How to place object files in separate subdirectory

Question

I'm having trouble with trying to use make to place object files in a separate subdirectory, probably a very basic technique. I have tried to use the information in this page: http://www.gnu.org/software/hello/manual/make/Prerequisite-Types.html#Prerequisite-Types

I get the following output from make:

make: *** No rule to make target `ku.h', needed by `obj/kumain.o'.  Stop.

However ku.h is a dependency not a target (although it's obviously #included within the c source files). When I don't try to use a subdirectory for object files (i.e. miss out the OBJDIR parts) it works fine. Why does make think ku.h is a target?

my makefile is this: (the style is after reading various sources of information)

.SUFFIXES:
.SUFFIXES: .c .o

CC=gcc 
CPPFLAGS=-Wall
LDLIBS=-lhpdf
VPATH=%.c src
VPATH=%.h src
VPATH=%.o obj
OBJDIR=obj

objects= $(addprefix $(OBJDIR)/, kumain.o kudlx.o kusolvesk.o kugetpuz.o kuutils.o \
  kurand.o kuASCboard.o kuPDFs.o kupuzstrings.o kugensud.o \
  kushapes.o )

ku : $(objects)
  $(CC) $(CPPFLAGS) -o ku $(objects) $(LDLIBS)

$(objects) : ku.h kudefines.h kuglobals.h kufns.h | $(OBJDIR)

$(OBJDIR):
  mkdir $(OBJDIR)

.PHONY: clean
clean :
  rm $(objects)

Edit: I applied the change to use the vpath directive. My version was a bad mixture of VPATH=xxx and vpath %.c xxx. However I now get another problem (which was the original problem before I added the wrong vpath). This is now the output:

    gcc  -o ku -lhpdf obj/kumain.o obj/kudlx.o obj/kusolvesk.o ..etc
    gcc: obj/kumain.o: No such file or directory
    gcc: obj/kudlx.o: No such file or directory
    gcc: obj/kusolvesk.o: No such file or directory
    gcc: obj/kugetpuz.o: No such file or directory
    gcc: obj/kuutils.o: No such file or directory
    gcc: obj/kurand.o: No such file or directory
    gcc: obj/kuASCboard.o: No such file or directory
    gcc: obj/kuPDFs.o: No such file or directory
    gcc: obj/kupuzstrings.o: No such file or directory
    gcc: obj/kugensud.o: No such file or directory
    gcc: obj/kushapes.o: No such file or directory
    make: *** [ku] Error 1

It appears that make is not applying the implicit rule for an object file although the manual says "Implicit rules tell make how to use customary techniques so that you do not have to specify them in detail when you want to use them. For example, there is an implicit rule for C compilation. File names determine which implicit rules are run. For example, C compilation typically takes a .c file and makes a .o file. So make applies the implicit rule for C compilation when it sees this combination of file name endings." and also "The search through the directories specified in VPATH or with vpath also happens during consideration of implicit rules (see Using Implicit Rules)."

Again here "For example, when a file foo.o has no explicit rule, make considers implicit rules, such as the built-in rule to compile foo.c if that file exists. If such a file is lacking in the current directory, the appropriate directories are searched for it. If foo.c exists (or is mentioned in the makefile) in any of the directories, the implicit rule for C compilation is applied."

Any assistance in getting implicit rules to work for my makefile would be greatly appreciated.

Edit no 2: Thanks to Jack Kelly I have made an explicit rule to compile the .c files since I couldn't get anywhere trying to use implicit rules. Also thanks to al_miro for the vpath info.

Here is the working makfile:

.SUFFIXES:
.SUFFIXES: .c .o

CC=gcc 
CPPFLAGS=-Wall
LDLIBS=-lhpdf
OBJDIR=obj
vpath %.c src
vpath %.h src

objects = $(addprefix $(OBJDIR)/, kumain.o kudlx.o kusolvesk.o kugetpuz.o kuutils.o \
  kurand.o kuASCboard.o kuPDFs.o kupuzstrings.o kugensud.o \
  kushapes.o )

ku : $(objects)
  $(CC) $(CPPFLAGS) -o ku $(objects) $(LDLIBS)

$(OBJDIR) obj/%.o : %.c ku.h kudefines.h kuglobals.h kufns.h 
  $(CC) -c $(CPPFLAGS) $< -o $@

.PHONY : clean
clean :
  rm $(objects)

Answer 1

Since you're using GNUmake, use a pattern rule for compiling object files:

$(OBJDIR)/%.o: %.c
    $(CC) $(CFLAGS) $(CPPFLAGS) -c -o $@ $<

Answer 2

This is the makefile that I use for most of my projects,

It permits putting source files, headers and inline files in subfolders, and subfolders of subfolders and so-forth, and will automatically generate a dependency file for each object This means that modification of headers and inline files will trigger recompilation of files which are dependent.

