Calling C function from Python (No Return Value)

Question

I used SWIG to connect my C++ program with Python. From cFunctions.py I call C function named checkVal like following:

lib = cdll.LoadLibrary('./test.so')

    xml_bytesTrx = bytearray(b'<xml>...</xml>')
    a1  = (ctypes.c_byte*513)(*xml_bytesTrx)

    class EFT_TRX(Structure):
        _fields_ = [
                    ("a2",c_ulong),
                    ("a3",c_char*16),
                    ("a4",c_char*4),
                    ("a5",c_ushort),
                    ("a6",c_char*41),
                    ("a7",c_char*13),
                    ("a8",c_char*21),
                    ("a1",c_byte*513)
                    ]


    Trx = EFT_TRX(0,"0",'CHF',0,'4506445638161117',"123456123456","202020",
                   a1)
def check(arg1, arg2):

    eftTransactionRes = lib.checkVal(arg1,arg2,Trx.a4,Trx.a5,
                                   Trx.a6,Trx.a7,Trx.a8,
                                   Trx.a1)

    Trx.a3 = arg2
    return eftTransactionRes

And in C header file (test.h) is defined like following:

long TEST_API checkVal(             
        _IN____ const unsigned long a2, 
        _INOUT_ char a3[16], 
        _IN____ const char a4[4], 
        _IN____ const unsigned short a5, 
        _IN____ const char a6[41], 
        _IN____ const char a7[13], 
        _IN____ const char a8[21],
        _INOUT_ char a1[513]
        );

Now I wrote a test Python code to call C functions (accessible from cFunctions.py). The issue is that , when I call "check" from my test code ( cFunctions.check(10,20) ) I reach it, but it never returns anything!

But if I call check from within cFunctions.py itself like this:

check(10,Trx.a2)

It returns the result. What am I doing wrong? Why check doesn't return anything when I'm calling it from my test.py ?

hm I am not sure. In your definition of check, the first two arguments to lib.checkVal(a2,a3,Trx.a4... are called without Trx reference. Maybe in cFunctions.py a2 and a3 are properly defined but not in the test code. Also you make never use of kulTrxType is this intended? — Bort
– Bort, Commented Apr 28, 2015 at 17:09
Thanks @Bort, That's a good point, I am going to try this.Also I have edited my question for kulTrxType. — AJ-B29
– AJ-B29, Commented Apr 28, 2015 at 17:33

Curious · Accepted Answer · 2015-04-28 18:32:48Z

Does this help? A little Googling led me here!

Calling C functions from Python - By Christian Stigen Larsen

Reproducing the above link here for reference. I have not tested the code listed here. Also all the credit belongs to the author of the above link.

Here’s a small tutorial on how to call your C functions from Python.

Let's make some simple functions in C. We'll call the file

myModule.c

#include <Python.h>

/*
 * Function to be called from Python
 */
static PyObject* py_myFunction(PyObject* self, PyObject* args)
{
  char *s = "Hello from C!";
  return Py_BuildValue("s", s);
}

/*
 * Another function to be called from Python
 */
static PyObject* py_myOtherFunction(PyObject* self, PyObject* args)
{
  double x, y;
  PyArg_ParseTuple(args, "dd", &x, &y);
  return Py_BuildValue("d", x*y);
}

/*
 * Bind Python function names to our C functions
 */
static PyMethodDef myModule_methods[] = {
  {"myFunction", py_myFunction, METH_VARARGS},
  {"myOtherFunction", py_myOtherFunction, METH_VARARGS},
  {NULL, NULL}
};

/*
 * Python calls this to let us initialize our module
 */
void initmyModule()
{
  (void) Py_InitModule("myModule", myModule_methods);
}

Compiling dynamic libraries on Mac OS X is different from the usual gcc -shared you might be used to:

gcc -dynamiclib -I/usr/include/python2.3/ -lpython2.3 -o myModule.dylib myModule.c

Now you have to do something awkward; rename myModule.dylib to myModule.so, so that Python will find the correct file (this is a bug in Python, it should've been fixed, but that's as far as I know):

mv myModule.dylib myModule.so

If you are using a system that supports -shared you can simply do this:

gcc -shared -I/usr/include/python2.3/ -lpython2.3 -o myModule.so myModule.c

On Windows you can reportedly type

gcc -shared -IC:\Python27\include -LC:\Python27\libs myModule.c -lpython27 -o myModule.pyd

Here's a simple program in Python to call your functions:

from myModule import *

print "Result from myFunction:", myFunction()
print "Result from myOtherFunction(4.0, 5.0):", myOtherFunction(4.0, 5.0)

The output is:

Result from myFunction(): Hello from C!
Result from myOtherFunction(4.0, 5.0): 20.0

If you are going to make bigger libraries available from Python I strongly suggest you check out SWIG or Boost Python.

Collectives™ on Stack Overflow

Calling C function from Python (No Return Value)

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related