Compiling C++ project with libraries - c++

Question

My friends developed a C++ game in Visual Studio on Windows, and I would like to compile it on my Linux x64 machine. I'm not very familiar with C++ but I'm trying g++ on command-line. However I only get a bunch of undefined reference errors.
The basic file structure is:
Libraries/SFML-2.0/lib
Libraries/SFML-2.0/include
Libraries/SFML_Linux64/lib
Libraries/SFML_Linux64/include
Libraries/Box2D/lib
Libraries/Box2D/include
Libraries/Box2DLinux/lib
Libraries/Box2DLinux/include
Game
Game/lib
Game/includes
Game/... (other subdirectories)
I tried the following command:
g++ -Wall Multiplaya/app.cpp -I Libraries/SFML_Linux64/include/ -I Libraries/Box2DLinux/include/ -L Libraries/SFML_Linux64/lib/ -L Libraries/Box2DLinux/lib/
This is the kind of errors I get (some lines snipped and replaced with ...):
/usr/lib/gcc/x86_64-linux-gnu/4.7/../../../x86_64-linux-gnu/crt1.o: I funktionen "_start":
(.text+0x20): undefined reference to `main'
/tmp/ccFXe37c.o: I funktionen "mp::createNetworkThread(void*)":
app.cpp:(.text+0x10): undefined reference to `worldDataMutex'
app.cpp:(.text+0x15): undefined reference to `sf::Mutex::lock()'
...
/tmp/ccFXe37c.o: I funktionen "mp::App::exec()":
app.cpp:(.text+0x148): undefined reference to `mp::ResourceHandler::instance()'
app.cpp:(.text+0x15a): undefined reference to `mp::ResourceHandler::loadTexture(std::string)'
app.cpp:(.text+0x3d7): undefined reference to `mp::Window::Window(mp::WorldData*)'
app.cpp:(.text+0x406): undefined reference to `mp::Controller::Controller(mp::World*, mp::Window*)'
...
app.cpp:(.text+0x471): undefined reference to `sf::Mutex::unlock()'
app.cpp:(.text+0x4bb): undefined reference to `sf::Thread::launch()'
app.cpp:(.text+0x4d7): undefined reference to `sf::Clock::Clock()'
app.cpp:(.text+0x4e6): undefined reference to `sf::Clock::getElapsedTime() const'
...
collect2: fel: ld returnerade avslutningsstatus 1
(I hope you can look through the Swedish above.)

Answer 1

That's very courteous of you to provide library paths to the linker. However, linkers are ungrateful slobs and usually do not look for any library files on their own when they see an undefined reference to a function.
Like, undefined reference to `sf::Mutex::lock() — I bet there is libsfml-system.so.2.0 or whatever in the Libraries/SFML_Linux64/lib/ directory, with the definition for sf::Mutex::lock(). But the linker doesn't care. You have to say -lsfml-system at the end of your compile invocation.
This will make g++ understand to look for functions not only in libstdc++ library, but also in the libsfml-system file. And if g++ happen to find such a file in the default or additional (specified with -L flag) library directories, he will use it for resolving references to function calls.
But you have to say to it explicitly what library files you want to throw in, specifying only directories with libraries doesn't do much. So, try to use
g++ -Wall Multiplaya/app.cpp -I Libraries/SFML_Linux64/include/ -I Libraries/Box2DLinux/include/ -L Libraries/SFML_Linux64/lib/ -L Libraries/Box2DLinux/lib/ -lsfml-system
How to build C++ programs
C++ programs are built in two steps: the first step is compilation, and the second step is linking.
During compilation, you turn your source files into object files — things which contain compiled machine code. Now there is a trick you have to understand. Say, if you have a.cpp with
// a.cpp
int f(int x);
int main() {
return f(42);
}
you can compile it with g++ -c a.cpp and it will get you object file a.o (with compiled code), without any compilation errors. But wait! There is no actual definition of what f() is in a.cpp!
And now, when you go to the second step, linking, and invokes g++ -o test a.o, it will complain that there is undefined reference to f(). So let's make b.cpp with this text:
// b.cpp
int f(int x) {
return 2 * x - 3;
}
compile it with g++ -c b.cpp and then perform linking as g++ -o test a.o b.o — and wow, now it links without an error!
What happened? Well, when the compiler sees a function call, it puts in object file not an actual call instruction, but a placeholder that says "call a function with such and such name and such and such parameters". And then the linker takes a bunch of object files, and sews them together. When it sees such a placholder, it looks for the mentioned function in object files it was given, and puts actual call to it instead of the placeholder.
So, the building of C++ program looks something like this:
For each x.cpp file you have, call g++ -c x.cpp <bunch of flags> -I<include directories>
Then call g++ -o resultprogram a.o b.o c.o d.o ... <bunch of flags> -L<library directories> -l<additional libraries>
-l flag tells the linker that if he sees a call to a function, and there is no such function defined anywhere in the specified object files (a.o, b.o, etc), then it should look in this library. Please not, that the linker won't look in any object files and/or libraries except those you specified (okay, it will also look in standard C++ library libstdc++, but that's it).
However, this process is pretty boring to do by hand if you have 10 or more files. That's why people use "project files" and "build systems". When Visual Studio builds a project, it does all those steps I mentioned: it compiles every single file in a project, and then links the result together. On Linux, you don't have Visual Studio, but you have make utility. There are some utilities for converting VS projects to makefiles, I believe.