[yocto] do any yocto docs clearly distinguish between "+=" and "_append"?

[Paul Eggleton]

Hi Robert,

On Monday 11 November 2013 06:30:46 Robert P. J. Day wrote:
>   still slogging my way through the dev manual, and here:
> 
> http://www.yoctoproject.org/docs/latest/dev-manual/dev-manual.html#usingpoky
> -extend-customimage
> 
> section 5.2.1 recommends that recipe files use:
> 
> IMAGE_INSTALL += "strace"
> 
> while, further down in section 5.2.4, developers are encouraged to add
> to their personal local.conf:
> 
> IMAGE_INSTALL_append = " strace"

There is a good reason for this difference - it's likely that IMAGE_INSTALL 
will be set using = in the image recipe, and thus if you use += to add to it 
in local.conf it will have no effect. _append on the other hand is deferred 
until later and thus it will always work from local.conf. In the image recipe 
it's OK to use IMAGE_INSTALL += because you'll be able to put it in the right 
place with regard to any other assignment of the variable.

As I've noted more than once, modifying IMAGE_INSTALL or 
CORE_IMAGE_EXTRA_INSTALL from local.conf is really only appropriate for quick 
testing - for anything beyond that users should be creating their own image 
recipe(s).

>   is there somewhere in a yocto doc a clear explanation of the proper
> usage of "+=" versus "_append"? because, in my experience, that's a
> point of serious confusion for new users.

The proper place for the general discussion about how these operators work is 
really the BitBake manual; we're planning on promoting that to a first-class 
citizen and publishing it with the rest of our manuals for 1.6.

Cheers,
Paul

-- 

Paul Eggleton
Intel Open Source Technology Centre