[C++-sig] ctypes newbie question

[Roman Yakovenko]

On Thu, Jun 5, 2008 at 8:34 PM, Gary Robinson <garyrob at mac.com> wrote:
> On OS X 10.5.3, with Python 2.5.2, I have the following C function, called test2.c:
>
> ==============
> double testreturn(void)
> {
>  return  105.0;
> }
> ==============
>
> I also have the following Python script:
>
> ==============
> from ctypes import *
>
> testreturnproto = CFUNCTYPE(c_double)
> testreturn = testreturnproto(CDLL('test2.dylib').testreturn)
> print 'prototype version returned: ', testreturn()
>
> testdll = CDLL('test2.dylib')
> testdll.testreturn.restype = c_double
> print 'non-prototype version returned:', testdll.testreturn()
> =============
>
> Based on reading the ctypes docs, I would expect the output of the script to be:
>
> prototype version returned:  105.0
> non-prototype version returned: 105.0
>
> However, what I actually get is:
>
> prototype version returned:  872440.0
> non-prototype version returned: 105.0
>
> If I modify the python script by commenting out the assignment to restype, the new script and results are:
>
> ================
> from ctypes import *
>
> testreturnproto = CFUNCTYPE(c_double)
> testreturn = testreturnproto(CDLL('test2.dylib').testreturn)
> print 'prototype version returned: ', testreturn()
>
> testdll = CDLL('test2.dylib')
> #testdll.testreturn.restype = c_double
> print 'non-prototype version returned:', testdll.testreturn()
> ================
>
> prototype version returned:  872440.0
> non-prototype version returned: 872440
>
> I assume that this means that the double 105.0 is being interpreted as if it were an int, since the default result type as an int.
>
> What I don't understand is why the line
>
>  testreturnproto = CFUNCTYPE(c_double)
>
> is apparently not causing the return type to be properly interpreted as a double. We know that it's ultimately causing a Python float to be created, since the printed output ends with .0. So it has received the information that the number is supposed to be a float. But it's still interpreting it as an int before instantiating it as a float.
>
> The ctypes reference describes CFUNCTYPE as follows:
>
>> CFUNCTYPE(restype, *argtypes)
>> The returned function prototype creates functions that use the
>> standard C calling convention.
>
> I am passing c_double as restype. So why is it apparently interpreting the result as an int?
>
> I guess I don't understand the purpose of instantiating a prototype with CFUNCTYPE if it doesn't take care of interpreting the result as the type you give it.
>
> Any insight/help would be appreciated.
>

You'd better ask this question on ctypes mailing list (
https://lists.sourceforge.net/lists/listinfo/ctypes-users ).

You also can take a look on one of my projects(
http://pygccxml.svn.sourceforge.net/viewvc/pygccxml/pygccxml_dev/pygccxml/msvc/bsc/
).
I use ctypes to wrap bsc ( browse source code ) API.

HTH
-- 
Roman Yakovenko
C++ Python language binding
http://www.language-binding.net/