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Re: [PATCH 7/6] Makefile: Make Makefile depend on generated qga files, t

From: Eric Blake
Subject: Re: [PATCH 7/6] Makefile: Make Makefile depend on generated qga files, too
Date: Wed, 4 Dec 2019 06:58:16 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.2

On 12/4/19 12:56 AM, Markus Armbruster wrote:

+++ b/Makefile
@@ -130,6 +130,15 @@ GENERATED_QAPI_FILES += qapi/qapi-doc.texi
     generated-files-y += $(GENERATED_QAPI_FILES)
   +GENERATED_QGA_FILES := qga-qapi-types.c qga-qapi-types.h
+GENERATED_QGA_FILES += qga-qapi-visit.c qga-qapi-visit.h
+GENERATED_QGA_FILES += qga-qapi-commands.h qga-qapi-commands.c
+GENERATED_QGA_FILES += qga-qapi-init-commands.h qga-qapi-init-commands.c
+GENERATED_QGA_FILES += qga-qapi-doc.texi
+GENERATED_QGA_FILES := $(addprefix qga/qapi-generated/, $(GENERATED_QGA_FILES))

Would it be worth using two separate variable names (maybe
GENERATED_QGA_BASEFILES for the first list) rather than exploiting the
arcane knowledge that consecutive use of := causes GNU make to rewrite
an existing variable with new contents?

Our rules.mak relies on this already.  It's full of magic, which
admittedly diminishes its suitability to serve as a good example.

Your worry might be rooted in old "=" burns.  "=" makes the variable
recursively expanded, and

     GENERATED_QGA_FILES = $(addprefix qga/qapi-generated/, 

Indeed, but I have to refer to the manual to remind myself of whether = or := is what I want in a given situation.

would be an infinite loop.  ":=" makes it simply expanded; there's not
even a loop, let alone an infinite one.  The GNU Make manual explains
this clearly at

Aside: there's a reason one of the two flavors is called "simple".  It
could additionally be called "not as slow".  One of my pet makefile
peeves: unthinking use of recursively expanded variables, complicating
semantics and slowing down builds.

Back to this patch.  I had started to write the thing in longhand, but
got tired of repeating qga/qapi-generated/, so I factored that out.
Would longhand be easier to understand?

It's more verbose.  My suggestion was more:

GENERATED_QGA_BASENAMES := qga-qapi-types.c qga-qapi-types.h
GENERATED_QGA_BASENAMES += qga-qapi-visit.c qga-qapi-visit.h
GENERATED_QGA_FILES := $(addprefix qga/qapi-generated/, $(GENERATED_QGA_BASENAMES))

to avoid the reassignment-to-self issue altogether, while still remaining concise compared to longhand.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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