Question

I’m trying to port my application to Mac, but I don’t understand what’s causing this problem.

At some point, I try to load a couple of C++ libraries provided by a third party, using ctypes’ LoadLibrary. For Windows, I have dlls, and for Mac, dylibs. Loading the dylibs gives an error too which I’m also trying to solve, but that shouldn’t be an issue here, because I do have a try/except block for exactly that.

try:
    self.log('Lib exists? %s: %s' % (libpath, os.path.exists(libpath)))       
    origdir = os.getcwd()
    os.chdir(os.path.dirname(libpath))
    self.lib = cdll.LoadLibrary(os.path.basename(libpath))
    os.chdir(origdir)
    self.log("Loaded Library!")
except Exception as e:
    self.log('Error importing Library! %s' % e)
    self.lib_loaded = False

Any problems are written to a log file. The thing is, it is appending to the log file that somehow works for the first call, but doesn’t work the second time it’s called in the above fragment, at
self.log('Error importing Library! %s' % e).

The log method is pretty obvious:

def log(self, text):
    if self.debug:
        print text
        with open('logfile.log', 'a') as w:
            w.write('%s\n' % text)   

On Windows, this works without problems. When the library is found, it loads, and when it isn’t found, I get the appropriate message printed and written to the log. But on Mac, I get the error

Lib exists? /usr/local/lib/path/to/Mylib: True
Error importing Library! dlopen(lib.dylib, 6): no suitable image found.  Did find:
    lib.dylib: mach-o, but wrong architecture
    /usr/local/lib/lib.dylib: mach-o, but wrong architecture
Traceback (most recent call last):
  File "myapp.py", line 987, in <module>
    foo = Foo(pyqtapp, splash)
  File "myapp.py", line 83, in __init__
    self.thelibLink = libLink.libLink(0.05, a, b)
  File "libLink.py", line 100, in __init__
    self.log('Error importing Library! %s' % e)
  File "libLink.py", line 326, in log
    with open(self.logfile, 'a') as w:
IOError: [Errno 13] Permission denied: 'logfile.log'

logfile.log has the permissions -rw-r--r-- and is owned by me, so this doesn’t make any sense to me. Moreover, after running the program, the logfile contains this:

==Log of date/time==
Lib exists? /usr/local/lib/path/to/Mylib: True

So somehow I do have the permission to append to the file when self.log is called for the first time. Any ideas how I can solve this?

Answer 1

You are opening the log file with a relative path, but you change directories just before the failure:

os.chdir(os.path.dirname(libpath))

Open the logfile with an absolute path instead, since you obviously cannot write in os.path.dirname(libpath).