How do I pass a pointer to a c function in Cython?

Question

I'm trying call qsort in Cython with a custom compare function but I don't understand how to pass the function reference. First, I have a struct:

cdef struct Pair:
    int i,j
    float h

The compare function sorts by h:

cdef int compare(const_void *a, const_void *b):
    cdef float v = ((<Pair*>a)).h-((<Pair*>b)).h
    if v < 0: return -1
    if v > 0: return 1
    return 0

This is the part I'm having trouble with:

    cdef Pair[5] pa
    for i in range(5):
        pa[i].i = i;
        pa[i].j = i*2;
        pa[i].h = i*.5;
    qsort(pa,5,sizeof(Pair),compare)

The last line won't compile and generates this error which I believe is related to the fact that I can't figure out how to pass compare as a reference to qsort:

Cannot assign type 'int (const_void *, const_void *)' to 'int (*)(const_void *, const_void *) nogil'

Answer 1

I've not been able to reproduce your error. The code you're using is right, and working with Cython 0.15. The only thing i see that might be your error is the "gil" appended to the type. If you want to specifically declare a imported method as "gil safe", append "nogil" at the end of the declaration.

(note that you can check your python code with cython -a , then open web browser for )

cdef extern from "stdlib.h":
    ctypedef void const_void "const void"
    void qsort(void *base, int nmemb, int size,
            int(*compar)(const_void *, const_void *)) nogil

cdef struct Pair:
    int i,j
    float h

cdef int compare(const_void *a, const_void *b):
    cdef float v = ((a)).h-((b)).h
    print 'do something with', v
    if v  0: return 1
    return 0

def r():
    cdef Pair[5] pa
    for i in range(5):
        pa[i].i = i;
        pa[i].j = i*2;
        pa[i].h = i*.5;
    print 'call qsort'
    qsort(pa,5,sizeof(Pair),compare)
    print 'call qsort done'

r()

This snippet is compiled as:

$ cython --version
Cython version 0.15
$ cython --embed test.pyx
$ gcc -I/usr/include/python2.7 -Wall -std=c99 test.c -lpython2.7
$ ./a.out 
call qsort
do something with -0.5
do something with -0.5
do something with -0.5
do something with -1.0
do something with -0.5
call qsort done