Source files are detected via shell find command, so there is no need to explicitly specify, just keep coding to your hearts content.

It will also copy all files from a 'resources' folder, into the bin folder when the project is compiled, which I find handy most of the time.

To provide credit where it is due, the auto-dependencies feature was based largely off Scott McPeak's page that can be found HERE, with some additional modifications / tweaks for my needs.

Example Makefile

#Compiler and Linker
CC          := g++-mp-4.7

#The Target Binary Program
TARGET      := program

#The Directories, Source, Includes, Objects, Binary and Resources
SRCDIR      := src
INCDIR      := inc
BUILDDIR    := obj
TARGETDIR   := bin
RESDIR      := res
SRCEXT      := cpp
DEPEXT      := d
OBJEXT      := o

#Flags, Libraries and Includes
CFLAGS      := -fopenmp -Wall -O3 -g
LIB         := -fopenmp -lm -larmadillo
INC         := -I$(INCDIR) -I/usr/local/include
INCDEP      := -I$(INCDIR)

#---------------------------------------------------------------------------------
#DO NOT EDIT BELOW THIS LINE
#---------------------------------------------------------------------------------
SOURCES     := $(shell find $(SRCDIR) -type f -name *.$(SRCEXT))
OBJECTS     := $(patsubst $(SRCDIR)/%,$(BUILDDIR)/%,$(SOURCES:.$(SRCEXT)=.$(OBJEXT)))

#Defauilt Make
all: resources $(TARGET)

#Remake
remake: cleaner all

#Copy Resources from Resources Directory to Target Directory
resources: directories
    @cp $(RESDIR)/* $(TARGETDIR)/

#Make the Directories
directories:
    @mkdir -p $(TARGETDIR)
    @mkdir -p $(BUILDDIR)

#Clean only Objecst
clean:
    @$(RM) -rf $(BUILDDIR)

#Full Clean, Objects and Binaries
cleaner: clean
    @$(RM) -rf $(TARGETDIR)

#Pull in dependency info for *existing* .o files
-include $(OBJECTS:.$(OBJEXT)=.$(DEPEXT))

#Link
$(TARGET): $(OBJECTS)
    $(CC) -o $(TARGETDIR)/$(TARGET) $^ $(LIB)

#Compile
$(BUILDDIR)/%.$(OBJEXT): $(SRCDIR)/%.$(SRCEXT)
    @mkdir -p $(dir $@)
    $(CC) $(CFLAGS) $(INC) -c -o $@ $<
    @$(CC) $(CFLAGS) $(INCDEP) -MM $(SRCDIR)/$*.$(SRCEXT) > $(BUILDDIR)/$*.$(DEPEXT)
    @cp -f $(BUILDDIR)/$*.$(DEPEXT) $(BUILDDIR)/$*.$(DEPEXT).tmp
    @sed -e 's|.*:|$(BUILDDIR)/$*.$(OBJEXT):|' < $(BUILDDIR)/$*.$(DEPEXT).tmp > $(BUILDDIR)/$*.$(DEPEXT)
    @sed -e 's/.*://' -e 's/\\$$//' < $(BUILDDIR)/$*.$(DEPEXT).tmp | fmt -1 | sed -e 's/^ *//' -e 's/$$/:/' >> $(BUILDDIR)/$*.$(DEPEXT)
    @rm -f $(BUILDDIR)/$*.$(DEPEXT).tmp

#Non-File Targets
.PHONY: all remake clean cleaner resources

Answer 3

The VPATH lines are wrong, they should be

vpath %.c  src
vpath %.h  src

i.e. not capital and without the = . As it is now, it doesn't find the .h file and thinks it is a target to be made.

