typedef struct consistency between C and Cython

[Al Danial]

I'm trying to wrap a legacy C library with cython.  The C library

defines a custom data type with a 'typedef struct'.  The Python 

side of the code needs to initialize C variables having this

type but I haven't figured out how to declare the variables

correctly.  Cython complains that

  ./library.c:2:6: note: expected ‘legacy_struct’ but argument is of type ‘__pyx_t_10new_module_legacy_struct’

Platform:

  Python 2.7.3

  Cython 0.17.1

  Fedora 17 i686

Files that boil the problem down to the bare minimum are below and 

also in the attached tar file.  Anyone know where I'm going wrong?

      -- Al

-------------- \/ cyth.pyx

#!/usr/bin/python

ctypedef struct legacy_struct:

    int     *S_start

    double  *N      

    char    *data   

cdef extern from "library.c":

    int  legacy_function(int          nC ,

                        legacy_struct A)

import  numpy as np

cimport numpy as np

np.import_array()

def Save():

    cdef int nC = 0

    cdef legacy_struct m

    legacy_function( nC , <legacy_struct> m)

    return

-------------- /\ cyth.pyx

-------------- \/ header.h

typedef struct {

    int     *S_start  ;

    double  *N        ;

    char    *data     ;

} legacy_struct;

int legacy_function(int           nC ,

                    legacy_struct A  );

-------------- /\ header.h

-------------- \/ library.c

#include "header.h"

int  legacy_function(int           nC,

                    legacy_struct  A ) {

    A.S_start[nC] = 0;

    return 1;

} 

-------------- /\ library.c

-------------- \/ Makefile

CFLAGS= -g

LDFLAGS= -g

all: cyth.pyx library.c header.h

LDFLAGS=$(LDFLAGS) CFLAGS=$(CFLAGS) python setup.py build_ext --inplace

clean:

rm -rf build *.so cyth.c library.o

-------------- /\ Makefile

-------------- \/ setup.py

import numpy

from Cython.Distutils import build_ext

def configuration(parent_package='', top_path=None):

    from numpy.distutils.misc_util import Configuration

    config = Configuration('', parent_package, top_path)

    config.add_extension('new_module',

                         sources=['cyth.pyx'],

                         depends=['cyth.c'],

                         extra_compile_args=["-g"],

                         extra_link_args=["-g"],

                         include_dirs=[numpy.get_include()])

    return config

if __name__ == '__main__':

    params = configuration(top_path='').todict()

    params['cmdclass'] = dict(build_ext=build_ext)

    from numpy.distutils.core import setup

    setup(**params)

-------------- /\ setup.py

[Stefan Behnel]

Al Danial, 12.11.2012 05:30:

> I'm trying to wrap a legacy C library with cython. The C library
> defines a custom data type with a 'typedef struct'. The Python
> side of the code needs to initialize C variables having this
> type but I haven't figured out how to declare the variables
> correctly. Cython complains that
>
> ./library.c:2:6: note: expected ‘legacy_struct’ but argument is of type
> ‘__pyx_t_10new_module_legacy_struct’

It's the C compiler complaining here.

> Files that boil the problem down to the bare minimum are below and
> also in the attached tar file. Anyone know where I'm going wrong?
> -- Al
>
> -------------- \/ cyth.pyx
> #!/usr/bin/python
>
> ctypedef struct legacy_struct:
> int *S_start
> double *N
> char *data
>
> cdef extern from "library.c":
> int legacy_function(int nC ,
> legacy_struct A)

Since you're dealing with an externally defined type, you have to move the
ctypedef into the cdef extern block. Otherwise, Cython will define the type
itself in the C code, which is not what you want here.

Stefan

[Al Danial]

So simple!  And yet I'd have never figured it out on my own.  Thanks,

my code builds cleanly now.