some questions about GNU makefiles - makefile

Question

i was wondering if anyone have sometime to answer some questions about GNU makefiles...
how to create a directory if it doesn't exists ("./obj") for output?
i have one makefile, but i got 2 build methods "Debug" and "Release", can i have both in 1 makefile and how to tell it which one to build?
ive been using Code::Blocks which builds only changed files, but my makefile builds them everytime i call make command, without touching any files. how can i make it build changed files only?
here is my current makefile
OBJPATH=./obj
COMPILER=gcc
Output: main.o Base64.o
$(COMPILER) -o Output.exe $(OBJPATH)/main.o $(OBJPATH)/Base64.o
strip Output.exe
main.o: main.c main.h
$(COMPILER) -c main.c -o $(OBJPATH)/main.o
Base64.o: Base64.c Base64.h
$(COMPILER) -c Base64.c -o $(OBJPATH)/Base64.o
thanks.

Answer 1

For the first question, you can put a fake target before any of the others, along the lines of:
preamble:
-mkdir obj
main.o: preamble main.c
blah blah blah
That will automatically execute everything in the preamble (you have to make it the first dependency in every rule) before it builds anything else. The - at the start of the mkdir ignores failures if, for example, the directory already exists.
For the second question, you can provide something like:
all: debug release
debug: blah blah blah
release: blah blah blah
and actually put the debug and release code in separate subdirectories. That way, you can build either with make release or make debug and build them both with make all.
Third question: Your makefile builds every time because the rules tell it to. For example, Output: main.o Base64.o will always try to build since Output never exists (the correct target seems to be Output.exe).
Similarly your object file rules will always execute since neither main.o nor Base64.o are updated by their statements (they update the files in the obj directory instead).
You may be able to fix that case by making the target $(OBJPATH)/main.o but, to be honest, I don't usually worry about separating objects and executables into separate directories. I tend to just lump them all into one directory and let make -clean clean them up.
So the makefile I would start with would be:
COMPILER=gcc
# Meta rules
all: release debug
release: Output.exe
debug: Output-d.exe
# Release stuff
Output.exe: main.o Base64.o
$(COMPILER) -o Output.exe main.o Base64.o
strip Output.exe
main.o: main.c main.h
$(COMPILER) -c main.c -o main.o
Base64.o: Base64.c Base64.h
$(COMPILER) -c Base64.c -o Base64.o
# Debug stuff
Output-d.exe: main-d.o Base64-d.o
$(COMPILER) -g -o Output-d.exe main-d.o Base64-d.o
main-d.o: main.c main.h
$(COMPILER) -g -DDEBUG -c main.c -o main-d.o
Base64-d.o: Base64.c Base64.h
$(COMPILER) -g -DDEBUG -c Base64.c -o Base64-d.o
In response to your comment question:
Is there anyway I can re-set a variable based on the Target Selected? for example if selected release OBJPATH will be "./obj/Release" if selected debug OBJPATH = "./obj/Debug"?
GNU Make may be more powerful than the ones I'm used to but you can do that by setting an environment variable then re-running make as per the following:
all: release debug
release:
( export zzvar=release ; $(MAKE) zz_Output.exe )
debug:
( export zzvar=debug ; $(MAKE) zz_Output-d.exe )
zz_Output.exe:
echo $(zzvar)
touch zz_Output.exe
zz_Output-d.exe: zz_main-d.o zz_Base64-d.o
echo $(zzvar)
touch zz_Output-d.exe
which outputs:
( export zzvar=release ; make zz_Output.exe )
make[1]: Entering directory '/home/pax'
echo release
release <==
touch zz_Output.exe
make[1]: Leaving directory '/home/pax'
( export zzvar=debug ; make zz_Output-d.exe )
make[1]: Entering directory '/home/pax'
echo debug
debug <==
touch zz_Output-d.exe
make[1]: Leaving directory '/home/pax'
You can see the two separate variables marked with <== above.
As I said, there's probably an easier way to do it with GNU Make but that'll get you started.

Answer 2

how to create a directory if it doesn't exists ("./obj") for output?
rm -Rf ./obj && mkdir ./obj
i have one makefile, but i got 2 build methods "Debug" and "Release", can i have both in 1 makefile and how to tell it which one to build?
You can have multiple top level build targets. Output in your makefile is a top level target. Make two. One called "Debug" and the other "Release". You can then say make Debug for the debug build and make Release for the release build.
ive been using Code::Blocks which builds only changed files, but my makefile builds them everytime i call make command, without touching any files. how can i make it build changed files only?
I haven't used Code::Blocks (I don't know what it is) but if your Makefiles are written properly (i.e. with dependencies properly specified), it will only rebuild the required targets.

Answer 3

In response to your comment question:
Is there anyway I can re-set a variable based on the Target Selected?
for example if selected release OBJPATH will be "./obj/Release" if
selected debug OBJPATH = "./obj/Debug"?
Here is how you do it without recursion:
COMPILER=gcc
release: OBJPATH = obj
release: Output.exe
debug: OBJPATH = obj-dbg
debug: Outputd.exe
Output%.exe: main.o Base64.o
$(COMPILER) -o $# $(OBJPATH)/main.o $(OBJPATH)/Base64.o
strip $#
main.o: main.c main.h
$(COMPILER) -c main.c -o $(OBJPATH)/main.o
Base64.o: Base64.c Base64.h
$(COMPILER) -c Base64.c -o $(OBJPATH)/Base64.o