How do you call Python code from C code?

See the relevant chapter in the manual: http://docs.python.org/extending/

Essentially you’ll have to embed the python interpreter into your program.

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 haven’t used an IPC approach for Python<->C communication but it should work pretty well. I would have the C program do a standard fork-exec and use redirected stdin and stdout in the child process for the communication. A nice text-based communication will make it very easy to develop and test the Python program.

If I had decided to go with IPC, I’d probably splurge with XML-RPC — cross-platform, lets you easily put the Python server project on a different node later if you want, has many excellent implementations (see here for many, including C and Python ones, and here for the simple XML-RPC server that’s part the Python standard library — not as highly scalable as other approaches but probably fine and convenient for your use case).

It may not be a perfect IPC approach for all cases (or even a perfect RPC one, by all means!), but the convenience, flexibility, robustness, and broad range of implementations outweigh a lot of minor defects, in my opinion.

apparently Python need to be able to compile to win32 dll, it will solve the problem

In such a way that converting c# code to win32 dlls will make it usable by any development tool

This seems quite nice http://thrift.apache.org/, there is even a book about it.

Details:

The Apache Thrift software framework, for scalable cross-language
services development, combines a software stack with a code generation
engine to build services that work efficiently and seamlessly between
C++, Java, Python, PHP, Ruby, Erlang, Perl, Haskell, C#, Cocoa,
JavaScript, Node.js, Smalltalk, OCaml and Delphi and other languages.

I’ve used the “standard” approach of Embedding Python in Another Application. But it’s complicated/tedious. Each new function in Python is painful to implement.

I saw an example of Calling PyPy from C. It uses CFFI to simplify the interface but it requires PyPy, not Python. Read and understand this example first, at least at a high level.

I modified the C/PyPy example to work with Python. Here’s how to call Python from C using CFFI.

My example is more complicated because I implemented three functions in Python instead of one. I wanted to cover additional aspects of passing data back and forth.

The complicated part is now isolated to passing the address of api to Python. That only has to be implemented once. After that it’s easy to add new functions in Python.

interface.h

// These are the three functions that I implemented in Python.
// Any additional function would be added here.
struct API {
    double (*add_numbers)(double x, double y);
    char* (*dump_buffer)(char *buffer, int buffer_size);
    int (*release_object)(char *obj);
};

test_cffi.c

//
// Calling Python from C.
// Based on Calling PyPy from C:
// http://doc.pypy.org/en/latest/embedding.html#more-complete-example
//

#include <stdio.h>
#include <assert.h>

#include "Python.h"

#include "interface.h"

struct API api;   /* global var */

int main(int argc, char *argv[])
{
    int rc;

    // Start Python interpreter and initialize "api" in interface.py using 
    // old style "Embedding Python in Another Application":
    // https://docs.python.org/2/extending/embedding.html#embedding-python-in-another-application
    PyObject *pName, *pModule, *py_results;
    PyObject *fill_api;
#define PYVERIFY(exp) if ((exp) == 0) { fprintf(stderr, "%s[%d]: ", __FILE__, __LINE__); PyErr_Print(); exit(1); }

    Py_SetProgramName(argv[0]);  /* optional but recommended */
    Py_Initialize();
    PyRun_SimpleString(
            "import sys;"
            "sys.path.insert(0, '.')" );

    PYVERIFY( pName = PyString_FromString("interface") )
    PYVERIFY( pModule = PyImport_Import(pName) )
    Py_DECREF(pName);
    PYVERIFY( fill_api = PyObject_GetAttrString(pModule, "fill_api") )

    // "k" = [unsigned long],
    // see https://docs.python.org/2/c-api/arg.html#c.Py_BuildValue
    PYVERIFY( py_results = PyObject_CallFunction(fill_api, "k", &api) )
    assert(py_results == Py_None);

    // Call Python function from C using cffi.
    printf("sum: %fn", api.add_numbers(12.3, 45.6));

