Alejandro Colomar (Friday, July 22, 2022 14:05)
> I'd like to suggest a new feature: post-requisites.
> As the word suggests, as pre-requisites are satisfied before a given
> target, post-requisites would be satisfied _after_ a given target.

and just as a prerequisite can be built at any time before, but must be
built before, the post-requisite can be run at any time after, but must
be built after.  So if several targets that are being built require some
given post-requisite, it waits until all of them have been built.

> Why would someone ever need that in a Makefile:
> In an install target for a library, you may want to run ldconfig(8)
> after any install targets have executed.
> In an install target for the linux man-pages, you may want to run
> mandb(8)  after any pages have been installed in the system.

I think the traditional solution to this would be to make the install
depend on rules that get individual files installed and have its command
run the post-install steps, or have the install rule depend on a set of
post-install rules (each of which does one post-install task), that in
turn depend on all the files whose updates would trigger them.  However,
the problem with that is that if I want to just update the one man page,
the mandb(8) run that's needed following it won't be run unless I build
the mandb-run rule, that depends on *all* my man pages.  So I can't do
fine-grained targeted "build this one thing" parts of my build process.

Post-requisites would take care of that; if I build and install an
update to any man page, then mandb gets run, regardless of what else I
am also building, aside from mandb's run happening after any of those
that need it to be done later.

> The proposed syntax would be similar to that of order only
> prerequisites, but using '>' instead of '|':
> [
> [...]
> $(DESTDIR)$(man3dir)/foo.3: man3/foo.3 | $(DESTDIR)$(man3dir) > update-mandb
>         $(info INSTALL  $@)
>         @$(INSTALL_DATA) -T $< $@
> $(DESTDIR)$(man3dir)/bar.3: man3/bar.3 | $(DESTDIR)$(man3dir) > update-mandb
>         $(info INSTALL  $@)
>         @$(INSTALL_DATA) -T $< $@
> .PHONY: update-mandb
> update-mandb:
>         $(info MANDB)
>         @mandb
> .PHONY: install
> install: $(DESTDIR)$(man3dir)/foo.3 $(DESTDIR)$(man3dir)/bar.3
> ]
> So that `make install` would install foo.3 and bar.3, and after all of
> the pages have been installed, mandb(8) would be run.
> And `make /usr/local/share/man/man3/foo.3` would install foo.3, and
> after it, it would run mandb(8).
> Does that make sense to you?

and if my package (e.g. perl) has a whole library of man pages, I'd be
able to do updates to just one (or a few) of them at a time and get
mandb run after that update, regardless of which subset of pages I
update, rather than only being able to get that run when I install the
whole library.

I guess it would also be useful for tidying away scratch directories and
the like, used by rules for their transient or intermediate files.

It sounds like a neat idea.  I wonder how practical it would be to
implement ...


Reply via email to