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Documentation: coding-style: Document global variable guidelines

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Document the issue related to global variable dependencies and how to
avoid them.

Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
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Paul Elder 2020-09-21 14:00:19 +09:00
parent 6193e9c252
commit 7e59bccb35
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Documentation/coding-style.rst
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@ -187,6 +187,25 @@ These rules match the `object ownership rules from the Chromium C++ Style Guide`
long term borrowing isn't marked through language constructs, it shall be
documented explicitly in details in the API.
Global Variables
~~~~~~~~~~~~~~~~
The order of initializations and destructions of global variables cannot be
reasonably controlled. This can cause problems (including segfaults) when global
variables depend on each other, directly or indirectly. For example, if the
declaration of a global variable calls a constructor which uses another global
variable that hasn't been initialized yet, incorrect behavior is likely.
Similar issues may occur when the library is unloaded and global variables are
destroyed.
Global variables that are statically initialized and have trivial destructors
(such as an integer constant) do not cause any issue. Other global variables
shall be avoided when possible, but are allowed when required (for instance to
implement factories with auto-registration). They shall not depend on any other
global variable, should run a minimal amount of code in the constructor and
destructor, and code that contains dependencies should be moved to a later
point in time.
C Compatibility Headers
~~~~~~~~~~~~~~~~~~~~~~~