How to write a makefile which has to generate two executables - c++

Question

I am learning how to use make. Recently I wrote a makefile to compile one of my projects, which structure is: src (which contains file.cpp and main.cpp) and include (which contains file.h) folders and the makefile, which is written as follows:
TARGET_EXEC := main
CC := g++
BUILD_DIR := .
SRC_DIR := src
OBJ_DIR := obj
SRC := $(shell find $(SRC_DIR) -name '*.cpp')
OBJ := $(SRC:%=$(OBJ_DIR)/%.o)
DEPS := $(OBJ:.o=.d)
INC_DIR := $(shell find $(SRC_DIR) -type d)
INC_FLAGS := $(addprefix -I,$(INC_DIR))
CPPFLAGS := $(INC_FLAGS) -MMD -MP
$(BUILD_DIR)/$(TARGET_EXEC): $(OBJ)
$(CC) $(OBJ) -o $# $(LDFLAGS)
$(OBJ_DIR)/%.cpp.o: %.cpp
# mkdir -p $(dir $#)
$(CXX) $(CPPFLAGS) $(CXXFLAGS) -c $< -o $#
.PHONY: clean
clean:
rm -r $(OBJ_DIR) main
-include $(DEPS)
Now I want to add a new folder (test) in which I'll host a tests.cpp code with doctest code. In this case I would like to obtain two executables: main and tests (not only main as before), one for the main and one for the testing. I tried everything, but I don't understand how to modify the previous makefile in order to implement the new test folder informations.
EDIT 1
I tried adding Andreas suggestions and also an all command to create both the executables:
TARGET_EXEC := main
TEST_EXEC := tests
CC := g++
BUILD_DIR := .
SRC_DIR := src
OBJ_DIR := obj
TEST_DIR := test
SRC := $(shell find $(SRC_DIR) -name '*.cpp')
OBJ := $(SRC:%=$(OBJ_DIR)/%.o)
DEPS := $(OBJ:.o=.d)
INC_DIR := $(shell find $(SRC_DIR) -type d)
INC_FLAGS := $(addprefix -I,$(INC_DIR))
CPPFLAGS := $(INC_FLAGS) -MMD -MP
all: $(BUILD_DIR)/$(TARGET_EXEC) $(BUILD_DIR)/$(TEST_EXEC)
$(BUILD_DIR)/$(TARGET_EXEC): $(SRC_DIR)/$(TARGET_EXEC).o $(OBJ)
$(CC) $(OBJ) -o $# $(LDFLAGS)
$(BUILD_DIR)/$(TEST_EXEC): $(TEST_DIR)/$(TEST_EXEC).o $(OBJ)
$(CC) $(OBJ) -o $# $(LDFLAGS)
$(OBJ_DIR)/%.cpp.o: %.cpp
# mkdir -p $(dir $#)
$(CXX) $(CPPFLAGS) $(CXXFLAGS) -c $< -o $#
.PHONY: clean all
clean:
rm -r $(OBJ_DIR) main
-include $(DEPS)
Now two executables main and tests are created, but they do the same things (same of the previous main), so it is wrong. When compile my output is:
g++ -Isrc -MMD -MP -c src/osmanip.cpp -o obj/src/osmanip.cpp.o
g++ -Isrc -MMD -MP -c src/main.cpp -o obj/src/main.cpp.o
g++ obj/src/osmanip.cpp.o obj/src/main.cpp.o -o main
g++ obj/src/osmanip.cpp.o obj/src/main.cpp.o -o tests
I think that the error may be in:
$(OBJ_DIR)/%.cpp.o: %.cpp
# mkdir -p $(dir $#)
$(CXX) $(CPPFLAGS) $(CXXFLAGS) -c $< -o $#
What do you think?
EDIT 2
Ok, I think that I found a solution:
TARGET_EXEC := main
TEST_EXEC := tests
CC := g++
BUILD_DIR := .
SRC_DIR := src
OBJ_DIR := obj
TEST_DIR := test
SRC := $(shell find $(SRC_DIR) -name '*.cpp')
TEST := $(shell find $(TEST_DIR) -name '*.cpp')
OBJ := $(SRC:%=$(OBJ_DIR)/%.o)
TEST_OBJ := $(TEST:%=$(OBJ_DIR)/%.o)
DEPS := $(OBJ:.o=.d)
INC_DIR := $(shell find $(SRC_DIR) -type d)
INC_FLAGS := $(addprefix -I,$(INC_DIR))
CPPFLAGS := $(INC_FLAGS) -MMD -MP
.PHONY: clean all
all: $(BUILD_DIR)/$(TARGET_EXEC) $(BUILD_DIR)/$(TEST_EXEC)
$(BUILD_DIR)/$(TARGET_EXEC): $(OBJ)
$(CC) $(OBJ) -o $# $(LDFLAGS)
$(BUILD_DIR)/$(TEST_EXEC): $(TEST_OBJ)
$(CC) $(TEST_OBJ) -o $# $(LDFLAGS)
$(OBJ_DIR)/%.cpp.o: %.cpp
# mkdir -p $(dir $#)
$(CXX) $(CPPFLAGS) $(CXXFLAGS) -c $< -o $#
clean:
rm -r $(OBJ_DIR) main tests
-include $(DEPS)
With this makefile the compilation is good (I think), since the obj/src and obj/test folder are created. main executable is compiled correctly and works. Also tests executable is compiled (correclty I think), but I get another error, not related to makefile:
g++ -Isrc -MMD -MP -c src/osmanip.cpp -o obj/src/osmanip.cpp.o
g++ -Isrc -MMD -MP -c src/main.cpp -o obj/src/main.cpp.o
g++ obj/src/osmanip.cpp.o obj/src/main.cpp.o -o main
g++ -Isrc -MMD -MP -c test/tests.cpp -o obj/test/tests.cpp.o
g++ obj/test/tests.cpp.o -o tests
/usr/bin/ld: /usr/lib/gcc/x86_64-linux-gnu/9/../../../x86_64-linux-gnu/Scrt1.o: in function `_start':
(.text+0x24): undefined reference to `main'
collect2: error: ld returned 1 exit status
make: *** [makefile:29: tests] Error 1
I think this is due to the fact that my test/tests.cpp file is
#define DOCTEST_CONFIG_IMPLEMENT
#include "doctest/doctest.h"
#include <string>
I didn't give it any command for the moment. I implemented doctest without main and in fact it tells me that main function is missing (because it is implemented into the src/main.cpp file). I tried looking at this answer, but I didn't solve this problem.
Do you know how to correctly compile the doctest test.cpp file? Thanks.
EDIT 3
Thanks to Jarod42 answer I solved also this very last issue related to doctest. Now all seems working perfectly.
Many thanks to sll you guys! It is my first post on Stack Overflow and I alrealy love this site.

Answer 1

Have tried humble beginnings? Here is where I would start and build on, key being to separate the main.o from OBJ so that tests can have its main from tests.o:
# Adding include directories (weird way of doing it but ok)
CPPFLAGS := $(addprefix -I,$(shell find src -type d))
# object files
OBJ := src/file1.o src/file2.o
# main
main: src/main.o $(OBJ)
# the test program
tests: test/tests.o $(OBJ)
(Notice this relies on built-in recipes, should work out-of-the-box.)
From there you can re-introduce the other concepts step by step:
Out-of-tree build.
Overrated (at least for GNU Make for which it's all manual).
Glob source files.
If you know what files you have, best not glob.
Dependency file generation and inclusion.
Keep folders and such in variables.