problems when linking objects with g++ - c++

Question

I am trying to make a simple obj loader with opengl in c++.
Compiling everything with one command works fine,
g++ -o main main.cpp timer.cpp screen.cpp obj_loader.cpp `sdl-config --cflags --libs` `pkg-config --cflags --libs glu`
returns no errors.
compiling objects individually works fine too,
g++ main.cpp -o main.o -c -Wall `sdl-config --cflags --libs` `pkg-config glu --cflags --libs`
g++ obj_loader.cpp -o obj_loader.o -c -Wall `sdl-config --cflags --libs` `pkg-config glu --cflags --libs`
g++ timer.cpp -o timer.o -c -Wall `sdl-config --cflags --libs` `pkg-config glu --cflags --libs`
g++ screen.cpp -o screen.o -c -Wall `sdl-config --cflags --libs` `pkg-config glu --cflags --libs`
also returns no errors.
However, when running the final
g++ main.o obj_loader.o timer.o screen.o -o main
I get a bunch of undefined reference errors:
main.o: In function `draw()':
main.cpp:(.text+0x1d): undefined reference to `glColor3f'
main.cpp:(.text+0x27): undefined reference to `glBegin'
main.cpp:(.text+0x36): undefined reference to `glVertex2i'
main.cpp:(.text+0x45): undefined reference to `glVertex2i'
main.cpp:(.text+0x54): undefined reference to `glVertex2i'
main.cpp:(.text+0x63): undefined reference to `glVertex2i'
main.cpp:(.text+0x68): undefined reference to `glEnd'
main.o: In function `main':
main.cpp:(.text+0xf8): undefined reference to `SDL_PollEvent'
main.cpp:(.text+0x10b): undefined reference to `glClear'
main.cpp:(.text+0x115): undefined reference to `SDL_GL_SwapBuffers'
main.cpp:(.text+0x11a): undefined reference to `glFinish'
main.cpp:(.text+0x14e): undefined reference to `SDL_Delay'
timer.o: In function `Timer::start()':
timer.cpp:(.text+0x4d): undefined reference to `SDL_GetTicks'
timer.o: In function `Timer::pause()':
timer.cpp:(.text+0xa6): undefined reference to `SDL_GetTicks'
timer.o: In function `Timer::unpause()':
timer.cpp:(.text+0xe5): undefined reference to `SDL_GetTicks'
timer.o: In function `Timer::tick()':
timer.cpp:(.text+0x136): undefined reference to `SDL_GetTicks'
timer.o: In function `Timer::get_ticks()':
timer.cpp:(.text+0x172): undefined reference to `SDL_GetTicks'
screen.o: In function `init()':
screen.cpp:(.text+0xa): undefined reference to `SDL_Init'
screen.cpp:(.text+0x31): undefined reference to `SDL_SetVideoMode'
screen.cpp:(.text+0x64): undefined reference to `SDL_WM_SetCaption'
screen.o: In function `init_GL()':
screen.cpp:(.text+0x80): undefined reference to `glClearColor'
screen.cpp:(.text+0x8a): undefined reference to `glMatrixMode'
screen.cpp:(.text+0x8f): undefined reference to `glLoadIdentity'
screen.cpp:(.text+0xc0): undefined reference to `glOrtho'
screen.cpp:(.text+0xca): undefined reference to `glMatrixMode'
screen.cpp:(.text+0xcf): undefined reference to `glLoadIdentity'
screen.cpp:(.text+0xd4): undefined reference to `glGetError'
screen.o: In function `clean_up()':
screen.cpp:(.text+0xf4): undefined reference to `SDL_Quit'
collect2: ld returned 1 exit status
My included libraries are:
#include <iostream>
#include <fstream>
#include <string>
#include <vector>
#include "SDL/SDL.h"
#include "SDL/SDL_opengl.h"
#include "GL/gl.h"
#include "GL/glu.h"
and my Makefile:
CC=g++
SDL_FLAGS=`sdl-config --cflags --libs`
GL_FLAGS=`pkg-config glu --cflags --libs`
CFLAGS=-c -Wall
FLAGS=$(CFLAGS) $(SDL_FLAGS) $(GL_FLAGS)
LDFLAGS=
SOURCES=main.cpp obj_loader.cpp timer.cpp screen.cpp
OBJECTS=$(SOURCES:.cpp=.o)
EXECUTABLE=main
all: $(SOURCES) $(EXECUTABLE)
$(EXECUTABLE): $(OBJECTS)
$(CC) $(OBJECTS) -o $# $(LDFLAGS)
.cpp.o:
$(CC) $< -o $# $(FLAGS)
clean:
rm -f *o main

Answer 1

Yeah... all those things being returned by --libs? You need those in order to link the final executable correctly.

Answer 2

sdl-config and pkg-config take two flags: --cflags and --libs. The first of these flags is needed when you're compiling source files into object files. The second of these flags is needed when you're linking. When you compiled everything with a single command, both phases were handled by a single command, so it was natural that you should pass both --cflags and --libs to sdl-config and pkg-config at the same time. Now that you've separated those phases, you need to pass the right option to sdl-config and pkg-config at the right time.
g++ main.cpp -o main.o -c -Wall `sdl-config --cflags` `pkg-config glu --cflags`
g++ obj_loader.cpp -o obj_loader.o -c -Wall `sdl-config --cflags` `pkg-config glu --cflags`
g++ timer.cpp -o timer.o -c -Wall `sdl-config --cflags` `pkg-config glu --cflags`
g++ screen.cpp -o screen.o -c -Wall `sdl-config --cflags` `pkg-config glu --cflags`
g++ main.o obj_loader.o timer.o screen.o -o main `sdl-config --libs` `pkg-config glu --libs`
In your makefile, this means configuring the variables as follows: (I've also made a few other minor fixes to your Makefile)
CC=g++
LD=g++
CFLAGS=-c -Wall $(shell sdl-config --cflags) $(shell pkg-config glu --cflags)
LDFLAGS=$(shell sdl-config --libs) $(shell pkg-config glu --libs)
SOURCES=main.cpp obj_loader.cpp timer.cpp screen.cpp
OBJECTS=$(SOURCES:.cpp=.o)
EXECUTABLE=main
all: $(EXECUTABLE)
$(EXECUTABLE): $(OBJECTS)
$(LD) $(LDFLAGS) $(OBJECTS) -o $#
.cpp.o:
$(CC) $(CFLAGS) $< -o $#
clean:
rm -f *.o main
There's lots more to critique about your Makefile -- you could probably eliminate most of the action lines here, because GNU Make has them built-in as implicits. You probably need to add rules that specify the dependencies of the .o files -- each .cpp file probably includes .h files, and make needs to know to recompile the .cpp files when the .h files they depend on change.

Answer 3

I don't see any -llibraryname to tell g++ to link with opengl. I would have expected to see this on the g++ line that builds main itself (i.e. the line that ends in 'main'). As far as I know, the .o files by themselves do not contain information about where to find the libraries to resolve their references. That information must be passed in as -l switches on the command line.