[Cython] Header file bug

[Robert Bradshaw]

On Sat, Jun 4, 2011 at 10:36 AM, Seth Shannin <sshannin at stwing.upenn.edu> wrote:
> Hello all,
>
> First, I tried to post this earlier but am pretty sure it didn't go through.
>  If this is a double, I apologize.
>
> This comes out of a discussion on the cython-users list.  I am getting some
> warnings when using cython to compile a python module and then calling into
> the module from c.
>
> I was able to condense the problem down to a very succinct example.
>
> There are three files of interest:
>
> test.pyx:
> ----------------------------------
> cdef class foo:
>    cdef int a
>    cdef int b
>    def __init__(foo self, int a, int b):
>        self.a = a
>        self.b = b
>
> cdef public void bar(foo x):
>    print "My_foo", x.a, x.b
>
> cdef public foo make_foo(int a, int b):
>    return foo(a, b)
> ------------------------------------------
>
> setup.py:
> ------------------------------------------
> from distutils.core import setup
> from distutils.extension import Extension
> from Cython.Distutils import build_ext
>
> ext_modules = [Extension("test", ["test.pyx"])]
>
> setup(
>  name = 'Hello world app',
>  cmdclass = {'build_ext': build_ext},
>  ext_modules = ext_modules
> )
> ------------------------------------------
> and dummy.c:
> -----------------------------------------
> #include "Python.h"
> #include "test.h"
>
> int main(int argc, char** argv){
>  Py_Initialize();
>  inittest();
>  bar(make_foo(1,2));
>  return 0;
> }
> ---------------------------------------
>
> I use the following commands to compile and link:
> python setup.py build_ext --inplace
> gcc -I/usr/include/python2.6 -lpython2.6 -L/usr/include/python2.6/config -c
> dummy.c
>
> The second command generates the following error messages:
> In file included from dummy.c:2:
> test.h:11: warning: 'struct __pyx_obj_4test_foo' declared inside parameter
> list
> test.h:11: warning: its scope is only this definition or declaration, which
> is probably not what you want
> dummy.c: In function 'main':
> dummy.c:7: warning: passing argument 1 of 'bar' from incompatible pointer
> type
> test.h:11: note: expected 'struct __pyx_obj_4test_foo *' but argument is of
> type 'struct __pyx_obj_4test_foo *'
>
> After a bit of digging, I found the problem:
> The definition for the struct in question was created in test.c, not test.h.
> GCC is therefore treating the parameter in test.h as a completely separate
> (and empty) struct declaration.  Luckily, in this case struct pointers are
> being used, so enough mem gets allocated, etc. and everything actually ends
> up working.
>
> That being said, the compiler errors are quite bothersome, and it may be
> that this could cause real errors in more complex programs with more
> interdependencies of types.

True. Cython doesn't usually get used this way, so thanks for the
report. BTW, for embedding the --embed option is usually easier to
use.

> I would like to propose as a fix that the struct declaration be generated
> inside the header file instead of the .c file (and on a side note, I would
> personally prefer that the #includes be placed in the header file as well).
>
> Thanks for all the work that has been put into this project.  Cython is
> really quite a remarkable tool!
>
> -Seth
>
>
>
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