Re: ctypes and OS X CF types?

AFAIK you don't need to do anything special (i.e. write
"ctypes  
definitions") to call the CF functions with ctypes.
Just call them.  
Here's a snippet of code that I use to open a PDF document
and get  
the number of pages:


from ctypes import cdll, byref, c_void_p, c_int, c_uint,
c_float,  
c_double
from ctypes.util import find_library

cf =
cdll.LoadLibrary(find_library("ApplicatonServices"
))

filename = "/path/to/document.pdf"

kCFAllocatorDefault = None
kCFURLPOSIXPathStyle = 0

# open pdf document
fileref = None
fileurl = None
try:
     fileref = cf.CFStringCreateWithCString(None, filename,
0)
     fileurl = cf.CFURLCreateWithFileSystemPath(
         kCFAllocatorDefault,
         fileref,
         kCFURLPOSIXPathStyle,
         0
     )
     pdf = cf.CGPDFDocumentCreateWithURL(fileurl)
     if not pdf:
         raise PrintError("cannot open PDF document:
%s" % filename)
finally:
     if fileurl:
         cf.CFRelease(fileurl)
     if fileref:
         cf.CFRelease(fileref)

numpages = cf.CGPDFDocumentGetNumberOfPages(pdf)

if pdf:
     cf.CGPDFDocumentRelease(pdf)



My experience with developing code like this is that it
takes a lot  
of patience and trial and error to get it stable. Remember
that  
you're dealing with C functions and pointers (it's called
ctypes for  
a reason), which are not nearly as forgiving as normal
Python  
functions and objects. For example, calling a C function
with the  
wrong argument types will generally cause a cold hard
segfault rather  
than give you a nice stack trace with some indication of
what caused  
the error. When all else fails read the documentation for
the library  
you're calling...again.

~ Daniel


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