CODEWITHSUNDEEP
compilationmakefile
GXR
GXR

Reputation: 95

Preventing Makefile target recompiling

I have the following Makefile. For some reason when I do make it always rebuilds the target test_phantom_types.o even when I haven't modified the files.

GTEST_HEADERS = ${GTEST_DIR}/include/gtest/*.h \
                ${GTEST_DIR}/include/gtest/internal/*.h

BASE_DIR =  ..

BUILD_DIR = build

USER_DIR = $(BASE_DIR)/tests/src

CPPFLAGS += -isystem $(GTEST_DIR)/include

CXXFLAGS += -g -Wall -Wextra -pthread -std=c++11

TESTS = test_phantom_types

all : $(TESTS)

clean :
        cd $(BUILD_DIR); \
        rm -rf $(BUILD_DIR)/*.o; \
        cd ../

test_phantom_types.o : $(USER_DIR)/test_phantom_types.cpp \
                       $(BASE_DIR) $(GTEST_HEADERS)
        $(CXX) $(CPPFLAGS) $(CXXFLAGS) -I$(BASE_DIR) -c   $(USER_DIR)/test_phantom_types.cpp -o $(BUILD_DIR)/$@

test_phantom_types : test_phantom_types.o
        $(CXX) $(CPPFLAGS) $(CXXFLAGS) -lpthread $(addprefix     $(BUILD_DIR)/,$^) ${GTEST_DIR}/libgtest.a ${GTEST_DIR}/libgtest_main.a -o   $(addprefix $(BUILD_DIR)/, $@)

check : $(TESTS)
        ./$(addprefix $(BUILD_DIR)/,$^)

.PHONY : clean

Upvotes: 0

Views: 25

Answers (1)

Etan Reisner
Etan Reisner

Reputation: 80931

make is very literal.

You told it you have a target called test_phantom_types.o so it expects that target to generate that file.

In your case, however, it doesn't. It generates $(BUILD_DIR)/$@ (from -o $(BUILD_DIR)/$@) and that's not the same thing.

So when you run make and make determines (from the all -> test_phantom_types -> test_phantom_types.o sequence) that it needs to check if it needs to build the test_phantom_types.o file it goes to look for it, can't find it and rebuilds it.

You can see this in the output from make -d if you try it.

That's why the second Rule of Makefiles is

2.Every non-.PHONY rule must update a file with the exact name of its target.

Make sure every command script touches the file “$@“–not “../$@“, or “$(notdir $@)“, but exactly $@. That way you and GNU make always agree.

Upvotes: 2

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