Ctypes "symbol not found" for dynamic library in OSX

Question

Hello.

I have made a C++ library and have built a .dylib dynamic library from it. However when I load it with ctypes, it fails. Something doesn't seem to have linked properly. I have no idea why. The error (The relevant part):

    cscalelib.setup_framebuffer(flip,surface.frame_buffer,surface.texture,surface._scale[0],surface._scale[1])
  File "/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/ctypes/__init__.py", line 325, in __getattr__
    func = self.__getitem__(name)
  File "/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/ctypes/__init__.py", line 330, in __getitem__
    func = self._FuncPtr((name_or_ordinal, self))
AttributeError: dlsym(0x56ecd0, setup_framebuffer): symbol not found

Here's the C++ code which is still in progress but should work with what I have so far.

#include <OpenGL/gl.h>
#include <OpenGL/glu.h>
#include <vector.h>

void setup_framebuffer(bool flip,GLuint frame_buffer_id,GLuint texture_id,int width,int height){
    glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, frame_buffer_id);
    glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, GL_TEXTURE_2D, texture_id, 0);
    glPushAttrib(GL_VIEWPORT_BIT);
    glViewport(0,0,width,height);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity(); //Load the projection matrix
    if (flip){
        gluOrtho2D(0,width,height,0);
    }else{
        gluOrtho2D(0,width,0,height);
    }
}
void end_framebuffer(){
    glPopAttrib();
    glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity(); //Load the projection matrix
    gluOrtho2D(0,1280,720,0); //Set an orthorgraphic view
}
void add_lines(bool antialias,vector< vector<double> > coordinates,double w,double r,double g, double b,double a){
    glDisable(GL_TEXTURE_2D);
    if (antialias){
        glEnable(GL_LINE_SMOOTH); //Enable line smoothing.
    }
    glColor4d(r,g,b,a);
    glLineWidth(w);
    glBegin(GL_LINE_STRIP);
    for (int x = 0; x < coordinates.size(); x++) {
        glVertex2d(coordinates[x][0],coordinates[x][1]);
    }
    glEnd();
    if (antialias){
        glDisable(GL_LINE_SMOOTH); //Disable line smoothing.
    }
    glEnable(GL_TEXTURE_2D);
}

I compiled it with:

g++ -dynamiclib CPPEXTSCALELIB.cp -framework opengl -arch i386 -o CPPEXTSCALELIB.dylib

Here's the Python code with "..." to represent irrelevant parts.

...
from ctypes import *
...
cscalelib = CDLL(os.path.dirname(sys.argv[0]) + "/CPPEXTSCALELIB.dylib")
...
def setup_framebuffer(surface,flip=False):
    #Create texture if not done already
    if surface.texture is None:
        create_texture(surface)
    #Render child to parent
    if surface.frame_buffer is None:
        surface.frame_buffer = glGenFramebuffersEXT(1)
    cscalelib.setup_framebuffer(flip,surface.frame_buffer,surface.texture,surface._scale[0],surface._scale[1])
...

Thank you.

Answer 1

The problem is most likely the fact that you are using C++, and hence the function name will be mangled and use C++ calling conventions. If you declare the function with extern "C" then it should be exported in such a way as to callable from C code (and from Python's CTypes module).