Python/Cython/C and callbacks, calling a Python function from C using Cython

Question

I have the following question. We have to pass callback functions to the C code. If the function is a Cython function in the same module, the situation is quite simple

In Cython:

def callme(int x):
    c_callme(x, <int (*)(int)>&callbackme) 

cdef int callbackme(int x):
    print <int> x
    return <int>x

In C:

int c_callme(int x, int (*f)(int))
{
    printf("---%d\n",x);
    printf("--%d\n",f(x));
    return x;
}

The question is as follows: we want to generalize this code in the most Pythonic way so that it can accept also python functions as callback arguments (of course, some additional layer is needed), and also C/Cython functions from another module. I suppose, for C/Cython functions from a separate module one has to get the address of these functions (convert to long int?) and for Python function a certain wrapper is needed

If you want wrap different functions at the same time (so one global variable isn't enough), there is no way around code generated at run-time. See stackoverflow.com/a/51054667/5769463 — ead
– ead, Commented Feb 21, 2019 at 23:32

Saullo G. P. Castro · Accepted Answer · 2013-08-29 12:17:05Z

15

In this example, extracted from a Python wrapper to the Cubature integration C library, a Python function is passed to a C function that has the prototype given by cfunction. You can create a function with the same prototype, called cfunction_cb (callback), and returning the same type, int in this example):

cdef object f
ctypedef int (*cfunction) (double a, double b, double c, void *args)

cdef int cfunction_cb(double a, double b, double c, void *args):
    global f
    result_from_function = (<object>f)(a, b, c, *<tuple>args)
    for k in range(fdim):
        fval[k] = fval_buffer[k]
    return 0

When calling the C function, you can cast your callback wrapper using the C prototype:

def main(pythonf, double a, double b, double c, args): 
    global f
    f = pythonf
    c_function( <cfunction> cfunction_cb,
                double a,
                double b,
                double c,
                <void *> args )

In this example it is also shown how to pass arguments to your Python function, through C.

answered Aug 29, 2013 at 12:17

Saullo G. P. Castro

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2 Comments

dashesy Over a year ago

this should be accepted as answer, it works fine. The global instance keeps the instance alive and well.

ead Over a year ago

It should be c_function(&cfunction_cb,...) and not c_function(<cfunction>cfunction_cb,...) because the version with <cfunction> will cast any function to cfunction-functor and one will see the problems only during the run time. For version with &, the compiler will catch a possible type mismatch.

Fred Foo · Accepted Answer · 2012-07-28 13:44:30Z

-1

I think the easiest way would be to wrap C callbacks in

cdef class Callback(object):
    cdef int (*f)(int)        # I hope I got the syntax right...

    def __call__(int x):
        return self.f(x)

so you can pass objects of this type, as well as any other callable, to the function that must call the callback.

However, if you must call the callback from C, then you should may pass it an extra argument, namely the optional address of a Python object, perhaps cast to `void*.

(Please do not cast function pointers to long, or you may get undefined behavior. I'm not sure if you can safely cast a function pointer to anything, even void*.)

answered Jul 28, 2012 at 13:44

Fred Foo

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2 Comments

Ivan Oseledets Over a year ago

I was also thinking in that direction. But the code does not compile at all :( . Can you elaborate more on why we do not have to cast function pointers to long?

Fred Foo Over a year ago

@IvanOseledets: function pointer values are not guaranteed to fit in long, and vice versa. It might work on one machine, but fail on another. That this code doesn't compile is probably due to a syntax error, I don't have a Cython installation handy.

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