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bitbake: bitbake-user-manual: Clarified override-style operators. 

Fixes [YOCTO #9985]

Made the following changes:

 * Section Removal (Override Style Syntax):  Added a small
   qualifying sentence at the end to further define behavior

 * Added new section "Override Style Operation Advantages":
   This section provides some rationale behind the "_append"
   style operations.

 * Section "Examples": Changed an example to use the "="
   operator rather than the "+=" operator.

(Bitbake rev: 797d9627baad9ccd3d55e825c0d705311f631f78)

Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
This commit is contained in:
Scott Rifenbark 2016-07-21 10:37:09 -07:00 committed by Richard Purdie
parent d1e3f0bb16
commit 6aaf379119
1 changed files with 56 additions and 2 deletions
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58
bitbake/doc/bitbake-user-manual/bitbake-user-manual-metadata.xml
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@ -278,6 +278,60 @@
"789 123456" and <filename>FOO2</filename> becomes
"ghi abcdef".
</para>
<para>
Like <filename>_append</filename> and
<filename>_prepend</filename>, <filename>_remove</filename>
is deferred until after parsing completes.
</para>
</section>
<section id='override-style-operation-advantages'>
<title>Override Style Operation Advantages</title>
<para>
An advantage of the override style operations
"_append", "_prepend", and "_remove" as compared to the
"+=" and "=+" operators is that the override style
operators provide guaranteed operations.
For example, consider a class iilename>foo.bbclass</filename>
that needs to add the value "val" to the variable
<filename>FOO</filename>, and a recipe that uses
<filename>foo.bbclass</filename> as follows:
<literallayout class='monospaced'>
inherit foo
FOO = "initial"
</literallayout>
If <filename>foo.bbclass</filename> uses the "+=" operator,
as follows, then the final value of <filename>FOO</filename>
will be "initial", which is not what is desired:
<literallayout class='monospaced'>
FOO += "val"
</literallayout>
If, on the other hand, <filename>foo.bbclass</filename>
uses the "_append" operator, then the final value of
<filename>FOO</filename> will be "initial val", as intended:
<literallayout class='monospaced'>
FOO_append = " val"
</literallayout>
<note>
It is never necessary to use "+=" together with "_append".
The following sequence of assignments appepnds "barbaz" to
<filename>FOO</filename>:
<literallayout class='monospaced'>
FOO_append = "bar"
FOO_append = "baz"
</literallayout>
The only effect of changing the second assignment in the
previous example is to add a space before "baz" in the
appended value (due to how the "+=" operator works.
</note>
Another advantage of the override style operations is that
you can combine them with other overrides as described in the
"<link linkend='conditional-syntax-overrides'>Conditional Syntax (Overrides)</link>"
section.
</para>
</section>
<section id='variable-flag-syntax'>
@ -564,13 +618,13 @@
OVERRIDES = "foo"
A = "Y"
A_foo_append = "Z"
A_foo_append += "X"
A_foo_append = "X"
</literallayout>
For this case, before any overrides are resolved,
<filename>A</filename> is set to "Y" using an immediate assignment.
After this immediate assignment, <filename>A_foo</filename> is set
to "Z", and then further appended with
"X" leaving the variable set to "Z X".
"X" leaving the variable set to "ZX".
Finally, applying the override for "foo" results in the conditional
variable <filename>A</filename> becoming "Z X" (i.e.
<filename>A</filename> is replaced with <filename>A_foo</filename>).