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RE: Rules with multiple outputs

From: Lane Schwartz
Subject: RE: Rules with multiple outputs
Date: Mon, 12 Apr 2010 14:34:43 -0500

>> I am trying to solve an issue regarding rules with multiple outputs. I would 
>> like my makefile to correctly handle these rules, so that the generating 
>> commands are only run once, even when make is run with -j (for parallel). I 
>> am aware that a solution to handle this is using pattern rules, as in the 
>> case when generating files using bison (section 10.5.2 GNU Make manual). 
>> But, in my case the targets that are generated tend not to share named 
>> patterns.
>> Here's an example of what I would like to do:
>> a b c: d
>>      foo d
>> What I have done before is to make sure to only explicitly list the
>> commands for one target, and then list the others as a dependency.
>> Something like this.
> OUTPUTS:=a b c
> $(firstword $(OUTPUTS)):
>         foo d
> $(OUTPUTS): d
> $(firstword $(OUTPUTS)): $(filter-out $(firstword
> This will make sure the commands are run only once with make -j and
> keeps the proper dependencies for all the outputs.


The solution you present comes close to working for me. The main problem is 
that it won't work if a target lists b or c as dependencies, or if those 
targets are asked for at the command line. To solve that, I believe this works:

OUTPUTS:=a b c

$(firstword $(OUTPUTS)):
        ./foo d


$(filter-out $(firstword $(OUTPUTS)),$(OUTPUTS)): $(firstword $(OUTPUTS))

This works, but feels deeply hackish. Looking through the email archives, it 
looks like this is a fairly common problem. I would be very interested in 
adding a feature to allow multiple targets to explicitly be associated as 
outputs of a single command. What is the best process for requesting such a 
feature, or for submitting a patch to that effect? Also, do others who have 
struggled with this have suggestions for how other implementations of make use 
syntactic sugar to mark this sort of thing?


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