On Tue, 9 Mar 2021, Quentin Schulz wrote:

Hi Robert,

On Tue, Mar 09, 2021 at 09:39:14AM -0500, Robert P. J. Day wrote:

bitbake manual, chapter 3, examples of conditional syntax:

https://docs.yoctoproject.org/bitbake/bitbake-user-manual/bitbake-user-manual-metadata.html#examples

correctly distinguishes between A_foo_append and A_append_foo, but how
often would one use that first form, anyway?

most uses of conditional appending are either just straight
appending:

VAR_append = "fubar"

or used with an override thusly:

VAR_append_x86 = "snafu"

is there an actual practical usage of, say:

VAR_x86_append = "huh"

i can't remember the last time i saw something of that form and,
while it might be worth explaining, it seems that the reader might be
warned that that form is almost certainly *not* what they want.

Yes, in 99% of the cases, you want VAR_append_foo and not
VAR_foo_append.

VAR_foo_append makes sense when you want to append to VAR_foo which
is a way to override completely VAR for builds matching the foo
override. This happens in kernel-yocto recipes where branches and
defconfigs are different per machine for example.

The sneaky thing about VAR_foo_append is that it creates VAR_foo
even if it didn't exist beforehand, meaning you though you
*appended* something to VAR while you actually replaced VAR entirely
with what VAR_foo_append is set too.

i remember testing this a few years back, and making sure i
appreciated the subtleties. i might take a shot at rewriting that
section, since i think it has potential for confusion. mostly, warning
the reader that the first form is almost always what they want.

rday