python2.6 script.py 2698 <_FuncPtr object at 0x100419ae0> (2352, '/') host:dir user$ sudo /usr/bin/python2.5 script.py 2698 <_FuncPtr object at 0x19fdc0> (0, '') (PID 2698 is "Activity Monitor.app"). Note the different return values. Since this program strongly based on ctypes, I can't imagine any difference in Python itself that would cause this. The same behavior (as Python 2.5) occurs with my self-built Python 3.2. I'm not sure what versioning information I can give to help track down the weirdness -- or even come up with a solution for 2.5 -- but here's some stuff: host:dir user$ otool -L /usr/bin/python2.6 /usr/bin/python2.6: /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 125.2.0) host:dir user$ otool -L /usr/bin/python2.5 /usr/bin/python2.5 (architecture i386): /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 125.2.0) /usr/bin/python2.5 (architecture ppc7400): /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 125.2.0) host:dir user$ uname -a Darwin host.local 10.8.0 Darwin Kernel Version 10.8.0: Tue Jun 7 16:33:36 PDT 2011; root:xnu-1504.15.3~1/RELEASE_I386 i386 Thanks to anyone that has a clue about what's going on here:)