How can I store my .o files in a sepparate folder?

Question

INCDIR=include
SRCDIR=src


SRC = $(wildcard $(SRCDIR)/*.cpp)
DEPS = $(wildcard $(INCDIR)/*.h)
OBJ = $(SRC:.cpp=.o)


CFLAGS = -I$(INCDIR) -Wall -Weffc++ -Wextra -Wsign-conversion
CC=g++


preprocessor: $(OBJ) 
    $(CC) $(OBJ) -o $@ $(CFLAGS)

$(SRCDIR)/%.o: $(SRCDIR)/%.cpp
    $(CC) -c $(CFLAGS) $^ -o $@

clean: 
    rm $(SRCDIR)/*.o preprocessor

This is my current makefile, if I wanted to store my .o files in a sepparate directory, src/obj for example, how would I have to modify it?

Answer 1

Many ways to do it, but according to your own code, you could do:

INCDIR=include
SRCDIR=src
OBJDIR=obj

SRC= $(wildcard $(SRCDIR)/*.cpp)
DEPS= $(wildcard $(INCDIR)/*.h)
OBJ=$(patsubst %.cpp, $(OBJDIR)/%.o, $(notdir $(SRC)))
    
CFLAGS= -I$(INCDIR) -Wall -Weffc++ -Wextra -Wsign-conversion
CC=g++

$(OBJDIR):
    @if ! [ -d $(@) ]; then\
        echo "==> creating dir: $(@)";\
        mkdir $(@);\
    fi

preprocessor: $(OBJDIR) $(OBJ)
    $(CC) $(OBJ) -o $@ $(CFLAGS)

$(OBJDIR)/%.o: $(SRCDIR)/%.cpp
    $(CC) -c $(CFLAGS) $^ -o $@

clean:
    rm $(OBJDIR)/*.o preprocessor

printobj: $(OBJDIR)
    @echo "$(OBJ)"

will output:

$ gmake preprocessor
g++ -c -Iinclude -Wall -Weffc++ -Wextra -Wsign-conversion src/a.cpp -o obj/a.o
g++ -c -Iinclude -Wall -Weffc++ -Wextra -Wsign-conversion src/b.cpp -o obj/b.o
g++  obj/a.o  obj/b.o -o preprocessor -Iinclude -Wall -Weffc++ -Wextra -Wsign-conversion

NOTE: the target printobj is just here to output what you could expect, the target $(OBJDIR) ensure your directory exist before creating object files.

Answer 2

The are just a few things to change:

INCDIR := include
SRCDIR := src
OBJDIR := $(SRCDIR)/obj

SRC  := $(wildcard $(SRCDIR)/*.cpp)
DEPS := $(wildcard $(INCDIR)/*.h)
OBJ  := $(patsubst $(SRCDIR)/%.cpp,$(OBJDIR)/%.o,$(SRC))


CFLAGS := -I$(INCDIR) -Wall -Weffc++ -Wextra -Wsign-conversion
CC     := g++

preprocessor: $(OBJ) 
    $(CC) $^ -o $@ $(CFLAGS)

$(OBJDIR)/%.o: $(SRCDIR)/%.cpp | $(OBJDIR)
    $(CC) -c $(CFLAGS) $< -o $@

$(OBJDIR):
    mkdir -p "$@"

clean: 
    rm -f $(OBJ) preprocessor

Note some other minor modifications:

• the | $(OBJDIR) order-only prerequisite and the $(OBJDIR): rule to ensure the objects directory exists before compiling,
• := instead of = for all make variable assignments because you don't need recursively expanded variables here,
• $^ instead of $(OBJ) in the link recipe because using automatic variables in recipes makes them more generic,
• $< instead of $^ in the compile recipe because you compile only the first prerequisite, not all of them,
• rm -f $(OBJ) preprocessor instead of rm $(SRCDIR)/*.o preprocessor to remove only the object files of the project and avoid errors if none exists.