Why does undefined reference disappear after adding a .cpp file with empty main function when building shared library - makefile

Question

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?

Answer 1

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)