Makefile doesn't use implicit rule on file

Question

I just wanted to write a quick Makefile for a small C++ project, but when I try to build, make says:

No rule to make target "obj/main.o"

even though my implicit rule should cover that file. I have seen others who had forgotten a slash or something, but I dont't see the problem with this Makefile:

.PHONY: run, clean

SRCDIR:=./src
OBJDIR:=./obj
BINDIR:=./bin

CC:=gcc
CXX:=g++

SRC:=$(shell find -name *.cpp)
HXX:=$(shell find -name *.hpp)
OBJ:=$(subst src/,obj/,$(SRC:%.cpp=%.o))
EXE:=main

CXXFLAGS:=-std=c++14 -Wall

$(EXE): $(OBJ)
    @echo $(OBJ)
    $(CXX) $(CXXFLAGS) $^ -o $(BINDIR)/$@

$(OBJDIR)/%.o: $(SRCDIR)/%.cpp $(SRCDIR)/%.hpp
    $(CXX) $(CXXFLAGS) -c $< -o $@

I am using Raspberry Pi OS on my Raspberry Pi 3 b.

Answer 1

because of

$(OBJDIR)/%.o: $(SRCDIR)/%.cpp $(SRCDIR)/%.hpp

you can only compile file having both a cpp and a hpp , but your main does not have header (eg you have a src/main.cpp but no src/main.hpp )

Example:

pi@raspberrypi:/tmp/p $ find .
.
./bin
./src
./src/main.cpp
./Makefile
./obj
pi@raspberrypi:/tmp/p $ make
make: ***  Aucune règle pour fabriquer la cible « obj/main.o », nécessaire pour « main ». Arrêt.
pi@raspberrypi:/tmp/p $ touch src/main.hpp
pi@raspberrypi:/tmp/p $ make
g++ -std=c++14 -Wall -c src/main.cpp -o obj/main.o
./obj/main.o
g++ -std=c++14 -Wall obj/main.o -o ./bin/main
pi@raspberrypi:/tmp/p $ 

( Aucune règle pour fabriquer la cible... means No rule to make target )

---

Changing the line by

$(OBJDIR)/%.o: $(SRCDIR)/%.cpp

the compilation is done:

pi@raspberrypi:/tmp/p $ find .
.generally
./bin
./src
./src/main.cpp
./Makefile
./obj
pi@raspberrypi:/tmp/p $ make
g++ -std=c++14 -Wall -c src/main.cpp -o obj/main.o
./obj/main.o
g++ -std=c++14 -Wall obj/main.o -o ./bin/main
pi@raspberrypi:/tmp/p $ 

To add the dependency to the header is not enough to recompile when necessary when a source #include other header files which is often the case.

You can look at makedepend , to install it under raspbian/debian/ubuntu: apt-get install xutils-dev

---

Note the link is done each time you do make :

pi@raspberrypi:/tmp/p $ find .
.
./bin
./bin/main
./src
./src/main.hpp
./src/main.cpp
./Makefile
./obj
./obj/main.o
pi@raspberrypi:/tmp/p $ make
./obj/main.o
g++ -std=c++14 -Wall obj/main.o -o ./bin/main
pi@raspberrypi:/tmp/p $ make
./obj/main.o
g++ -std=c++14 -Wall obj/main.o -o ./bin/main
pi@raspberrypi:/tmp/p $