Re: how to create a single rule to build a directory as necessary

[Sam Steingold]

Thanks for the informative reply!

Paul Smith wrote:
> On Tue, 2009-09-22 at 11:44 -0400, Sam Steingold wrote:
> > I, too, am struggling with rebuilding a directory.
> > it appears that the rule:
> >
> > gllib: config.status
> >         mkdir -p gllib; cd gllib; make
> >
> > is wrong because the script "mkdir ..." is always executed, regardless of 
> > whether config.status has changed or not.
> > So, what is the right way to handle this?
>
> The short answer is that using a directory as a target in make is almost
> never correct, unless it's a .PHONY target or the equivalent.

so what is this SUBDIRS automake thingie?
how does it work?

> Why is that?  It's because make doesn't treat directories any different
> than files, when it computes whether the item is up to date: if the time
> modified on the directory is newer than all its prerequisites then the
> command is not executed.  If the modification time on the directory is
> older than any of its prerequisites, then the command is executed.
>
> The problem is that the modification time on a directory means something
> that is not really appropriate for make: the modification time on a
> directory changes whenever the DIRECTORY ITSELF changes.  When does a
> directory change?  It changes whenever any element in the directory is
> renamed or deleted, or a new element is created in the directory.  In
> short, if you run "ls -a" before and after some command and the output
> is identical, then the directory was not modified.

this makes perfect sense.
thanks.
how about this addition then:

gllib: config.status
        mkdir -p gllib; cd gllib; make
        touch gllib/.stamp; rm -f gllib/.stamp

adding and removing the .stamp file will modify the directory, right?

> which by the way, is not well-formed; it should be:
>
>         mkdir -p $@ && cd $@ && $(MAKE)

I am afraid I cannot use the "gnuisms" (as they were called by a NetBSD user 
who was bitten by these variables when I used them)

> Your problem here is more conceptual than anything.  What, in words, are
> you really trying to get make to do?  How does the upper level makefile
> know, really, that it does or doesn't need to invoke the sub-make?  It
> can't know that... unless it invokes the submake to see whether anything
> needs to be done.

I need a few files which are created in gllib by make.

Do I need to explicitly type

gllib/foo gllib/bar : config.status
        mkdir -p gllib; cd gllib; make

> In a standard recursive make environment, sub-makes are almost always
> set up as .PHONY, so they're always invoked, to be sure that their
> content is up to date.  That's why recursive makes are slower,
> typically, than non-recursive makes.

so, if I add gllib to .PHONY, the commands will be always executed, even with 
the .stamp trick?

Thanks!
Sam.