How do I link and built a dynamic link library correctly?

Question

I am in linux. My Makefile file is this

main2: main.cpp
g++ -c $(LIBS) $(CFLAGS) -fPIC main.cpp
g++ -shared main.o -o main.so 

Where,

SDL_CFLAGS := $(shell sdl-config --cflags)
SDL_LDFLAGS := $(shell sdl-config --libs)
CC = gcc
COPTS = -g -Wall
CFLAGS = $(SDL_CFLAGS)
LIBS = -lstdc++ -lSDL $(SDL_LDFLAGS) -L/usr/X11R6/lib -lGL -lGLU

which runs

g++ -c -lstdc++ -lSDL -L/usr/lib -lSDL -L/usr/X11R6/lib -lGL -lGLU -I/usr/include/SDL -D_GNU_SOURCE=1 -D_REENTRANT -fPIC main.cpp

g++ -shared main.o -o main.so

Now, this works without error. main.o and main.so files are produced.

However, when I try to link main.os with python ctypes

from ctypes import * import os
libtest = cdll.LoadLibrary(os.getcwd()+ '/main.so') libtest.main_loop()
libtest.main_loop()

I get this error

>>> libtest = cdll.LoadLibrary(os.getcwd() + '/main.so')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.6/ctypes/__init__.py", line 431, in LoadLibrary
    return self._dlltype(name)
  File "/usr/lib/python2.6/ctypes/__init__.py", line 353, in __init__
    self._handle = _dlopen(self._name, mode)
OSError: /home/atomos/DF/open_gl_client/ctypes_client/main.so: undefined symbol: glEnd

I am not sure if I am creating the linked library correctly. How do I create a linked library that I can load?

Do I have to create a .o and .os file for every library I import from main.cpp or is that taken care of automatically?

I do not understand what the compiler or linker is doing, however it works for a simple example with no imports, but for cpp files which import opengl libraries, it gives that error.

---- Update---- ldd against main.so yields

ldd main.so
        linux-gate.so.1 =>  (0xb7755000)
        libstdc++.so.6 => /usr/lib/libstdc++.so.6 (0xaf625000)
        libm.so.6 => /lib/tls/i686/cmov/libm.so.6 (0xaf5ff000)
        libc.so.6 => /lib/tls/i686/cmov/libc.so.6 (0xaf4a4000)
        libgcc_s.so.1 => /lib/libgcc_s.so.1 (0xaf485000)
        /lib/ld-linux.so.2 (0xb7756000)

---- Update ----

I ran g++ without -shared flag in the second compilation step and received this error

 g++ main.o -o main.so
main.o: In function `Texture_map_list::add_texture(std::basic_string<char, std::char_traits<char>, std::allocator<char> >)':
main.cpp:(.text+0xf0fe): undefined reference to `glGenTextures'
main.cpp:(.text+0xf2ba): undefined reference to `glBindTexture'
main.cpp:(.text+0xf2d7): undefined reference to `glTexParameterf'
main.cpp:(.text+0xf2f4): undefined reference to `glTexParameterf'
main.cpp:(.text+0xf310): undefined reference to `glTexParameteri'
main.cpp:(.text+0xf32c): undefined reference to `glTexParameteri'
main.cpp:(.text+0xf365): undefined reference to `gluBuild2DMipmaps'
main.o: In function `init()':
main.cpp:(.text+0xf457): undefined reference to `SDL_Init'
main.cpp:(.text+0xf46d): undefined reference to `SDL_WM_SetCaption'
main.cpp:(.text+0xf472): undefined reference to `SDL_GetVideoInfo'
main.cpp:(.text+0xf480): undefined reference to `SDL_GetError'
main.cpp:(.text+0xf497): undefined reference to `SDL_Quit'
main.cpp:(.text+0xf4e2): undefined reference to `SDL_GL_SetAttribute'
main.cpp:(.text+0xf505): undefined reference to `SDL_SetVideoMode'
main.cpp:(.text+0xf513): undefined reference to `SDL_GetError'
main.cpp:(.text+0xf52a): undefined reference to `SDL_Quit'
main.cpp:(.text+0xf559): undefined reference to `glClearColor'
main.cpp:(.text+0xf565): undefined reference to `glEnable'
main.cpp:(.text+0xf571): undefined reference to `glMatrixMode'
main.cpp:(.text+0xf576): undefined reference to `glLoadIdentity'
main.cpp:(.text+0xf5a2): undefined reference to `gluPerspective'
main.o: In function `process_keypresses()':
main.cpp:(.text+0x10678): undefined reference to `SDL_PollEvent'
main.cpp:(.text+0x109a1): undefined reference to `SDL_PollEvent'
main.o: In function `main_loop':
main.cpp:(.text+0x10d76): undefined reference to `SDL_GetKeyState'
main.cpp:(.text+0x10d9f): undefined reference to `SDL_Quit'
main.o: In function `render()':
main.cpp:(.text+0x10e00): undefined reference to `SDL_GetMouseState'
main.cpp:(.text+0x10e90): undefined reference to `SDL_GetMouseState'
main.cpp:(.text+0x10f94): undefined reference to `glClear'
main.cpp:(.text+0x10fa0): undefined reference to `glMatrixMode'
main.cpp:(.text+0x10fa5): undefined reference to `glLoadIdentity'
main.cpp:(.text+0x11081): undefined reference to `gluLookAt'
main.cpp:(.text+0x11120): undefined reference to `glTranslatef'
main.cpp:(.text+0x1114d): undefined reference to `glRotatef'
main.cpp:(.text+0x1117a): undefined reference to `glRotatef'
main.cpp:(.text+0x1119e): undefined reference to `SDL_GL_SwapBuffers'
main.o: In function `draw_plane(float, float, float)':
main.cpp:(.text+0x111d8): undefined reference to `glColor3f'
main.cpp:(.text+0x111e4): undefined reference to `glBegin'
main.cpp:(.text+0x1120b): undefined reference to `glVertex3f'

....

Answer 1

First, note that this is easier to debug if you don't build a shared library. You are building with -shared so the linker errors don't happen until you dynamically load the library in Python. Turn off -shared while you are debugging, and you will see the errors on the command-line when you try to link:

g++ main.o -o main
main.o: In function `main':
main.cpp:(.text+0x1d): undefined reference to `glBegin'
main.cpp:(.text+0x22): undefined reference to `glEnd'
collect2: ld returned 1 exit status

Now the problem is that you are passing linker arguments to the compiler. I see that you have nicely separated CFLAGS and LIBS in the Makefile. Good. But you are passing both $(LIBS) and $(CFLAGS) on the first line, to build main.o. The LIBS will be ignored on that line, because they are link flags.

On the second line, where you are actually building the final executable, you do not pass $(LIBS), so the program is not linked with libGL or any other libraries.

Therefore, simply fix your makefile thusly:

main2: main.cpp
    g++ -c $(CFLAGS) -fPIC main.cpp
    g++ -shared main.o $(LIBS) -o main.so

Edit: I have since realised that in GCC, you must always put the libraries after the object files, so I have changed the last line of the makefile accordingly.