Re: What is wrong with this simple rule in makefile?

[Lin George]

Thanks John,


The problem I am meeting with in my situation is that,
another guy has defined the dependency of %.o to
related %.cpp and header files by using g++ -M to
generate prerequisites automatically (they defined the
rules and the prerequisites, and the body of the rule
is empty to check whether all prerequisites are
up-to-date).

I am not sure there will be
any conflicts (or any re-definition issues) since the
target %.o is defined twice.

Have I made myself understood? :-)


regards,
George

--- John Graham-Cumming <address@hidden> wrote:

> Lin George wrote:
> > I want to use g++ $(CFLAGS) to compile any .cpp
> files
> > into .o files. For example, using g++ $(CFLAGS)
> > foo.cpp to generate foo.o, using g++ $(CFLAGS)
> goo.cpp
> > to generate goo.o. CFLAGS is a variale which I
> defined
> > before. I write a rule in Makefile as,
> > 
> > %.o :
> >         g++ $(CFLAGS) %.cpp
> > 
> > But when executing the Makefile, it reports error
> > message "g++: %.cpp: No such file or directory".
> 
> There are two important things wrong with this rule:
> 
> 1. You've specified no prerequisite so this rule
> means "in order to 
> build any file ending in .o do this".  That could be
> incorrect if there 
> were .o's that were not built from .cpp files as you
> intend.
> 
> 2. You can't say %.cpp in the rule body.  That's the
> wrong syntax.  What 
> you need to do is use the automatic variable $<.
> 
> Here's the correct rule:
> 
>      %.o : %.cpp
>          g++ $(CFLAGS) $<
> 
> You probably also want to set a bunch of other g++
> options (such as -o), 
> but that's a bit out of the range of what you asked.
> 
> The other question I would have is why you need to
> define this rule at 
> all.  Don't GNU Make's built-in rules do what you
> need? GNU Make has the 
> following as a built-in:
> 
>      %.o: %.cpp
>          $(COMPILE.cpp) $(OUTPUT_OPTION) $<
> 
> where COMPILE.cpp = $(COMPILE.cc) and COMPILE.cc =
> $(CXX) $(CXXFLAGS) 
> $(CPPFLAGS) $(TARGET_ARCH) -c and CXX = g++.
> 
> So you could use GNU Make's built-in rule and just
> do CXXFLAGS += 
> $(CFLAGS) somewhere in your Makefile and you'd be
> golden.
> 
> John.
> 


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