Python2’s Building Python with C or C++

If you want make extension module with C or C++ above python 2.7

First You have to include Python.h on top of your c source file which gives you access to the internal Python API used to hook your module into interpreter. it is call CPython C-API.

Before including Python.h, install python2.7-dev wit apt on ubuntu like “sudo apt install python2.7-dev”

Then You need to connect c source file to python. Normally The signature of the C implementation of your function always takes one of the following three times:

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static PyObject *MyFunction( PyObject *self, PyObject *args );

static PyObject *MyFunctionWithKeywords(PyObject *self,
                                 PyObject *args,
                                 PyObject *kw);

static PyObject *MyFunctionWithNoArgs( PyObject *self );

i.e. You need to define python module function, its c function usually are named by combination the Python module and function names together like this :

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static PyObject *module_func(PyObject *self, PyObject *args) {
   /* Do your stuff here. */
   Py_RETURN_NONE;
}

The following example :

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static PyObject * spam_system(PyObject *self, PyObject *args)
{
    const char *command;
    int sts;

    if (!PyArg_ParseTuple(args, "s", &command))
        return NULL;
    sts = system(command);
    return Py_BuildValue("i", sts);
}

Second, you need to define method structure like this :

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struct PyMethodDef {
   char *ml_name;
   PyCFunction ml_meth;
   int ml_flags;
   char *ml_doc;
};

  • ml_name − This is the name of the function as the Python interpreter presents when it is used in Python programs.

  • ml_meth − This must be the address to a function that has any one of the signatures described in previous seection.

  • ml_flags − This tells the interpreter which of the three signatures ml_meth is using.

    • This flag usually has a value of METH_VARARGS.

    • This flag can be bitwise OR’ed with METH_KEYWORDS if you want to allow keyword arguments into your function.

    • This can also have a value of METH_NOARGS that indicates you do not want to accept any arguments.

  • ml_doc − This is the docstring for the function, which could be NULL if you do not feel like writing one.

The following is example.

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static PyMethodDef SpamMethods[] = {
    ...
    {"system",  spam_system, METH_VARARGS,
     "Execute a shell command."},
    ...
    {NULL, NULL, 0, NULL}        /* Sentinel */
};

As you can see above thing, this method table needs to be terminated a sentinel that consists of NULL and 0 Value for the appropriate members.

Finally, You need to define initname function like this :

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PyMODINIT_FUNC initspam(void)
{
    (void) Py_InitModule("spam", SpamMethods);
}

Be careful of the final function, initname have to use method table in python3

When the python program imports module spam for first time, initspam() is called.

Let’s sum up the previous process :

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#include <Python.h>

static PyObject *module_func(PyObject *self, PyObject *args) {
   /* Do your stuff here. */
   Py_RETURN_NONE;
}

static PyMethodDef module_methods[] = {
   { "func", (PyCFunction)module_func, METH_NOARGS, NULL },
   { NULL, NULL, 0, NULL }
};

PyMODINIT_FUNC initModule(void) {
   (void)Py_InitModule(func, module_methods);
}

But If you want to use multi-phase initialization as Python source distribution as Modules/xxmodule.c like this :

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static struct PyModuleDef xxmodule = {
    PyModuleDef_HEAD_INIT,
    "xx",
    module_doc,
    0,
    xx_methods,
    xx_slots,
    NULL,
    NULL,
    NULL
};

/* Export function for the module (*must* be called PyInit_xx) */

PyMODINIT_FUNC
PyInit_xx(void)
{
    return PyModuleDef_Init(&xxmodule);
}

This is using PyModuleDef_Init(&xxmodule) and look at module table, xxmodule,above.

From now on, just see an example.

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## In Example_of_Python_C_API.c

#include <Python.h>

static PyObject *
spam_system(PyObject *self, PyObject *args)
{
    const char *command;
    int sts;

    if (!PyArg_ParseTuple(args, "s", &command))
        return NULL;
    sts = system(command);
    return Py_BuildValue("i", sts);
}


static PyMethodDef SpamMethods[] = {
    {"system",  spam_system, METH_VARARGS,
     "Execute a shell command."},

    {NULL, NULL, 0, NULL}        /* Sentinel */
};

PyMODINIT_FUNC
initspam(void)
{
   (void) Py_InitModule("spam", SpamMethods);
}

python3 setup.py build_ext --inplace

The following is the result of the command above

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# hyunyoung2 @ hyunyoung2-desktop in ~/Labs/Konltk/CPython/Python2/Normal on git:master x [22:38:25] 
$ ./run.sh 
running build_ext
building 'spam' extension
x86_64-linux-gnu-gcc -pthread -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -fno-strict-aliasing -Wdate-time -D_FORTIFY_SOURCE=2 -g -fstack-protector-strong -Wformat -Werror=format-security -fPIC -I/usr/include/python2.7 -c Example_of_Python_C_API.c -o build/temp.linux-x86_64-2.7/Example_of_Python_C_API.o
x86_64-linux-gnu-gcc -pthread -shared -Wl,-O1 -Wl,-Bsymbolic-functions -Wl,-Bsymbolic-functions -Wl,-z,relro -fno-strict-aliasing -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -Wdate-time -D_FORTIFY_SOURCE=2 -g -fstack-protector-strong -Wformat -Werror=format-security -Wl,-Bsymbolic-functions -Wl,-z,relro -Wdate-time -D_FORTIFY_SOURCE=2 -g -fstack-protector-strong -Wformat -Werror=format-security build/temp.linux-x86_64-2.7/Example_of_Python_C_API.o -o /home/hyunyoung2/Labs/Konltk/CPython/Python2/Normal/spam.so

Let’s see setup.py

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from distutils.core import setup, Extension

setup(name="test",  version="1.0",\
     ext_modules=[Extension("spam", ["Example_of_Python_C_API.c"])])

The following is the result using spam.so file to import like this:

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# hyunyoung2 @ hyunyoung2-desktop in ~/Labs/Konltk/CPython/Python2/Normal on git:master x [22:38:28] 
$ python
Python 2.7.12 (default, Dec  4 2017, 14:50:18) 
[GCC 5.4.0 20160609] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import spam
>>> spam.system("ls -l")
total 64
drwxrwxr-x 4 hyunyoung2 hyunyoung2  4096  4월 18 22:38 build
-rw-rw-r-- 1 hyunyoung2 hyunyoung2   513  4월 18 22:38 Example_of_Python_C_API.c
-rw-rw-r-- 1 hyunyoung2 hyunyoung2  4665  4월 18 22:34 README.md
-rwxr-xr-x 1 hyunyoung2 hyunyoung2    58  4월 18 22:37 run.sh
-rw-rw-r-- 1 hyunyoung2 hyunyoung2   150  4월 18 22:19 setup.py
-rwxrwxr-x 1 hyunyoung2 hyunyoung2 17120  4월 18 22:36 spam.cpython-35m-x86_64-linux-gnu.so
-rwxrwxr-x 1 hyunyoung2 hyunyoung2 17464  4월 18 22:38 spam.so
0

Reference