How do you tell Make to look into a separate object dir?

Question

FINAL UPDATE:

Problem solved thanks to @Useless!

Changed $(EXET): $(OBJS) to $(EXET): $(OSRC).

UPDATE: Sept 29, 2020

Following @MadScientist's suggestions, I edited and removed the unnecessary parts from the Makefile. However, the line OSRC := $(addprefix $(ODIR)/, $(OBJS)) doesn't seem to have any effect because the *.o object files are now compiled into the workspace folder, not the dedicated ./obj folder, which is my origin problem.

#############################################################
## THIS IS A TEST PROJECT                                  ##
#############################################################
vpath %.cpp src
vpath %.h hdr

#############################################################
## BUILD TASKS                                             ##
#############################################################
HDIR            := hdr
#SRCS           := src
ODIR            := obj

EXET            := a
OBJS            := main.o mainhdr.o testcode.o
OSRC            := $(addprefix $(ODIR)/, $(OBJS))

CXX             := g++
CXXFLAGS        := -I$(HDIR) -g -Wall -std=c++17

## BUILD DIRECTIVE:
all: $(EXET)

$(EXET): $(OBJS)
        $(CXX) $(CXXFLAGS) $^ -o $@

$(ODIR)/%.o: %.cpp
        $(CXX) -c $(CXXFLAGS) $^ -o $@


#############################################################
## CLEAN TASK                                              ##
#############################################################
.PHONY: clean
clean:
        rm -r $(EXET) *.o

ORIGINAL QUESTION:

I've been pulling my hair out trying to no avail to get Make to link prebuilt objects from a specific directory. I got my Makefile to compile the object files into a different directory, but I'm basically stuck after that.

#############################################################
## THIS IS A TEST PROJECT                                  ##
#############################################################
.SUFFIXES:
.SUFFIXES: .o .h .cpp

vpath %.cpp src
vpath %.h hdr
vpath %.o out

%.h : %.cpp

#############################################################
## BUILD TASKS                                             ##
#############################################################

HDIR            := hdr
#SRCS            := src #UNUSED
ODIR            := obj

EXE             := a
OBJS            := main.o mainhdr.o testcode.o

## This line added in reference to this thread:
## https://stackoverflow.com/questions/9178285/how-can-makefile-use-separate-directories-for-source-code-and-binaries
## But it doesn't seem to work either
OSRC            := $(addprefix $(ODIR)/, $(OBJS))

CPP             := g++
CPPFLAGS        := -I$(HDIR) -c -g -Wall -std=c++17

## BUILD DIRECTIVE:
all: $(EXE)

## PROBLEM: Doesn't seem to work when prefixed with the output dir:
#$(EXE): $(ODIR)/$(OBJS)
$(EXE): $(OBJS)
        $(CPP) -o %@ %^

## This compiles the objects into the output dir just fine.
%.o: %.cpp
#       $(CPP) $(CPPFLAGS) $< -o $@
        $(CPP) $(CPPFLAGS) $< -o $(ODIR)/$@

#############################################################
## CLEAN TASK                                              ##
#############################################################
.PHONY: clean
clean:
        rm -r $(EXE) $(ODIR)/*.o

## End of Makefile ##########################################

Debugging effort so far:

I added the .o filetype to .SUFFIXES: but I doubt that it does anything. Make only seems to care about the headers and source files.
I did some more research and I added this part later:

## This line added in reference to this thread:
## https://stackoverflow.com/questions/9178285/how-can-makefile-use-separate-directories-for-source-code-and-binaries
## But it doesn't seem to work either
OSRC            := $(addprefix $(ODIR)/, $(OBJS))

However, it doesn't seem to have any effect at all. Make just couldn't find the object directory either.

Even though it works, this line bothers me somehow:

$(CPP) $(CPPFLAGS) $< -o $(ODIR)/$@

Shouldn't it be fixed somehow to be consistent with the $(addprefix) directive?