Answer 4

In general, you either have to specify $(OBJDIR) on the left hand side of all the rules that place files in $(OBJDIR), or you can run make from $(OBJDIR). VPATH is for sources, not for objects.

Take a look at these two links for more explanation, and a "clever" workaround.

• http://mad-scientist.net/make/vpath.html
• http://mad-scientist.net/make/multi-arch.html

Answer 5

Build from the output directory

Instead of building from the top-level directory, build from the output directory. You can access the source directories by setting the vpath. This option has the advantage that the built-in rules can be used.

build.sh

#!/bin/bash
mkdir -p obj
cp Makefile.template obj/Makefile
cd obj
make "$*"

Makefile

.SUFFIXES:
.SUFFIXES: .c .o

CC=gcc 
CPPFLAGS=-Wall
LDLIBS=-lhpdf
VPATH=%.c ../src
VPATH=%.h ../src

objects=kumain.o kudlx.o kusolvesk.o kugetpuz.o kuutils.o \
  kurand.o kuASCboard.o kuPDFs.o kupuzstrings.o kugensud.o \
  kushapes.o

ku : $(objects)

$(objects) : ku.h kudefines.h kuglobals.h kufns.h

.PHONY: clean
clean :
  rm $(objects)

The disadvantage is that error messages do not match the CWD. This can be solved by skipping build.sh and directly building from the obj directory.

Another advantage of this approach is that it's somewhat popular. cmake works in a similar fashion.

Create Rule based on output option

The following solution isn't nice in my opinion, as I really love the built-in rules. However, GNU make doesn't support something like vpath for output directories. And the built-in rules cannot match, as the % in %.o would match obj/foo of obj/foo.o, leaving make with a search in vpath %.c src/ for stuff like src/obj/foo.c, but not src/foo.c.

But this is as close to the built-in rules as you can get, and therefore to my best knowledge the nicest solution that's available.

$(OBJDIR)/%.o: %.c
        $(COMPILE.c) $(OUTPUT_OPTION) $<

Explanation: $(COMPILE.c) $(OUTPUT_OPTION) $< actually is how .c.o is implemented, see http://git.savannah.gnu.org/cgit/make.git/tree/default.c (and it's even mentioned in the manual)

Besides, if $(OBJDIR) would only ever contain auto-gererated files, you could create it on-the-fly with an order-only prerequisite, making the clean rule slightly simpler:

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

$(OBJDIR)/%.o: %.c | $(OBJDIR)
        $(COMPILE.c) $(OUTPUT_OPTION) $<

.PHONY: clean
clean:
        $(RM) -r $(OBJDIR)

This requires that the feature order-only is available, which you can check using $(filter order-only, $(.FETAURES)). I've checked on Kubuntu 14.04 GNU make 3.81 and OpenSUSE 13.1 GNU make 3.82. Both were built with order-only enabled, and am now left puzzled why Kubuntu 14.04 comes with an older version of GNU make than OpenSUSE 13.1. Anyways, gonna download make 4.1 now :)

Answer 6

For all those working with implicit rules (and GNU MAKE). Here is a simple makefile which supports different directories:

#Start of the makefile

VPATH = ./src:./header:./objects

OUTPUT_OPTION = -o objects/$@

CXXFLAGS += -Wall -g -I./header

Target = $(notdir $(CURDIR)).exe

Objects := $(notdir $(patsubst %.cpp,%.o,$(wildcard src/*.cpp)))



all: $(Target)

$(Target): $(Objects)
     $(CXX) $(CXXFLAGS) -o $(Target) $(addprefix objects/,$(Objects))


#Beware of -f. It skips any confirmation/errors (e.g. file does not exist)

.PHONY: clean
clean:
     rm -f $(addprefix objects/,$(Objects)) $(Target)

Lets have a closer look (I will refer to the current Directory with curdir):

This line is used to get a list of the used .o files which are in curdir/src.

Objects := $(notdir $(patsubst %.cpp,%.o,$(wildcard src/*.cpp)))
#expands to "foo.o myfoo.o otherfoo.o"

Via variable the output is set to a different directory (curdir/objects).

OUTPUT_OPTION = -o objects/$@
#OUTPUT_OPTION will insert the -o flag into the implicit rules

To make sure the compiler finds the objects in the new objects folder, the path is added to the filename.

