ctypes own library loads but can't access functions/methods

Question

I am trying to write a c routine to speed up a python script.

In order to route prove my method I first wrote a short test c function and compiled it into a library: the code for the test function is:

void cttest(void);
#include <stdio.h>

void cttest()
{
    printf("This is the shared module|n");
    return;
}

I saved this code into a file 'ct_test.cc'.

I then compiled this code as follows:

g++ -shared -fPIC -o /home/paula/libs/libcttest.so ct_test.cc

which gave no errors or warnings.

I then loaded ipython and typed the following:

import ctypes as ct

test=ct.CDLL("/home/paula/libs/libcttest.so")

if I now try to access the function (by typing 'test.' and then hitting tab) nothing happens.

If I do this:

What am I doing wrong?

Oh, just in case it makes any difference:

OS: xubuntu 14.10 Python: 2.7.8 ipython: 2.3.0 g++ : 4.9.1

The syntax used at docs.python.org/2/library/… is different. You could try their suggestion. — R Sahu
– R Sahu, Commented Jan 10, 2015 at 5:56
Thank you very much for taking the time to comment and for your suggestion. I have now tried the 'cdll.LoadLibrary' syntax and the result is the same. — Paula Thomas
– Paula Thomas, Commented Jan 10, 2015 at 6:26
Using C++, the name stored in the library will differ from just 'cttest' (there's name mangling). To prevent this, add extern "C" just before your first line to inform g++ that the C-style name should be stored instead. — brm
– brm, Commented Jan 10, 2015 at 9:50
I am delighted to tell you all that brm is right! Thankyou all for your help — Paula Thomas
– Paula Thomas, Commented Jan 10, 2015 at 10:42

Community · Accepted Answer · 2017-05-23 11:46:46Z

You have two problems:

Firstly as has already been noted tab auto-complete won't work for ctypes until a function has been used for the first time. (It wouldn't be impossible to make it work on most platforms, but it would add a fair overhead to loading the library I suspect).
You've written C and you're trying to call it like C, but you used a C++ compiler so you have a name mangling problem.

The solution to the first problem is trivial - use the full name and don't rely on auto-completion. There are at least 3 solutions to the second, I've written them here from best to worst:

Use a C compiler instead of C++ compiler. (gcc instead of g++).

Use extern "C" to force the C++ compiler to not mangle the function name:

extern "C" void cttest(void);

extern "C" 
{
    void cttest()
    {
        printf("This is the shared module\n");
        return;
    }
}

Use the mangled C++ name in Python, in this case it would be _Z6cttestv

To prove that this works I tried it out on Ubuntu 14.04:

gcc -shared -fPIC -o libcttest.so test.c

And using it in ipython:

ipython
Python 2.7.6 (default, Mar 22 2014, 22:59:38) 
Type "copyright", "credits" or "license" for more information.

IPython 1.2.1 -- An enhanced Interactive Python.
?         -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help      -> Python's own help system.
object?   -> Details about 'object', use 'object??' for extra details.

In [1]: import ctypes as ct

In [2]: test=ct.CDLL('./libcttest.so')

In [3]: test.cttest()
This is the shared module
Out[3]: 26

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