Static multiple target rules

[tom honermann]

I've been struggling for some time now with how to write rules for 
commands that generate multiple targets and I have so far been unable to 
find an elegant solution that works reliably.  I know pattern rules have 
support for specifying multiple targets, but they don't seem to be 
appropriate for all scenarios.

As an example, lets consider the venerable 'yacc' command:

   yacc -d -v grammar.y

This will produce the following files:  y.tab.h, y.tab.c, and y.output

Myself and many others have naively tried a rule like the one below and 
then been surprised to see 'yacc' get invoked multiple times (especially 
in parallel builds).

   y.tab.h y.tab.c y.output: grammar.y
       yacc -d -v $^

The fine GNU make manual describes this syntax and its behavior well 
(section 4.10 - "Multiple Targets in a Rule" in the manual I'm looking 
at).  This syntax and behavior are very useful - just not for specifying 
multiple targets for a single command!

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

This solves the problem of 'yacc' getting run multiple times while 
preserving the desired behavior that 'yacc' is only invoked when its 
targets are out of date.  However, if 'yacc.ts' exists, is newer than 
'grammar.y', and any of 'y.tab.h', 'y.tab.c', or 'y.output' do not 
exist, then 'yacc' won't get run to re-generate them (since make sees 
that 'yacc.ts' is up to date and concludes that the other targets must 
be up to date as well).

A solution for that last problem might look something like this:

   $(shell [ -f y.tab.h -a -f y.tab.c -a -f y.output ] || rm -f yacc.ts)

   yacc.ts: grammar.y
       yacc -d -v $^
       touch $@

   y.tab.h y.tab.c y.output: yacc.ts

This works, but is rather inelegant - especially if there are many more 
output files than in my simple example.  Is there a better way to write 
rules for generating multiple targets such as this?  Has anyone proposed 
extensions to GNU make to support static multiple target rules?

Tom.