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Date: Thu, 26 Jul 2012 13:55:29 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 09/11] qapi: add qapi-errors.py
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To: Markus Armbruster <armbru@redhat.com>
Cc: mdroth@linux.vnet.ibm.com, aliguori@us.ibm.com, Eric Blake <eblake@redhat.com>, qemu-devel@nongnu.org, Luiz Capitulino <lcapitulino@redhat.com>

Il 26/07/2012 13:50, Markus Armbruster ha scritto:
>> > +qapi-errors.h qapi-errors.c :\
>> > +$(SRC_PATH)/qapi-schema-errors.json $(SRC_PATH)/scripts/qapi-errors.py
>> > +	$(call quiet-command,$(PYTHON) $(SRC_PATH)/scripts/qapi-errors.py -o "." < $<, "  GEN   $@")
> I'm afraid this isn't quite what you want.  It's shorthand for two
> separate rules with the same recipe[*].  Therefore, it's prone to run
> the recipe twice, with make blissfully unaware that each of the two runs
> clobbers the other file, too.  Could conceivably lead to trouble with
> parallel execution.
> 
> Paolo, Eric, maybe you can provide advice on how to best tell make that
> a recipe generates multiple files.

Hmm, I would just do

qapi-errors.h: qapi-errors.c
qapi-errors.c: $(SRC_PATH)/qapi-schema-errors.json $(SRC_PATH)/scripts/qapi-errors.py
	$(call quiet-command,$(PYTHON) $(SRC_PATH)/scripts/qapi-errors.py -o "." < $<, "  GEN   $@")

I think that's what I usually saw for bison (which creates both .h and .c).

A perhaps cleaner alternative is to add a stamp file, and make both files
depend on it.

Paolo