Quick references from GNU Make in case I'm missing something:

$@ target
%< first prerequisite
$^ prerequisites

The following is simple test code:

// cat hdr/mainhdr.h
#ifndef MAINHDR_H
#define MAINHDR_H
#include "testcode.h"
#define BIGCONST 777
void testFunc();
#endif

// cat hdr/testcode.h
#ifndef TESTCODE_H
#define TESTCODE_H
#include <iostream>
#include <string>

#define SMALLCONST 25

class NameTag {
        private:
        std::string f;
        std::string l;
        public:
        NameTag(std::string first="", std::string last="");
        void setFirstN(std::string );
        void setLastN(std::string );

        std::string getFirstN() const;
        std::string getLastN() const;

        void operator=(const NameTag &);
};

#endif

// cat src/main.cpp
#include "mainhdr.h"

int main () {
        testFunc();

        return 0;
}

// cat src/mainhdr.cpp
#include "mainhdr.h"

void testFunc() {
        std::cout << "THIS IS A TEST STRING!\n";
        std::cout << "BIGCONST = " << BIGCONST << std::endl;
        std::cout << "SMALLCONST = " << SMALLCONST << std::endl;

        NameTag someDude("Good", "Dude");

        std::cout << someDude.getFirstN() << " " << someDude.getLastN() << std::endl;
}

// cat src/testcode.cpp
#include "testcode.h"

NameTag::NameTag(std::string first, std::string last) {
        f = first;
        l = last;
}

void NameTag::setFirstN(std::string first) {
        f = first;
}

void NameTag::setLastN(std::string last) {
        l = last;
}

std::string NameTag::getFirstN() const {
        return f;
}

std::string NameTag::getLastN() const {
        return l;
}

void NameTag::operator=(const NameTag &right) {
        f = right.f;
        l = right.l;
}

Your `EXET` still depends on `OBJS`, and not on the `OSRC` where you added the prefix. If nothing depends on `OSRC`, it never gets used. — Useless, Sep 29 '20 at 16:59

score 1 · Accepted Answer · answered Sep 29 '20 at 05:18

There are a number of issues here, some real problems and some just not useful things.

.SUFFIXES:
.SUFFIXES: .o .h .cpp

These lines are useless because you're using pattern rules. .SUFFIXES is only used if you're writing suffix rules.

vpath %.o out

This is not useful. You should not ever use vpath to find targets. You can only use it to find source files. Also, you never seem to use out anywhere else in the makefile.

%.h : %.cpp

I'm not sure what this is trying to do, but it's actually not doing anything.

OBJS := main.o mainhdr.o testcode.o

OSRC := $(addprefix $(ODIR)/, $(OBJS))

This is good, but it's useless because you never actually use the variable OSRC anywhere else in your makefile.

CPP             := g++
CPPFLAGS        := -I$(HDIR) -c -g -Wall -std=c++17

FYI, common practice in makefiles is to use CXX for the C++ compiler, not CPP. CPP (in common makefile parlance) refers to the C preprocessor. Of course this is just a suggestion.

## PROBLEM: Doesn't seem to work when prefixed with the output dir:
#$(EXE): $(ODIR)/$(OBJS)

Sure this won't work. So $(ODIR) is obj and $(OBJS) is main.o mainhdr.o testcode.o. That means that this:

$(ODIR)/$(OBJS)

obviously expands to this:

obj/main.o mainhdr.o testcode.o

which is clearly not what you want. The above version, using addprefix to prepend the directory to each word in the variable not just putting it at the start of the entire variable expansion, is what you want.

$(EXE): $(OBJS)
        $(CPP) -o %@ %^

You set OSRC to the right thing, but then you still use $(OBJS) here so it's not helping you.

Also, obviously you mean $@ and $^ here not %@ and %^.

## This compiles the objects into the output dir just fine.
%.o: %.cpp
        $(CPP) $(CPPFLAGS) $< -o $(ODIR)/$@

No, it doesn't. I mean yes, it does compile objects into the output dir, but not "just fine". Any time you have a recipe that builds some other file which is not exactly $@, that recipe is wrong. Here your recipe builds $(ODIR)/$@, not $@, so it's definitely wrong.

You want to write it like this:

$(ODIR)/%.o: %.cpp
        $(CPP) $(CPPFLAGS) $< -o $@

Also just a note, IMO it's poor practice to put an output control flag like -c into the CPPFLAGS (or, using the standard names, CXXFLAGS). You should put flags like that into the recipe directly, not in the variable.

Usually it's wise to include the $(CXXFLAGS) variable in your link line, as well as in your compile line, so any compiler flags which are relevant to the linker are seen. That can't work if the variable contains -c as well.

I updated the origin post with a new edited Makefile. The problem remains since the objects are compiled into the workspace folder, instead of the intended `./obj` directory. Any suggestion? — karlncreate, Sep 29 '20 at 16:54
Problem solved! Thank you for the extensive explanation and suggestions! — karlncreate, Sep 29 '20 at 17:18

How do you tell Make to look into a separate object dir?

1 Answers1