i want to make absolutely sure i understand the mechanics of the
different types of assignment so i'm looking at this snippet from
core-image.bbclass:
... snip ...
PACKAGE_GROUP_qt4-pkgs = "task-core-qt-demos"
CORE_IMAGE_BASE_INSTALL = '\
task-core-boot \
task-base-extended \
\
${CORE_IMAGE_EXTRA_INSTALL} \
'
CORE_IMAGE_EXTRA_INSTALL ?= ""
IMAGE_INSTALL ?= "${CORE_IMAGE_BASE_INSTALL}"
... snip ...
first, if you're assigning a fixed string (as in the first line
shown in the snippet above), i assume there is no functional
difference in using "=" versus ":=", correct?
next, is there any value in that null line in the assignment to
CORE_IMAGE_BASE_INSTALL?
next, is it technically necessary to conditionally do this?
CORE_IMAGE_EXTRA_INSTALL ?= ""
if that variable wasn't set, does it really need to be set to the
empty string?
finally, what are the evaluation mechanics of the "?=" operator? in
this line,
IMAGE_INSTALL ?= "${CORE_IMAGE_BASE_INSTALL}"
is the right-hand side evaluated first? i ask since i find the
snippet above a bit confusing in that you need to follow backwards to
deduce what IMAGE_INSTALL is eventually set to. i would think this
would be easier to read:
CORE_IMAGE_BASE_INSTALL = '\
task-core-boot \
task-base-extended
IMAGE_INSTALL = "${CORE_IMAGE_BASE_INSTALL} ${CORE_IMAGE_EXTRA_INSTALL}"
in this rewriting, i can *immediately* see what values are used to
construct IMAGE_INSTALL. in the current code, it's nowhere near as
clear, and you have to backtrack to see how CORE_IMAGE_BASE_INSTALL is
assigned. or would that not be equivalent based on a misunderstanding
of how assignment works?
rday
--
========================================================================
Robert P. J. Day Ottawa, Ontario, CANADA
http://crashcourse.ca
Twitter: http://twitter.com/rpjday
LinkedIn: http://ca.linkedin.com/in/rpjday
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