[bitbake-devel] still confused about _append and OVERRIDES and = and +=, argh

[Robert P. J. Day]

  i'm trying to write a more comprehensive tutorial about what's
currently in the bitbake manual, section 2.2.1, "Override and append
ordering" and it's becoming clear that i still don't have a handle on
this.

  by way of intro, this is what got me wondering this morning so does
it make any sense?

meta/recipes-extended/chkconfig/chkconfig_1.3.58.bb:FILES_${PN}_append_linuxstdbase += "${libdir}/lsb"

note the combination of "_append_"? and "+=", which strikes me as
redundant, or so i thought until i played with this and now i'm more
confused than ever, so i ran some tests wherein i added this to my
local.conf:

OVERRIDES .= ":rday"

and started playing with chris larson's "bitbake-env" utility and
here's what i learned (for better or worse) based on adding different
sets of test settings underneath that in local.conf.

	test 1:

BLAH = "A"

$ bitbake-env BLAH
BLAH="A"
$

  totally predictable.  movin on ...

	test 2:

BLAH = "A"
BLAH_rday = "B"

$ bitbake-env BLAH
BLAH="B"
$

  that looks good, it's exactly what i expected given the override.
keep going ...

	test 3:

BLAH = "A"
BLAH_rday = "B"
BLAH_rday = "C"

$ bitbake-env BLAH
BLAH="C"
$

  again, so far, so good -- the second override superseded the first,
correct?

	test 4:

BLAH = "A"
BLAH_rday = "B"
BLAH_rday_append = "C"

$ bitbake-env BLAH
BLAH="B"
$

   ?????  that confuses me.  according to section 2.2.1 in bitbake
manual (very first example):

OVERRIDES = "foo"

A_foo_append = "X"

"In this case, X is unconditionally appended to the variable A_foo.
Since foo is an override, A_foo would then replace A."

according to that, i should expect BLAH to have the value "BC", no?

  more to the point, i don't see a lot of assignments of the form:

  VAR_<overridevalue>_append =

i see *loads* of

  VAR_append_<overridevalue> =

which makes much more sense.  so how is one supposed to interpret the
above based on the explanation in the current bitbake manual?

rday

-- 

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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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