Nsryjdtyk

I have makefile that builds shared library libsimpletron.so:

#shell

MKDIR_P = mkdir -p

#compiler

CC = g++

#cpp flags

FLAGS      = -std=c++1z

CPPFLAGS   = -fPIC -Wall -Wextra 

LDFLAGS    = -shared

#directories

INC_DIR = ../inc

LIB_DIR = ../lib

BIN_DIR = ../bin

OBJ_DIR = ./obj

ALG_DIR = ./algebra 





SOURCES = $(shell echo *.cpp)

HEADERS = $(shell echo $(INC_DIR)/*.h)

_OBJECTS = $(SOURCES:.cpp=.o)

OBJECTS = $(patsubst %,$(OBJ_DIR)/%,$(_OBJECTS))



ALGEBRA = $(LIB_DIR)/libalgebra.so

TARGET = $(LIB_DIR)/libsimpletron.so



.PHONY: directories



all : directories $(ALGEBRA) $(TARGET)





directories: $(OBJ_DIR) $(BIN_DIR) $(LIB_DIR)



$(OBJ_DIR):

    $(MKDIR_P) $(OBJ_DIR)



$(BIN_DIR):

    $(MKDIR_P) $(BIN_DIR)



$(LIB_DIR):

    $(MKDIR_P) $(LIB_DIR)



$(ALGEBRA):

    $(MAKE) -C $(ALG_DIR)







$(OBJ_DIR)/%.o: %.cpp $(HEADERS)

    $(CC) -c $(FLAGS) $(CPPFLAGS) -o $@ $<



$(TARGET) : $(OBJECTS)

    $(CC) $(LDFLAGS) -o $@ $^





.PHONY: clean

clean:

    rm -f $(OBJ_DIR)/*.o

make command builds library successfully (I think) :

g++ -c -std=c++1z -fPIC -Wall -Wextra  -o obj/builder.o builder.cpp

...similar lines for each .cpp file...

g++ -shared -o ../lib/libsimpletron.so obj/builder.o obj/gradient_trainer.o obj/layer.o obj/neuron.o obj/perceptron.o obj/simpletron.o obj/trainer.o

Then I am trying to use this library. I compile my test xor as follows:

#compiler

CC = g++

#cpp flags

FLAGS       = -std=c++1z

PUGI        = -l pugixml

#directories

LIB_DIR = /home/lrdprdx/Projects/SimplePerceptron/lib

SIMPLETRON  = -lsimpletron

ALGEBRA     = -lalgebra



XOR     = xor

SOURCE  = xor.cpp



#config file

CONFIG  = config.xml





$(XOR) : $(SOURCE) $(CONFIG)

    $(CC) $(FLAGS) -L$(LIB_DIR) $(SIMPLETRON) $(ALGEBRA) $(PUGI) -o $(XOR) $(SOURCE)

But when I try to compile this I get errors of undefined reference to .... OK, though I do not understand why those errors exist, I found that adding a .cpp file with an empty main function to the directory with other .cpp files fixes all the stuff:

//empty.cpp

int main()

{

    return 0;

}

And after rebuild the shared library I make and execute xor successfully.
The question is: what is going on here?

I doubt that adding the extra file really fixed anything. Most likely some other side-effect helped.

Your problem is that your link line is incorrect. GCC, like most UNIX linkers, is a single-pass linker. That means it only walks all the libraries one time looking for unresolved symbols. And that means that the order of the arguments to the linker is critically important: you have to ensure that if item A references symbols in item B, that A comes before B on the link line.

In your example, you are putting all your libraries first, and your source files last. So, when make starts looking at your libraries it hasn't seen your sources yet, and there are no symbols it needs to link. By the time it compiles your sources, there are no libraries left to resolve symbols from.

Your link line should be arranged with sources and object files first, then libraries (in referencer ... referencee order):

$(CC) $(FLAGS) $(SOURCE) -L$(LIB_DIR) $(SIMPLETRON) $(ALGEBRA) $(PUGI) -o $(XOR)