Re: Help me understand Openembedded variable expansion

[Richard Purdie <richard.purdie@linuxfoundation.org>]

On Tue, 2014-10-28 at 10:11 -0700, Matt Schuckmann wrote:
> So there is something I just don't get about how variables get
> created, expanded, interpreted, etc in Bitbake
> 
> For example I'm trying to copy some of the techniques in the Poky Tiny
> distribution to reduce my image size and build time. 
> One of the fragments is this one. 
> 
> # The mtrace script included by eglibc is a perl script. This means
> the system
> # will build perl in case this package is installed. Since we don't
> care about
> # this script, remove the dependency from here.
> RDEPENDS_${PN}-mtrace_pn-eglibc = ""
> 
> I put this in my distribution .conf file and I doesn't seem to do
> anything perl is still getting built. 
> Of course it doesn't help that I don't understand what does,
> especially in the context of the distribution .conf file. 
> 
> I also see hints in the code in debug output that the machine, image,
> or distribution names can automatically get appended to paths, or
> variables but I don't really understand how that works. 
> 
> I'm not even sure how to phrase a question around this, immediately
> I'd like to figure out why Perl is still getting built but ultimately
> I'd like to understand the variable rules better so I that I can
> better take advantage of this power tool. 

Perhaps have a look at "overrides" in the bitbake manual.

You can split RDEPENDS_${PN}-mtrace_pn-eglibc into components. Starting
on the RHS, "_pn-eglibc" is an override that applies to recipes where PN
= "eglibc". In those recipes, the variable becomes:

RDEPENDS_${PN}-mtrace = ""

and is basically overriding a dependency on perl that would other be
there, something like:

RDEPENDS_${PN}-mtrace = "perl".

Does that help?

Cheers,

Richard