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build system: Document --warn-undefined-variables and add a config item for it

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Angus Gratton
2017-09-05 16:10:00 +10:00
committed by Angus Gratton
parent 8670844acf
commit 5f3cb9f9dc
4 changed files with 39 additions and 3 deletions
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docs/api-reference/kconfig.rst
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@@ -25,4 +25,13 @@ By convention, all option names are upper case with underscores. When Kconfig ge
.. include:: /_build/inc/kconfig.inc
.. _Kconfig: https://www.kernel.org/doc/Documentation/kbuild/kconfig-language.txt
Customisations
==============
Because IDF builds by default with :ref:`warn-undefined-variables`, when the Kconfig tool generates Makefiles (the ``auto.conf`` file) its behaviour has been customised. In normal Kconfig, a variable which is set to "no" is undefined. In IDF's version of Kconfig, this variable is defined in the Makefile but has an empty value.
(Note that ``ifdef`` and ``ifndef`` can still be used in Makefiles, because they test if a variable is defined *and has a non-empty value*.)
When generating header files for C & C++, the behaviour is not customised - so ``#ifdef`` can be used to test if a boolean config item is set or not.
.. _Kconfig: https://www.kernel.org/doc/Documentation/kbuild/kconfig-language.txt