I am trying to extend Python with some C code. I made a trivial
"Hello World" program in C that I am trying to wrap in "boilerplate"
for inclusion in a Python program. But I can't compile the C code. The
C compiler cannot find the required function `Py_BuildValue'.
My C code looks like this:
#include "/usr/include/python2.5/Python.h"
/* Python wrapper for 'main'. */
static PyObject* mpi_main() {
int res = main();
PyObject* retval = (PyObject*)Py_BuildValue("i",res);
}
/* Main. A 'Hello World' function. */
int main() {
printf ("Hello, World. I am a C function.\n");
}
And my error message looks like this:
[ore@localhost Opgave03]$ gcc ctest.c
/tmp/ccH46bs8.o: In function `mpi_main':
ctest.c:(.text+0x1d): undefined reference to `Py_BuildValue'
collect2: ld returned 1 exit status
[ore@localhost Opgave03]$
I searched the newsgroup and found this thread:
It recommended that I add the option "-Wl,--export-dynamic" when
calling gcc. That makes no difference. The thread also has some more
stuff that I don't quite understand.
Can anyone help? I am including Python.h, so why does it not find
Py_BuildValue?
Thanks in advance.
Read your error message again. It says the dynamic linker (ld) can't
find the name. You have to link against the Python library.
Christian
Hi! Your C code contains too many errors. I'm lazy to comment them all.
Here's my take on the trivial "Hello world" in Python C API:
1. create 'hello.c' file with the following content:
#include <Python.h>
PyObject*
hello(PyObject* self)
{
printf("Hello world!\n");
Py_RETURN_NONE;
}
static PyMethodDef functions[] = {
{"hello", (PyCFunction)hello, METH_NOARGS},
{NULL, NULL, 0, NULL},
};
DL_EXPORT(void)
init_hello(void)
{
Py_InitModule("_hello", functions);
}
2. create 'buildme.py' file with this content:
import os
import sys
from distutils.core import Extension, setup
os.chdir(os.path.dirname(os.path.abspath(__file__)))
sys.argv = [sys.argv[0], 'build_ext', '-i']
setup(ext_modules = [Extension('_hello', ["hello.c"])])
3. run "python buildme.py"
That's all.
>>> from _hello import hello
>>> hello()
Hello world!
-- Ivan
Thanks for the replies.
Maybe I should have read the rest of the guide to extending Python
with C before whining here. I hadn't noticed the part where I was
supposed to create a script to compile my C code rather than just call
gcc. I fixed that, and now it works. Thanks.
Yeah, my C code is probably full of bugs, too. I am a sucky C
programmer. :P
Have you considered using Cython instead of C?
Stefan