[yocto] which yocto doc explains bitbake assignment variations?

[Rudolf Streif]

1. yes, = will always override any prior ?= or ??= and any subsequent ?= or
??= will not change the setting anymore.

2. yes, a ?= following a ??= will set the variable. Only a = will be
override the setting of the ?=. Any following ?= and ??= will not change
the setting.

3. yes, a ?= or = following a ??= will set the variable.


Just try it. Put a variable (FOO) in local.conf and use bitbake -e | grep
FOO

FOO ??= "bar"
FOO ?= "barbar"

:rjs

On Thu, Mar 29, 2012 at 3:34 PM, Robert P. J. Day <rpjday at crashcourse.ca>wrote:

>
>  just to be absolutely clear so i don't misunderstand:
>
> On Thu, 29 Mar 2012, Rudolf Streif wrote:
>
> > VAR = "value"
> >
> >     In this example, VAR is set to value.
>
>   so the "=" assignment will *always* set the variable to that value,
> regardless of what combination of =, ?= or ??= has come before it,
> correct?
>
> >     Early Default Value Assignment (?=)
> >
> >     A variable can be early-assigned a default value using
> the ?= assignment operator:
> >
> > A ?= "1"
> > B ?= "2"
> > B ?= "3"
> >
> >     In this example A will contain 1 if it was not previously
> set. B will contain 2 because
> >     the ?= assignment operator is immediate meaning that if there are
> multiple ?= assignments
> >     to a single variable, the first one will be used.
>
>   and the first ?= will be processed, even if there were earlier ??=
> assignments, correct?  (obviously not if there were earlier =
> assignments.)
>
> > A ??= "1"
> > B ??= "2"
> > B ??= "3"
> >
> >     In this example A will contain 1 if it was not previously
> set. B will contain 3 because
> >     the ??= assignment operator is a late or lazy assignment operator
> and assignment will not
> >     take place until the end of the parsing process. If there are
> multiple ??= assignments to a
> >     single variable, the last one will be used.
>
>   however, this "weak" assignment is cancelled by the appearance of
> either a = or a ?= assignment at any time, yes?
>
> 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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