is it possible to pass a python function as c function pointer?

[Jiajun Huang]

I'm trying to binding a c library to python. The library uses lots of generic function pointers:

int arraylist_index_of(ArrayList *arraylist,

                       ArrayListEqualFunc callback,

                       ArrayListValue data)

{

unsigned int i;

for (i=0; i<arraylist->length; ++i) {

if (callback(arraylist->data[i], data) != 0)

return (int) i;

}

return -1;

}

and the prototype of ArrayListEqualFunc is: 

typedef int (*ArrayListEqualFunc)(ArrayListValue value1, ArrayListValue value2);

when I do this:

# carraylist.pxd

cdef extern from "calg/src/arraylist.h":

    # ...

    ctypedef bint (*ArrayListEqualFunc)(

        ArrayListValue value1, ArrayListValue value2

    )

# arraylist.pyx

cdef class ArrayList:

    cdef int _indexof(self, EQUALFUNC cmp_func, object item):

        return <int>carraylist.arraylist_index_of(

                self.__c_array_list, cmp_func, <void *>item

            )

    def indexof(self, item):

        return self._indexof(<EQUALFUNC>operator.eq, item)

compile it, and I got:

Compiling calgdqueue.pyx because it changed.

Compiling arraylist.pyx because it changed.

[1/2] Cythonizing arraylist.pyx

Error compiling Cython file:

------------------------------------------------------------

...

        return <int>carraylist.arraylist_index_of(

                self.__c_array_list, cmp_func, <void *>item

            )

    def indexof(self, item):

        return self._indexof(<EQUALFUNC>operator.eq, item)

                            ^

------------------------------------------------------------

arraylist.pyx:58:29: Python objects cannot be cast to pointers of primitive types

Traceback (most recent call last):

  File "setup.py", line 15, in <module>

    ext_modules=cythonize(ext_modules)

  File "/usr/lib/python3.6/site-packages/Cython/Build/Dependencies.py", line 934, in cythonize

    cythonize_one(*args)

  File "/usr/lib/python3.6/site-packages/Cython/Build/Dependencies.py", line 1056, in cythonize_one

    raise CompileError(None, pyx_file)

Cython.Compiler.Errors.CompileError: arraylist.pyx

is it possible to pass a python function(e.g. operator.eq) as c function pointers by cython?

[Jiajun Huang]

I've done it by wrap the function in a cdef function: https://github.com/jiajunhuang/pyalgorithms/blob/master/arraylist.pyx but I wonder is there any better solution?

在 2017年1月16日星期一 UTC+8下午3:31:48，Jiajun Huang写道：

[Robert Bradshaw]

A well written library will accept an arbitrary void* argument along
with the callback function pointer which is then returned when the
callback is called (as C doesn't have language support for closures).
You can then use this to stash away your Python callable. See the
example at

https://github.com/cython/cython/blob/master/Demos/callback/cheese.pyx

If the function you're wrapping is fixed, as in your case, just
defining a cdef function is probably sufficient.

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