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Re: Future plans for Autotools

From: Gavin Smith
Subject: Re: Future plans for Autotools
Date: Sun, 31 Jan 2021 22:14:11 +0000
User-agent: Mutt/1.9.4 (2018-02-28)

On Mon, Jan 25, 2021 at 03:15:47PM -0800, Paul Eggert wrote:
> On 1/25/21 2:59 PM, Gavin Smith wrote:
> > Does this
> > work or does GNU make have other ways of doing this?  (It would be
> > better if it could be done without communicating via files.)  Is there
> > any way to set a Makefile variable from within a rule and then depend upon
> > that variable being set in other rules?
> GNU Make supports variables defined via ::=, and an earlier section of a GNU
> makefile could compute CC and assign to it that way. This would handle the
> simple case you gave. Unfortunately there are other cases that would require
> intermediate files with current GNU Make. Still, the overhead in using files
> would be worth it (at least for me) if we could use make -j.
> The ::= feature was added to POSIX ten years ago or so, so it could even be
> used in Makefiles intended to be portable to other new-enough
> POSIX-compatible platforms. It wouldn't work on older platforms, though.

Another issue is how to record the results of tests from a a Makefile.
With autoconf, autoconf variables are recorded with AC_CONFIG_FILES
and AC_CONFIG_HEADERS.  Presumably the same would have to be done,
substituting in an output file with the values of these variables.
However, there can be many hundreds of them (just look at a
file).  As far as I can tell, there isn't an easy way to export
hundreds of Makefile variables to a process.  Is there some feature or
trick I'm missing?

You could build up a long command line, but it could end up being
extremely long.  The code below shows how this could be done with
GNU Make (for just 3 variables).  The substitution step would need to
be done after all the variables had been set, unless there was some
kind of locking/synchronisation to allow Makefile rules to record
configuration variables in parallel.  It's possible to create a
small file for every variable but this would lead to hundreds of
small files being created.


VARS::=$(VARS) A=$$(A)
        $(eval A ::= $(shell echo 1))

VARS::=$(VARS) B=$$(B)
        $(eval B ::= $(shell echo 2))

VARS::=$(VARS) C=$$(C)
        $(eval C ::= $(shell echo 3))

finish-configure: set-A set-B set-C
        $(eval EXVARS=$(VARS))
        export $(EXVARS) ; echo "$$A $$B $$C"

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