When loading a Python module written in C language, it prompts that the function in .so is not found?

Question

I try to write a Python module in C language, but my C program itself depends on a third-party library (libwiringPi.so). When I import the library I generated in the Python source program, I will be prompted. The function is undefined. These functions are all in that third-party library. How should I compile so that the module I compile can dynamically link to that library?

I also tried to use gcc to manually compile the dynamic link library, and then used ctyes, but the same error was reported; the C code and setup.py code of the generated module are both based on the demo program in the Python source package.

My C program code

/* Example of embedding Python in another program */

#include "python2.7/Python.h"
#include <wiringPi.h>

void initdht11(void); /* Forward */

int main(int argc, char **argv)
{
    /* Initialize the Python interpreter.  Required. */
    Py_Initialize();

    /* Add a static module */
    initdht11();

    /* Exit, cleaning up the interpreter */
    Py_Exit(0);
    return 0;
}

/* A static module */

/* 'self' is not used */
static PyObject *
dht11_foo(PyObject *self, PyObject* args)
{
    wiringPiSetup();
    return PyInt_FromLong(42L);
}

static PyMethodDef dht11_methods[] = {
    {"foo",             dht11_foo,      METH_NOARGS,
     "Return the meaning of everything."},
    {NULL,              NULL}           /* sentinel */
};

void
initdht11(void)
{
    PyImport_AddModule("dht11");
    Py_InitModule("dht11", dht11_methods);
}

setup.py

from distutils.core import setup, Extension

dht11module = Extension('dht11',
                    library_dirs = ['/usr/lib'],
                    include_dirs = ['/usr/include'],
                    sources = ['math.c'])

setup (name = 'dht11',
       version = '1.0',
       description = 'This is a demo package',
       author = 'Martin v. Loewis',
       author_email = 'martin@v.loewis.de',
       url = 'https://docs.python.org/extending/building',
       long_description = '''
This is really just a demo package.
''',
       ext_modules = [dht11module])

error message

Traceback (most recent call last):
  File "test.py", line 1, in <module>
    import dht11
ImportError: /usr/local/lib/python2.7/dist-packages/dht11.so: undefined symbol: wiringPiSetup

Answer 1

Hey, I woke up in the morning and suddenly thought of it, so I quickly tried it.

This problem occurs because you need to add -lwiringPi 选项来引用这个库，但是我仔细看了以下执行 python setup.py build 之后执行的编译命令，根本就没有加这个选项，解决方式很简单，只需要修改一下setup.py，在Extension里面加上 libraries = ['wiringPi'] this parameter when compiling. The modified setup.py becomes as follows

from distutils.core import setup, Extension

dht11module = Extension('dht11',
                    library_dirs = ['/usr/lib'], #指定库的目录
                    include_dirs = ['/usr/include'], #制定头文件的目录
                    libraries = ['wiringPi'], #指定库的名称
                    sources = ['math.c'])

setup (name = 'dht11',
       version = '1.0',
       description = 'This is a demo package',
       author = 'Martin v. Loewis',
       author_email = 'martin@v.loewis.de',
       url = 'https://docs.python.org/extending/building',
       long_description = '''
This is really just a demo package.
''',
       ext_modules = [dht11module])