Skip to main content

Swig with C++ and external libs

This is the kind of problem I work out how to solve, then I forget... and eventually have to do again... So more for my sake here is how to compile a python extension in c++ that has external libraries like opencv. Automatically wrapped with swig from distutils...

So first up I have a random library of C++ that I want to use from python. One such C file is doing some low level math on an opencv IPL image. Since I want to use this in python but require it to be fast, I need to create an extension.

// The header file I want to wrap.
#include <stdio.h>

#include <stdlib.h>
#include <math.h>

#include "cv.h"

void remove_corneal_reflection(IplImage *image, IplImage *threshold_image, int sx, int sy, int window_size,
     int biggest_crr, int &crx, int &cry, int& crr, int *valid_point_calc);

This has a few normal includes and "cv.h". This is refering to the opencv installed NOT in the default dir but in /usr/local/.

Next is the instructions for swig to use when creating the wrapper for the file. This uses the autodoc feature so in python the docstrings have some (almost) usefull information about what the parameters are.

/* Interface to the C file remove_corneal_reflection.c */

%module removeCornealReflection
    /* Put header files here or function declarations like below */
    #include "opencv/cv.h"
    #include "remove_corneal_reflection.h"
// Tell swig to put type information into the functions docstrings...
%feature("autodoc", "1");

// Tell swig to parse the header file.
%include "remove_corneal_reflection.h"

To get distutils to do all the compiling and linking we need a file:

import distutils
from distutils.core import setup, Extension
    name = "Brian's C++ Library wrapped up all nice for python",
    author = 'Brian Thorne',
    author_email = '',
    license='GPL v3 :: GNU General Public License',  
    version = "0.1",
    ext_modules = [
            sources = ["interfaceFile.i","Hello World.cpp"],
            sources = ["remove_corneal_reflection.i","remove_corneal_reflection.cpp"],
            include_dirs = ['/usr/local/include/opencv']

And for the first time I also made a setup.cfg file with a whole two lines:


And then the whole thing can be wrapped up with the familiar command:

brian@brian-hitlab:~/dev/swig/linkedLib$ python build
running build
running build_ext
building '_helloWorld' extension
swigging interfaceFile.i to interfaceFile_wrap.cpp
swig -python -c++ -o interfaceFile_wrap.cpp interfaceFile.i
creating build
creating build/temp.linux-x86_64-2.6
gcc -pthread -fno-strict-aliasing -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -fPIC -I/usr/include/python2.6 -c interfaceFile_wrap.cpp -o build/temp.linux-x86_64-2.6/interfaceFile_wrap.o
cc1plus: warning: command line option "-Wstrict-prototypes" is valid for Ada/C/ObjC but not for C++
gcc -pthread -fno-strict-aliasing -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -fPIC -I/usr/include/python2.6 -c Hello World.cpp -o build/temp.linux-x86_64-2.6/Hello World.o
cc1plus: warning: command line option "-Wstrict-prototypes" is valid for Ada/C/ObjC but not for C++
g++ -pthread -shared -Wl,-O1 -Wl,-Bsymbolic-functions build/temp.linux-x86_64-2.6/interfaceFile_wrap.o build/temp.linux-x86_64-2.6/Hello World.o -o
building '_removeCornealReflection' extension
swigging remove_corneal_reflection.i to remove_corneal_reflection_wrap.cpp
swig -python -c++ -o remove_corneal_reflection_wrap.cpp remove_corneal_reflection.i
gcc -pthread -fno-strict-aliasing -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -fPIC -I/usr/local/include/opencv -I/usr/include/python2.6 -c remove_corneal_reflection_wrap.cpp -o build/temp.linux-x86_64-2.6/remove_corneal_reflection_wrap.o
cc1plus: warning: command line option "-Wstrict-prototypes" is valid for Ada/C/ObjC but not for C++
gcc -pthread -fno-strict-aliasing -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -fPIC -I/usr/local/include/opencv -I/usr/include/python2.6 -c remove_corneal_reflection.cpp -o build/temp.linux-x86_64-2.6/remove_corneal_reflection.o
cc1plus: warning: command line option "-Wstrict-prototypes" is valid for Ada/C/ObjC but not for C++
g++ -pthread -shared -Wl,-O1 -Wl,-Bsymbolic-functions build/temp.linux-x86_64-2.6/remove_corneal_reflection_wrap.o build/temp.linux-x86_64-2.6/remove_corneal_reflection.o -L/usr/local/lib/opencv -lcv -lhighgui -lcvaux -lcvaux -o

This creates two python extensions - one called helloWorld and one called removeCornealReflection. Ideal! Not sure why the warning message comes up, it looks like it is using gcc for the linking when surely g++ would be better... anyhow can't argue with positive results!

Popular posts from this blog

Driveby contribution to Python Cryptography

While at PyConAU 2016 I attended the Monday sprints and spent some time looking at a proposed feature I hoped would soon be part of cryptography. As most readers of this blog will know, cryptography is a very respected project within the Python ecosystem and it was an interesting experience to see how such a prominent open source project handles contributions and reviews.

The feature in question is the Diffie-Hellman Key Exchange algorithm used in many cryptography applications. Diffie-Helman Key Exchange is a way of generating a shared secret between two parties where the secret can't be determined by an eavesdropper observing the communication. DHE is extremely common - it is one of the primary methods used to provide "perfect forward secrecy" every time you initiate a TLS connection to an HTTPS website. Mathematically it is extremely elegant and the inventors were the recipients of the 2015 Turing award.

I wanted to write about this particular contribution because man…

My setup for downloading & streaming movies and tv

I recently signed up for Netflix and am retiring my headless home media pc. This blog will have to serve as its obituary. The box spent about half of its life running FreeNAS, and half running Archlinux. I’ll briefly talk about my experience with FreeNAS, the migration, and then I’ll get to the robust setup I ended up with.

The machine itself cost around $1000 in 2014. Powered by an AMD A4-7300 3.8GHz cpu with 8GB of memory. A SilverStone DS380 case is both functional, quiet and looks great. The hard drives have been updated over the last two years until it had a full compliment of 6 WD Green 4TiB drives - all spinning bits of metal though.

Initially I had the BSD based FreeNAS operating system installed. I had a single hard drive in its own ZFS pool for TV and Movies, and a second ZFS pool comprised of 5 hard drives for documents and photos.

FreeNAS is straight forward to use and setup, provided you only want to do things supported out of the box or by plugins. Each plugin is install…

Markdown Editor Component for Angular2

Thought I'd share a component I've been hacking on for angular2: a syntax highlighted markdown editor with rendered preview.

The code including a basic example is available on github. Because Angular2 hasn't yet been released this is really just me kicking the tyres.

This component relies on two libraries:

- marked for rendering markdown as html
- and ace editor for editing markdown
Basic Usage Example Add to your html template:
<markdown-editor (save)="updatedText($event)" [initial-text]="markdownContent"></markdown-editor> Remember to include the Markdowndirective in your @Component annotation:
@Component({ selector:'about', directives: [CORE_DIRECTIVES, Markdown] }) Another Example You can also control the component with external ui:
<button (click)="md.editMode = true">Custom Edit Button</button><markdown-editor [initial-text]="myMarkdownText" [show-edit-but…