makefile linking does not work (although no error message) - c++

Question

I am having issue with Makefile that I produced. It consists of one .cpp file with main() inside and I want to create executable from it. While putting in terminal make command I get following:
g++ STutorial.o -o MyExecutable
g++: error: STutorial.o: No such file or directory
g++: fatal error: no input files
While putting first make STutorial.o (.o created) and then make get this:
g++ STutorial.o -o MyExecutable
STutorial.o: In function `main':
STutorial.cpp:(.text+0x47a): undefined reference to `alcOpenDevice'
Firstly, why make does not go from the beginning?
Secondly, why this reference is undefined as if I did not include library, I did that in Makefile aswell as in STutorial.cpp file.
Can you please help me out? I was reading up what could I do wrong and see no clue. (I am beginner and maybe mistake is a rookie one, I apologise in advance but cannot understand it alone)
Makefile:
FLAGS += -std=c++11
CCX=g++
FLAGS +=-c -Wall #for compilation, for warning
FLAGS += -lopenal -lSDL
all: MyExecutable
MyExecutable:
$(CXX) STutorial.o -o MyExecutable
STutorial.o: STutorial.cpp
$(CXX) $(FLAGS) STutorial.cpp

Answer 1

Your makefile should be like this:
CCX=g++
FLAGS +=-c -Wall #for compilation, for warning
LINK_FLAGS += -lopenal -lSDL
all: MyExecutable
MyExecutable: Stutorial.o
$(CXX) STutorial.o -o MyExecutable $(LINK_FLAGS)
STutorial.o: STutorial.cpp
$(CXX) $(FLAGS) STutorial.cpp
Explanation:
Your MyExecutable depends on Stutorial.o which inturn depends on Stutorial.cpp
Now -c flag should be used only with .cpp file to create an object file and not with already created .o file.
Therefore you should have two flags: FLAGS for compiling and LINK_FLAGS for linking libraries during making executable file.

Answer 2

Your executable rule is the issue:
MyExecutable:
$(CXX) STutorial.o -o MyExecutable
It has a target (MyExecutable) and it has a recipe ($(CXX) ...), that all looks good. But what are its prerequisites? MyExecutable does have prerequisites - it needs STutorial.o in order to generate the binary! You need to explicitly tell make about this:
MyExecutable: STutorial.o
$(CXX) STutorial.o -o MyExecutable
Otherwise, you are telling make that you want to build all. all depends on MyExecutable. MyExecutable doesn't depend on anything, so the rule for STutorial.o never gets run.
As for the linker error, you're not linking in the library you need, so you should define something like:
LFLAGS += -lopenal -lSDL
MyExecutable: STutorial.o
$(CXX) STutorial.o $(LFLAGS) -o MyExecutable

Answer 3

You have a few problem in your Makefile starting with:
FLAGS +=-c -Wall #for compilation, for warning
FLAGS += -lopenal -lSDL
You are redefining the FLAGS variable here.
So what you should have is a different variable for your compiler and linker flags:
CFLAGS +=-c -Wall #for compilation, for warning
LDFLAGS += -lopenal -lSDL
Now, for the sake of giving a complete answer, and not solving your immediate problem only I'll try to show how to make the Makefile more flexible:
Start with the sources - you should have a variable for them as well; it's useful when adding/removing source files to/from the project:
SOURCES = STutorial.cpp
define a variable for your object files (this will come in handy at link-time):
OBJ = $(SOURCES:.cpp=.o)
Compile all source files into object files:
.cpp.o:
$(C++) $(CFLAGS) -o $# $<
Link your binary file using the compiled object files:
$(MyExecutable): $(OBJ)
$(C++) $(LDFLAGS) $(OBJ) -o $#
Add a clean command for completeness (removes the binary and object files):
clean:
$(RM) $(EXECUTABLE) $(OBJ)
Now, putting it all together:
CCX=g++
CFLAGS +=-c -Wall -std=c++11#for compilation, for warning
LDFLAGS += -lopenal -lSDL
SOURCES = STutorial.cpp
OBJ = $(SOURCES:.cpp=.o)
all: $(MyExecutable)
$(MyExecutable): $(OBJ)
$(CCX) $(LDFLAGS) $(OBJ) -o $#
.cpp.o:
$(CCx) $(CFLAGS) -o $# $<
clean:
$(RM) $(EXECUTABLE) $(OBJ)
This should allow you to flexibly build, rebuild, clean you project.

Answer 4

This is how you should do:
g++ -std=c++11 -Wall -lopenal -lSDL STutorial.cpp -o MyExecutable