Re: Multiple targets generated by one rule

[Paul Smith]

On Mon, 2007-04-09 at 16:20 -0700, Ben Wilhelm wrote:

> a b c d: source.flag
>    @true
> 
> source.flag: source
>    ( touch source.flag && blackbox_generate a b c d ) \
>      || ( rm -f source.flag && false )   # eugh
> 
> but this is horribly ugly, although it does work. (I think the @true is 
> necessary but to be honest I'm hazy as to why, although it doesn't 
> rebuild the things that depend on a b c d without it.)

You can write this much more cleanly, like this:

        a b c d: .source_flag
                @:
        
        .source_flag: source
                blackbox_generate a b c d
                @touch .source_flag

> Is there a good solution to this? Ideally one without extra temporary 
> files? While I know why the first case doesn't work ("A rule with 
> multiple targets is equivalent to writing many rules, each with one 
> target, and all identical aside from that." from 4.10 in the make 
> manual) I haven't been able to figure out how to get the behavior I want.

There are only two ways to do what you want.  The one above works for
all types of targets.

The other way is to use pattern rules: if your targets and a
prerequisite share a stem that can be matched by a pattern ("%"), then
you can write:

        %a %b %c %d: %source
                blackbox_generate $*a $*b $*c $*d

Unlike explicit rules, pattern rules with multiple targets behave as you
are looking for: they build all the targets with one invocation of the
command script.

If you can't use pattern rules then you must use temporary files, sorry.
Although the method I show above is much cleaner, and users will
probably not notice the difference.

-- 
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