Makefile - place object files in obj directory

[wolfv]

I am trying to write a Makefile that will place object files in obj directory.

This Makefile creates a f1.o file as expected:

CXX = g++
CXXFLAGS = -Wall -g

f1.o: f1.cpp

output:

$ make
g++ -Wall -g   -c -o f1.o f1.cpp

I changed Makefile target to "obj/f1.o":

CXX = g++
CXXFLAGS = -Wall -g

obj/f1.o: f1.cpp

output:

$ make
make: Nothing to be done for 'obj/f1.o'.

The obj directory is empty.

What is the Make syntax for putting object files in obj directory?
Thank you.

[ne555]

http://stackoverflow.com/a/5188496

[wolfv]

Thanks for the response ne555.

I saw that page before, but I couldn't figure out the "%" syntax.
This Makefile does not work:

CXX = g++
CXXFLAGS = -Wall -g

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

output:

$ make
make: *** No targets.  Stop.

But this Makefile works:

CXX = g++
CXXFLAGS = -Wall -g

f2.o: f2.c

output:

$ make
cc   -c -o f2.o f2.c

[wolfv]

The Makfile works without the '%':

obj/f2.o: f2.c
	$(CC) $(CFLAGS) $(CPPFLAGS) -c -o $@ $<

output:

$ make
cc   -c -o obj/f2.o f2.c

[ne555]

if you have several sources then you'll need to write that same rule for each file
the idea is to use % to avoid that
http://www.gnu.org/software/make/manual/make.html#Pattern-Rules

In your first example you've got no target. You have defined the pattern rule but never specified what should be built.
For example, your object file may be a dependency of you main program

main: obj/f2.o
	$(CXX) $(LDLIBS) $^ -o $@

so in order to build main `obj/f2.o' is needed, and it will find the `obj/%.o: %.c' rule to make it.


Here you have a full example, with mechanism to obtain all the sources in a directory (it uses ./src/, but you may easily change it to ./)
http://stackoverflow.com/a/2908351