Makefile - change variable value depending on a target.

Question

I am not proficient with makefiles, but am used to simple ones. Right now, I have a task on hand.

I need to compile and link a test application with a different library and different include path based on the given target. If target is TARGET1, then link against LIB1 and include INCLUDEPATH1 during compilation. Similarly, if given target is TARGET2, then compile with INCLUDEPATH2 in CFLAGS and link with LIB2.

%.o: %.c
    @echo [CC]  $< ...
    $(CC) $(CFLAGS) -o $*.o $<

Now I have a rule as above which compiles my test application. Now how can the CFLAGS be changed based on the target.

Answer 1

I don't think you can alter variable depending on a target. Assume you invoke

make TARGET1 TARGET2

What value would the CFLAGS have then?

In this case, you can use non-pattern rules to distinguish targets.

TARGET1: a.c
    @echo [CC] $< ...
    $(CC) -I INCLUDEPATH1 ...

TARGET2: a.c
    @echo [CC] $< ...
    $(CC) -I INCLUDEPATH2 ...

To decrease repetition, you may also use variables and "functions". Then, you could re-use what would be the body of your pattern rule in different rules.

define compile_cmd
    @echo [CC] $< ...
    $(CC) -I $1 -l$2 $(CFLAGS)
end

TARGET1: a.c
    $(call compile_cmd,INCLUDEPATH1,LIB1) -o $@ $<

TARGET2: a.c
    $(call compile_cmd,INCLUDEPATH2,LIB2) -o $@ $<

%.o: %.c
    $(call compile_cmd,INCLUDEPATH_DEFAULT,LIB_DEFAULT) -o $@ $<

That would make a nice enough and flexible makefile that suits your needs.

Answer 2

If you are using GNU Make, you can use target-specific variables:

target1: CFLAGS = -IINCLUDEPATH1
target1: LDLIBS = -lLIB1

target2: CFLAGS = -IINCLUDEPATH2
target2: LDLIBS = -lLIB2

all: target1 target2

target1: target1.o misc.o
target2: target2.o

However this doesn't work quite as well as you'd like: if target1 and target2 share some source files, you'll need to arrange for them to each be compiled twice and to differently-named .o files -- which will rather complexify your makefile.

Also, if you type make target1 then -IINCLUDEPATH1 will be propagated to the compilation of misc.c, as desired. However if you type make misc.o it has no way to know that this is eventually destined for target1 and the compilation of misc.c will get no special $CFLAGS value (though it'll get the global one, if there is one).

So this is really only useful in simple cases. But maybe your case is sufficiently simple.