Use generated header file from Cython

Question

According to the documentation it is possible to use C header files generated from Cython. I have followed the Hello World example with no problem and now I want to try something different. I want to use a public declaration to be able to use a custom method. My code structure is the following:

hello.pyx
setup.py
main.c

hello.pyx

cdef public void say_hello():
    print("Hello World")

setup.py

from distutils.core import setup
from distutils.extension import Extension
from Cython.Distutils import build_ext

ext_modules = [
    Extension("hello", ["hello.pyx", "main.c"]),
]

setup(
  name='Hello app',
  cmdclass={'build_ext': build_ext},
  ext_modules=ext_modules
)

main.c

#include "hello.h"

int main(void){
    say_hello();
}

The main.c acts as a test file to verify that the say_hello() method works as intended. Building the setup file python3 setup.py build_ext produces the following output.

    running build_ext
    skipping 'hello.c' Cython extension (up-to-date)
    building 'hello' extension
    x86_64-linux-gnu-gcc -pthread -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -g -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -fPIC -I/usr/include/python3.5m -c hello.c -o build/temp.linux-x86_64-3.5/hello.o
    x86_64-linux-gnu-gcc -pthread -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -g -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -fPIC -I/usr/include/python3.5m -c main.c -o build/temp.linux-x86_64-3.5/main.o
    In file included from main.c:1:0:
    hello.h:26:1: error: unknown type name ‘PyMODINIT_FUNC’
     PyMODINIT_FUNC inithello(void);
     ^
    error:

 command 'x86_64-linux-gnu-gcc' failed with exit status 1

The hello.h file contains the following

    /* Generated by Cython 0.25.2 */

#ifndef __PYX_HAVE__hello
#define __PYX_HAVE__hello


#ifndef __PYX_HAVE_API__hello

#ifndef __PYX_EXTERN_C
  #ifdef __cplusplus
    #define __PYX_EXTERN_C extern "C"
  #else
    #define __PYX_EXTERN_C extern
  #endif
#endif

#ifndef DL_IMPORT
  #define DL_IMPORT(_T) _T
#endif

__PYX_EXTERN_C DL_IMPORT(void) say_hello(void);

#endif /* !__PYX_HAVE_API__hello */

#if PY_MAJOR_VERSION < 3
PyMODINIT_FUNC inithello(void);  // <-- Line 26
#else
PyMODINIT_FUNC PyInit_hello(void);
#endif

#endif /* !__PYX_HAVE__hello */

To my understanding, it seems that gcc in not able to get the right version of Python (I'm using Python 3.5). Is there a way to set that somehow? Also, if that is indeed the case, why isn't this linking taken care of when I run the python3 setup.py build_ext command?

I don't have much experience with C so I might be missing something.

user_7343721 · Answer

You can use the api keyword

cdef api void hello():
    print("hello")
    return;

This will autogenerate a "hello.h" when compiling.

Use generated header file from Cython

2 Answers