How do I check dependencies when invoking a sub-make to build when there are changes?

Question

If I have a makefile that calls another makefile, how to I get the master makefile to correctly check if the dependencies of the subordinate makefile have changed?

For example, if I have the rule

server:
     @cd $(SERVERDIR) && $(MAKE)

That invokes make in the subdirectory in which I build an executable "server". However, if I change one of the files that make up server, the parent make doesn't see the changes and refuses to rebuild server - "make: `server' is up to date."

How can I get the master makefile to correctly detect when there's a change in one of the dependent files (something like $(SERVERDIR)/server.c, for example?

Answer 1

Your target name matches the name of one of the files or directories in your main Makefile directory.

Assuming you need to build everything in a subdirectory called server, this rule:

server:
    $(MAKE) -C server

will not work, as the target server is a directory, has no source files and doesn't need to be built then.

This one:

srv:
    $(MAKE) -C server

will work, as long as there is no file or directory called srv.

Answer 2

It looks like you want to use a phony target

.PHONY: server
server:
     @cd $(SERVERDIR) && $(MAKE)

There's a detailed description of the Phony target here, but the short description is you're telling the makefile that there will never be a file that corresponds with this server target, and therefore it won't consider server up to date if there is a file named server in the directory.

Answer 3

You don't:

• Recursive Make Considered Harmful
• Implementing non-recursive make

But yes, if you have no choice, e.g. because you don't control the sub-makefile, a .PHONY target is what you are looking for.