Make rebuilds when it is not necessary

[Rupal Desai]

I am writing a rules.mk file which will have rules for compile for the whole code base.  The problem I am having is that whenever I give the make command it recompiles everything without checking the timestamp.  How can I only build things that are out of date and not everything all the time.  I am attaching my rules.mk file.

 

VPATH = $(OBJDIR) $(MODDIR)
vpath %.cpp $(MODDIR)
vpath %.o $(OBJDIR)
CXXFLAGS =
ALLAPPLIBS = $(DEFAULT_LIBS) $(addprefix -l, c dl \
        comm utilcpp)

 

.PHONY: all

 

all: $(OBJDIR) subdirs $(OBJS) $(STATIC_LIB) $(SHARED_LIB)

 

$(OBJS):$(OBJDIR)/%.o: %.cpp $(OBJDIR)
        $(CXX) $(INCLUDES) $(CXXFLAGS) -o $@ -c $<

 

$(OBJDIR):
        if [ ! -d $(OBJDIR) ]; then mkdir -p $(OBJDIR); fi

 

subdirs:
ifneq ($(SUBDIR),)
        for d in $(SUBDIR); do \
                cd "$$d" && $(MAKE) $(MAKECMDGOALS) && cd ..; done
endif

 

clean: subdirs
        rm -rf $(OBJDIR)

 

$(SHARED_LIB): $(OBJS)
        $(CXX) $(LDFLAGS) $(OBJS) $(LIBS) $(DFLDFLAGS) -o $(OBJDIR)/$@

 

$(STATIC_LIB): $(OBJS)
        $(AR) $(OBJDIR)/$@ $(OBJS)