$(Target): $(Objects)
     $(CXX) $(CXXFLAGS) -o $(Target) $(addprefix objects/,$(Objects))
#                                    ^^^^^^^^^^^^^^^^^^^^    

This is meant as an example and there is definitly room for improvement.

For additional Information consult: Make documetation. See chapter 10.2

Or: Oracle: Programming Utilities Guide

Answer 7

You can specify the -o $@ option to your compile command to force the output of the compile command to take on the name of the target. For example, if you have:

• sources: cpp/class.cpp and cpp/driver.cpp
• headers: headers/class.h

...and you want to place the object files in:

• objects: obj/class.o obj/driver.o

...then you can compile cpp/class.cpp and cpp/driver.cpp separately into obj/class.o and obj/driver.o, and then link, with the following Makefile:

CC=c++
FLAGS=-std=gnu++11

INCS=-I./headers
SRC=./cpp
OBJ=./obj
EXE=./exe

${OBJ}/class.o:    ${SRC}/class.cpp
                   ${CC} ${FLAGS} ${INCS} -c $< -o $@

${OBJ}/driver.o:    ${SRC}/driver.cpp ${SRC}/class.cpp
                    ${CC} ${FLAGS} ${INCS} -c $< -o $@

driver: ${OBJ}/driver.o ${OBJ}/class.o
        ${CC} ${FLAGS} ${OBJ}/driver.o ${OBJ}/class.o -o ${EXE}/driver

Answer 8

For anyone that is working with a directory style like this:

project
    > src
        > pkgA     
        > pkgB
        ...
    > bin
        > pkgA
        > pkgB
        ...

The following worked very well for me. I made this myself, using the GNU make manual as my main reference; this, in particular, was extremely helpful for my last rule, which ended up being the most important one for me.

My Makefile:

PROG := sim
CC := g++
ODIR := bin
SDIR := src
MAIN_OBJ := main.o
MAIN := main.cpp
PKG_DIRS := $(shell ls $(SDIR))
CXXFLAGS = -std=c++11 -Wall $(addprefix -I$(SDIR)/,$(PKG_DIRS)) -I$(BOOST_ROOT)
FIND_SRC_FILES = $(wildcard $(SDIR)/$(pkg)/*.cpp) 
SRC_FILES = $(foreach pkg,$(PKG_DIRS),$(FIND_SRC_FILES)) 
OBJ_FILES = $(patsubst $(SDIR)/%,$(ODIR)/%,\
$(patsubst %.cpp,%.o,$(filter-out $(SDIR)/main/$(MAIN),$(SRC_FILES))))

vpath %.h $(addprefix $(SDIR)/,$(PKG_DIRS))
vpath %.cpp $(addprefix $(SDIR)/,$(PKG_DIRS)) 
vpath $(MAIN) $(addprefix $(SDIR)/,main)

# main target
#$(PROG) : all
$(PROG) : $(MAIN) $(OBJ_FILES) 
    $(CC) $(CXXFLAGS) -o $(PROG) $(SDIR)/main/$(MAIN) 

# debugging
all : ; $(info $$PKG_DIRS is [${PKG_DIRS}])@echo Hello world

%.o : %.cpp
    $(CC) $(CXXFLAGS) -c $< -o $@

# This one right here, folks. This is the one.
$(OBJ_FILES) : $(ODIR)/%.o : $(SDIR)/%.h
    $(CC) $(CXXFLAGS) -c $< -o $@

# for whatever reason, clean is not being called...
# any ideas why???
.PHONY: clean

clean :
    @echo Build done! Cleaning object files...
    @rm -r $(ODIR)/*/*.o

By using $(SDIR)/%.h as a prerequisite for $(ODIR)/%.o, this forced make to look in source-package directories for source code instead of looking in the same folder as the object file.

I hope this helps some people. Let me know if you see anything wrong with what I've provided.

BTW: As you may see from my last comment, clean is not being called and I am not sure why. Any ideas?

Answer 9

None of these answers seemed simple enough - the crux of the problem is not having to rebuild:

makefile

OBJDIR=out
VPATH=$(OBJDIR)

# make will look in VPATH to see if the target needs to be rebuilt
test: moo
    touch $(OBJDIR)/$@

example use

touch moo
# creates out/test
make test
# doesn't update out/test
make test
# will now update test
touch moo
make test