RE: Static multiple target rules

[Martin Dorey]

> instead of a touch-file, use a tar-file !

Yeah, one of my company's makefiles uses a similar intermediate file to good 
effect.  We've been using the touch-file or "sentinel" solution elsewhere for 
years.  I'd previously suggested replacing the sentinels with tar files so we 
could get rid of all the hackery we have to address the problem Tom raised 
about non-existent targets.  We have a macro that's used to convert missing 
source file names to the name of the corresponding sentinel file and then code 
to force that sentinel to rebuild.  It's complicated, ugly and fragile.

What really motivated me to reply, however, was:

> Note the crucial -m in the extracting tar

I would have made that mistake, so thanks for pointing it out before I stepped 
on the rake!

-----Original Message-----
From: address@hidden [mailto:address@hidden On Behalf Of Edward Welbourne
Sent: Tuesday, March 02, 2010 02:45
To: tom honermann
Cc: address@hidden
Subject: Re: Static multiple target rules

> I've been struggling for some time now with how to write rules for 
> commands that generate multiple targets

A familiar and annoying problem: make really believes in commands that
generate just one (relevant) file, and doesn't fit so well with ones
that generate several.

> The next thing to try is using a proxy or timestamp file:
>
>    yacc.ts: grammar.y
>        yacc -d -v $^
>        touch $@
>
>    y.tab.h y.tab.c y.output: yacc.ts

A quarter of an hour after reading your mail, I had a mildly perverted
idea: instead of a touch-file, use a tar-file !  The problem you point
to is when the touch file exists but the files we want don't: we need
a command for the rule that declares their dependency on it; and that
command needs to be able to generate the outputs from the fake file.

    yacc.ts: grammar.y
        yacc -d -v $^
        tar cf $@ y.tab.h y.tab.c y.output

    y.tab.h y.tab.c y.output: yacc.ts
        tar xf $< -m

Note the crucial -m in the extracting tar, so that we touch the
outputs after the tar-file has been created, thereby avoiding
re-extracting every time we run make.

Not sure how well that'd work but it *looks* like it should ... and it
should generalise reasonably well.  Unfortunately I can't, just yet,
see how to turn it into a pattern rule for general .y file processing,

        Eddy.


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