compilation - How to add suffix to a string in a Makefile rule when having multiple targets?

Question

My goal is to have a single Makefile that compiles multiple executables all in the same directory. OBJ contains all the .o files except the mains.o which I plan to add only when compiling the specific executable. The logic is this: calling "make all" will try to build main1 first using OBJ and main1.o, but I don't know how to specify main1.o! I thought of using automatic variable $@ to get the target and append to it the .o extension but it does not work.

EXE_ALL = main1 main2 main3
all: $(EXE_ALL)

$(EXE_ALL): $(OBJ) $($@:=.o) # <--- here I don't know how to add the current mains.o?

I know that a multiple target should expand to.

main1: $(OBJ) main1.o
main2: $(OBJ) main2.o
main3: $(OBJ) main3.o

For now it just builds everything but doesn't build the mains.o files, it will build the executables directly (they all work, but I want also the mains.o) like this: main1.cpp OBJ -> main1.

Any help is appreciated. Thanks.

question from:https://stackoverflow.com/questions/65920041/how-to-add-suffix-to-a-string-in-a-makefile-rule-when-having-multiple-targets

Answer 1

In order to resolve $@ in a prerequisite list you need to turn on secondary expansion, like so:

$ cat Makefile
EXE_ALL = main1 main2 main3
OBJ = whatever.o

.PHONY: all
all: $(EXE_ALL)

.SECONDEXPANSION:
$(EXE_ALL): $(OBJ) [email protected]
        @echo Making $@ from $^

%.o:
        @echo Making $@

Output:

$ make
Making whatever.o
Making main1.o
Making main1 from whatever.o main1.o
Making main2.o
Making main2 from whatever.o main2.o
Making main3.o
Making main3 from whatever.o main3.o