    // More complex example.
    char buffer[20];
    char * result = api.dump_buffer(buffer, sizeof buffer);
    assert(result != 0);
    printf("buffer: %sn", result);

    // Let Python perform garbage collection on result now.
    rc = api.release_object(result);
    assert(rc == 0);

    // Close Python interpreter.
    Py_Finalize();

    return 0;
}

interface.py

import cffi
import sys
import traceback

ffi = cffi.FFI()
ffi.cdef(file('interface.h').read())

# Hold references to objects to prevent garbage collection.
noGCDict = {}

# Add two numbers.
# This function was copied from the PyPy example.
@ffi.callback("double (double, double)")
def add_numbers(x, y):
    return x + y

# Convert input buffer to repr(buffer).
@ffi.callback("char *(char*, int)")
def dump_buffer(buffer, buffer_len):
    try:
        # First attempt to access data in buffer.
        # Using the ffi/lib objects:
        # http://cffi.readthedocs.org/en/latest/using.html#using-the-ffi-lib-objects
        # One char at time, Looks inefficient.
        #data = ''.join([buffer[i] for i in xrange(buffer_len)])

        # Second attempt.
        # FFI Interface:
        # http://cffi.readthedocs.org/en/latest/using.html#ffi-interface
        # Works but doc says "str() gives inconsistent results".
        #data = str( ffi.buffer(buffer, buffer_len) )

        # Convert C buffer to Python str.
        # Doc says [:] is recommended instead of str().
        data = ffi.buffer(buffer, buffer_len)[:]

        # The goal is to return repr(data)
        # but it has to be converted to a C buffer.
        result = ffi.new('char []', repr(data))

        # Save reference to data so it's not freed until released by C program.
        noGCDict[ffi.addressof(result)] = result

        return result
    except:
        print >>sys.stderr, traceback.format_exc()
        return ffi.NULL

# Release object so that Python can reclaim the memory.
@ffi.callback("int (char*)")
def release_object(ptr):
    try:
        del noGCDict[ptr]
        return 0
    except:
        print >>sys.stderr, traceback.format_exc()
        return 1

def fill_api(ptr):
    global api
    api = ffi.cast("struct API*", ptr)

    api.add_numbers = add_numbers
    api.dump_buffer = dump_buffer
    api.release_object = release_object

Compile:

gcc -o test_cffi test_cffi.c -I/home/jmudd/pgsql-native/Python-2.7.10.install/include/python2.7 -L/home/jmudd/pgsql-native/Python-2.7.10.install/lib -lpython2.7

Execute:

$ test_cffi
sum: 57.900000
buffer: 'Tx9ex04x08xa8x93xffxbf]x86x04x08x00x00x00x00x00x00x00x00'
$ 

Few tips for binding it with Python 3

1. file() not supported, use open()

ffi.cdef(open('interface.h').read())

3. PyObject* PyStr_FromString(const char *u)

    Create a PyStr from a UTF-8 encoded null-terminated character buffer.
    Python 2: PyString_FromString
    Python 3: PyUnicode_FromString

Change to: PYVERIFY( pName = PyUnicode_FromString("interface") )

4. Program name

    wchar_t *name = Py_DecodeLocale(argv[0], NULL);
    Py_SetProgramName(name); 

5. for compiling

   gcc cc.c -o cc -I/usr/include/python3.6m -I/usr/include/x86_64-linux-gnu/python3.6m -lpython3.6m

6. I butchered dump def .. maybe it will give some ideas

def get_prediction(buffer, buffer_len):
    try:
        data = ffi.buffer(buffer, buffer_len)[:]
        result = ffi.new('char []', data)
      
        print('n I am doing something here here........',data )

        resultA = ffi.new('char []', b"Failed")  ### New message 

        ##noGCDict[ffi.addressof(resultA)] = resultA 
        return resultA
    except:
        print >>sys.stderr, traceback.format_exc()
        return ffi.NULL
} 

Hopefully it will help and save you some time