How do I write a smart makefile? - c++

Question

I'm new to linux and new to makefiles so I'm a little stumped at this point. I've been staring the make manual for an hour now and figured I would just ask:
I want to create a makefile for a relatively small project, whose source files are in proj/src/, and within subdirectories of that location. I have written a makefile at proj/makefile and want to use it to gather up all the source files, find their dependencies, and compile the result into proj/build/. Here is what I've written:
# here are my files and directories (there are also header files that are not
# listed here, but these are referred to from within the .cpp files with
# respect to the proj/src directory)
# proj/makefile
# proj/src/
# proj/src/main.cpp
# proj/src/dir1/
# proj/src/dir1/source1.cpp
# proj/src/dir2/
# proj/src/dir2/source2.cpp
# proj/build/
srcDir = src/
buildDir = build/
# This is a list of all the object files (can I get this programatically?)
objects = main.o source1.o source2.o
all: prog
# Here I want to compile all source (.cpp) files from src/ and all of its subdirectories, and to find the dependencies I want to call g++ -MM to automatically generate the list:
%.o: %.cpp
$(CXX) $(CXXFLAGS) -MM -I $(srcDir)
clean:
rm -rf *o $(buildDir)/prog
This is far from functional but I wanted to give it ago before asking. Running this gives
make: * No rule to make target main.o', needed byprog'. Stop.
So presumably my attempt at automatically writing the object files failed. If someone could give me some direction that would be great, I'm hoping that between the comments and code you can make out the intention.
EDIT: I have now tried the following:
SHELL := /bin/bash
srcDir = src
buildDir = bin
sources := $(shell find $(srcDir) -name *.cpp)
objects := $(sources:%.cpp=%.o)
-include $(sources:%.cpp=%.d)
# This is a list of all the object files (can I get this programatically?)
all: prog
prog: $(objects)
g++ $(objects) -o $(buildDir)/prog
# Here I want to compile all source (.cpp) files from src/ and all of its subdirectories, and to find the dependencies I want to call g++ -MM to automatically generate the list:
%.o: %.cpp
$(CXX) $(CXXFLAGS) -MMD -MP -c -I $(srcDir)
clean:
rm -rf *o $(buildDir)/hello
But when I run this I get the following error:
g++ -MMD -MP -I src
g++: fatal error: no input files
compilation terminated.
make: *** [src/core/Cell.o] Error 1

Answer 1

It wants to build main.o from main.cpp, but there is no main.cpp at the top level. You could specify the full paths of the object files, if they are to be built alongside the source files:
objects = $(srcDir)/main.o $(srcDir)/dir1/source1.o $(srcDir)/dir2/source2.o
can I get this programatically?
I do it using gnumake's shell function:
SHELL := /bin/bash
sources := $(shell find $(srcDir) -name *.cpp)
objects := $(sources:%.cpp=%.o)
to find the dependencies I want to call g++ -MM to automatically generate the list
Use -MMD to generate dependency files as well as compiling. It's also a good idea to specify -MP here - that makes sure the dependencies are correctly updated when you remove a header. Given the variables defined above, you can include the dependency files with
-include $(sources:%.cpp=%.d)