Makefile, header dependencies

Question

Let's say I have a makefile with the rule

%.o: %.c  gcc -Wall -Iinclude ... 

I want *.o to be rebuilt whenever a header file changes. Rather than work out a list of dependencies, whenever any header file in /include changes, then all objects in the dir must be rebuilt.

I can't think of a nice way to change the rule to accomodate this, I'm open to suggestions. Bonus points if the list of headers doesn't have to be hard-coded

Answer 1

If you are using a GNU compiler, the compiler can assemble a list of dependencies for you. Makefile fragment:

depend: .depend  .depend: $(SRCS)         rm -f "$@"         $(CC) $(CFLAGS) -MM $^ -MF "$@"  include .depend 

or

depend: .depend  .depend: $(SRCS)         rm -f "$@"         $(CC) $(CFLAGS) -MM $^ > "$@"  include .depend 

where SRCS is a variable pointing to your entire list of source files.

There is also the tool makedepend, but I never liked it as much as gcc -MM

Answer 2

Most answers are surprisingly complicated or erroneous. However simple and robust examples have been posted elsewhere [codereview]. Admittedly the options provided by the gnu preprocessor are a bit confusing. However, the removal of all directories from the build target with -MM is documented and not a bug [gpp]:

By default CPP takes the name of the main input file, deletes any directory components and any file suffix such as ‘.c’, and appends the platform's usual object suffix.

The (somewhat newer) -MMD option is probably what you want. For completeness an example of a makefile that supports multiple src dirs and build dirs with some comments. For a simple version without build dirs see [codereview].

CXX = clang++ CXX_FLAGS = -Wfatal-errors -Wall -Wextra -Wpedantic -Wconversion -Wshadow  # Final binary BIN = mybin # Put all auto generated stuff to this build dir. BUILD_DIR = ./build  # List of all .cpp source files. CPP = main.cpp $(wildcard dir1/*.cpp) $(wildcard dir2/*.cpp)  # All .o files go to build dir. OBJ = $(CPP:%.cpp=$(BUILD_DIR)/%.o) # Gcc/Clang will create these .d files containing dependencies. DEP = $(OBJ:%.o=%.d)  # Default target named after the binary. $(BIN) : $(BUILD_DIR)/$(BIN)  # Actual target of the binary - depends on all .o files. $(BUILD_DIR)/$(BIN) : $(OBJ)     # Create build directories - same structure as sources.     mkdir -p $(@D)     # Just link all the object files.     $(CXX) $(CXX_FLAGS) $^ -o $@  # Include all .d files -include $(DEP)  # Build target for every single object file. # The potential dependency on header files is covered # by calling `-include $(DEP)`. $(BUILD_DIR)/%.o : %.cpp     mkdir -p $(@D)     # The -MMD flags additionaly creates a .d file with     # the same name as the .o file.     $(CXX) $(CXX_FLAGS) -MMD -c $< -o $@  .PHONY : clean clean :     # This should remove all generated files.     -rm $(BUILD_DIR)/$(BIN) $(OBJ) $(DEP) 

This method works because if there are multiple dependency lines for a single target, the dependencies are simply joined, e.g.:

a.o: a.h a.o: a.c     ./cmd 

is equivalent to:

a.o: a.c a.h     ./cmd 

as mentioned at: Makefile multiple dependency lines for a single target?