You should call C from Python by writing a ctypes wrapper. Cython is for making python-like code run faster, ctypes is for making C functions callable from python. What you need to do is the following:
- Write the C functions you want to use. (You probably did this already)
- Create a shared object (.so, for linux, os x, etc) or dynamically loaded library (.dll, for windows) for those functions. (Maybe you already did this, too)
- Write the ctypes wrapper (It's easier than it sounds, I wrote a how-to for that)
- Call a function from that wrapper in Python. (This is just as simple as calling any other python function)
You should call C from Python by writing a ctypes wrapper. Cython is for making python-like code run faster, ctypes is for making C functions callable from python. What you need to do is the following:
- Write the C functions you want to use. (You probably did this already)
- Create a shared object (.so, for linux, os x, etc) or dynamically loaded library (.dll, for windows) for those functions. (Maybe you already did this, too)
- Write the ctypes wrapper (It's easier than it sounds, I wrote a how-to for that)
- Call a function from that wrapper in Python. (This is just as simple as calling any other python function)
If I understand well, you have no preference for dialoging as c => python or like python => c.
In that case I would recommend Cython. It is quite open to many kinds of manipulation, specially, in your case, calling a function that has been written in Python from C.
Here is how it works (public api) :
The following example assumes that you have a Python Class (self is an instance of it), and that this class has a method (name method) you want to call on this class and deal with the result (here, a double) from C. This function, written in a Cython extension would help you to do this call.
cdef public api double cy_call_func_double(object self, char* method, bint *error):
if (hasattr(self, method)):
error[0] = 0
return getattr(self, method)();
else:
error[0] = 1
On the C side, you'll then be able to perform the call like so :
PyObject *py_obj = ....
...
if (py_obj) {
int error;
double result;
result = cy_call_func_double(py_obj, (char*)"initSimulation", &error);
cout << "Do something with the result : " << result << endl;
}
Where PyObject is a struct provided by Python/C API
After having caught the py_obj (by casting a regular python object, in your cython extension like this : <PyObject *>my_python_object), you would finally be able to call the initSimulation method on it and do something with the result.
(Here a double, but Cython can deal easily with vectors, sets, ...)
Well, I am aware that what I just wrote can be confusing if you never wrote anything using Cython, but it aims to be a short demonstration of the numerous things it can do for you in term of merging.
By another hand, this approach can take more time than recoding your Python code into C, depending on the complexity of your algorithms. In my opinion, investing time into learning Cython is pertinent only if you plan to have this kind of needs quite often...
Hope this was at least informative...
How do you call Python code from C code? - Stack Overflow
What's the typical way to call a Python program within a C program?
Calling a python method from C/C++, and extracting its return value - Stack Overflow
Calling C communication functions via Python?
What is the advantage of calling Python code from C?
What are the different ways of compiling Python code from C?
I recommend the approaches detailed here. It starts by explaining how to execute strings of Python code, then from there details how to set up a Python environment to interact with your C program, call Python functions from your C code, manipulate Python objects from your C code, etc.
EDIT: If you really want to go the route of IPC, then you'll want to use the struct module or better yet, protlib. Most communication between a Python and C process revolves around passing structs back and forth, either over a socket or through shared memory.
I recommend creating a Command struct with fields and codes to represent commands and their arguments. I can't give much more specific advice without knowing more about what you want to accomplish, but in general I recommend the protlib library, since it's what I use to communicate between C and Python programs (disclaimer: I am the author of protlib).
Have you considered just wrapping your python application in a shell script and invoking it from within your C application?
Not the most elegant solution, but it is very simple.
I have a C program and have to do some HTML parsing. I can do it in C it's just that I can write a Python script to parse what I need in way less time.
My thought is to save the HTML in a string in the C program, call my python script with the html string as an arg with exec(), and then use what it returns in the C program.
my question is, is this way best practices or is it a hacky way to do such a thing?
As explained before, using PyRun_SimpleString seems to be a bad idea.
You should definitely use the methods provided by the C-API (http://docs.python.org/c-api/).
Reading the introduction is the first thing to do to understand the way it works.
First, you have to learn about PyObject that is the basic object for the C API. It can represent any kind of python basic types (string, float, int,...).
Many functions exist to convert for example python string to char* or PyFloat to double.
First, import your module :
PyObject* myModuleString = PyString_FromString((char*)"mytest");
PyObject* myModule = PyImport_Import(myModuleString);
Then getting a reference to your function :
PyObject* myFunction = PyObject_GetAttrString(myModule,(char*)"myabs");
PyObject* args = PyTuple_Pack(1,PyFloat_FromDouble(2.0));
Then getting your result :
PyObject* myResult = PyObject_CallObject(myFunction, args)
And getting back to a double :
double result = PyFloat_AsDouble(myResult);
You should obviously check the errors (cf. link given by Mark Tolonen).
If you have any question, don't hesitate. Good luck.
Here is a sample code I wrote (with the help of various online sources) to send a string to a Python code, then return a value.
Here is the C code call_function.c:
#include <Python.h>
#include <stdlib.h>
int main()
{
// Set PYTHONPATH TO working directory
setenv("PYTHONPATH",".",1);
PyObject *pName, *pModule, *pDict, *pFunc, *pValue, *presult;
// Initialize the Python Interpreter
Py_Initialize();
// Build the name object
pName = PyString_FromString((char*)"arbName");
// Load the module object
pModule = PyImport_Import(pName);
// pDict is a borrowed reference
pDict = PyModule_GetDict(pModule);
// pFunc is also a borrowed reference
pFunc = PyDict_GetItemString(pDict, (char*)"someFunction");
if (PyCallable_Check(pFunc))
{
pValue=Py_BuildValue("(z)",(char*)"something");
PyErr_Print();
printf("Let's give this a shot!\n");
presult=PyObject_CallObject(pFunc,pValue);
PyErr_Print();
} else
{
PyErr_Print();
}
printf("Result is %d\n",PyInt_AsLong(presult));
Py_DECREF(pValue);
// Clean up
Py_DECREF(pModule);
Py_DECREF(pName);
// Finish the Python Interpreter
Py_Finalize();
return 0;
}
Here is the Python code, in file arbName.py:
def someFunction(text):
print 'You passed this Python program '+text+' from C! Congratulations!'
return 12345
I use the command gcc call_function.c -I/usr/include/python2.6 -lpython2.6 ; ./a.out to run this process. I'm on redhat. I recommend using PyErr_Print(); for error checking.