[PATCH 2/4] coding-style: show how reusing macros prevents naming collisions

[Yueh-Shun Li <shamrocklee@posteo.net>]

In section "18) Don't re-invent the kernel macros" in "Linux kernel
coding style":

Show how reusing macros from shared headers prevents naming collisions
using "stringify", the one of the most widely reinvented macro, as an
example.

This patch aims to provide a stronger reason to reuse shared macros,
by showing the risk of improvised macro variants.

Signed-off-by: Yueh-Shun Li <shamrocklee@posteo.net>
---
 Documentation/process/coding-style.rst | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/Documentation/process/coding-style.rst b/Documentation/process/coding-style.rst
index 2504cb00a961..1e79aba4b346 100644
--- a/Documentation/process/coding-style.rst
+++ b/Documentation/process/coding-style.rst
@@ -1070,6 +1070,28 @@ Similarly, if you need to calculate the size of some structure member, use
 There are also ``min()`` and ``max()`` macros in ``include/linux/minmax.h``
 that do strict type checking if you need them.
 
+Using existing macros provided by the shared headers also prevents naming
+collisions. For example, if one developer define in ``foo.h``
+
+.. code-block:: c
+
+	#define __stringify(x) __stringify_1(x)
+	#define __stringify_1(x) #x
+
+and another define in ``bar.h``
+
+.. code-block:: c
+
+	#define stringify(x) __stringify(x)
+	#define __stringify(x) #x
+
+When both headers are ``#include``-d into the same file, the facilities provided
+by ``foo.h`` might be broken by ``bar.h``.
+
+If both ``foo.h`` and ``bar.h``  use the macro ``__stringify()`` provided by
+``include/linux/stringify.h``, they wouldn't have stepped onto each other's
+toes.
+
 Feel free to search across and peruse the header files to see what else is
 already defined that you shouldn't reproduce in your code.
 
-- 
2.42.0