python - ctypes - Beginner

Question

I have the task of "wrapping" a c library into a python class. The docs are incredibly vague on this matter. It seems they expect only advanced python users would implement ctypes. Well i'm a beginner in python and need help.

Some step by step help would be wonderful.

So I have my c library. What do I do? What files do I put where? How do I import the library? I read that there might be a way to "auto wrap" to Python?

(By the way I did the ctypes tutorial on python.net and it doesn't work. Meaning I'm thinking they are assuming I should be able to fill in the rest of the steps.

In fact this is the error I get with their code:

File "importtest.py", line 1
   >>> from ctypes import *
   SyntaxError: invalid syntax

I could really use some step by step help on this! Thanks~

Answer 1

Here's a quick and dirty ctypes tutorial.

First, write your C library. Here's a simple Hello world example:

testlib.c

#include <stdio.h>

void myprint(void);

void myprint()
{
    printf("hello world
");
}

Now compile it as a shared library (mac fix found here):

$ gcc -shared -Wl,-soname,testlib -o testlib.so -fPIC testlib.c

# or... for Mac OS X 
$ gcc -shared -Wl,-install_name,testlib.so -o testlib.so -fPIC testlib.c

Then, write a wrapper using ctypes:

testlibwrapper.py

import ctypes

testlib = ctypes.CDLL('/full/path/to/testlib.so')
testlib.myprint()

Now execute it:

$ python testlibwrapper.py

And you should see the output

Hello world
$

If you already have a library in mind, you can skip the non-python part of the tutorial. Make sure ctypes can find the library by putting it in /usr/lib or another standard directory. If you do this, you don't need to specify the full path when writing the wrapper. If you choose not to do this, you must provide the full path of the library when calling ctypes.CDLL().

This isn't the place for a more comprehensive tutorial, but if you ask for help with specific problems on this site, I'm sure the community would help you out.

PS: I'm assuming you're on Linux because you've used ctypes.CDLL('libc.so.6'). If you're on another OS, things might change a little bit (or quite a lot).