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| author | Pierrick Bouvier <pierrick.bouvier@linaro.org> | 2025-01-16 16:02:53 +0000 |
|---|---|---|
| committer | Alex Bennée <alex.bennee@linaro.org> | 2025-01-17 10:45:27 +0000 |
| commit | ecbf3567e217bc7de320bfe165c8ce72eea51b2c (patch) | |
| tree | fbc9e5473a0dfebbcb0780fdf45b23dbe9abb0dc /docs/devel | |
| parent | 8f5a4cfc7ed9e06e07fdd8e8fdf50ef3ea783f63 (diff) | |
| download | qemu-ecbf3567e217bc7de320bfe165c8ce72eea51b2c.zip qemu-ecbf3567e217bc7de320bfe165c8ce72eea51b2c.tar.gz qemu-ecbf3567e217bc7de320bfe165c8ce72eea51b2c.tar.bz2 | |
docs/devel/style: add a section about bitfield, and disallow them for packed structures
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Tested-by: Stefan Weil <sw@weilnetz.de>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20250110203401.178532-3-pierrick.bouvier@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20250116160306.1709518-25-alex.bennee@linaro.org>
Diffstat (limited to 'docs/devel')
| -rw-r--r-- | docs/devel/style.rst | 20 |
1 files changed, 20 insertions, 0 deletions
diff --git a/docs/devel/style.rst b/docs/devel/style.rst index 2f68b50..d025933 100644 --- a/docs/devel/style.rst +++ b/docs/devel/style.rst @@ -416,6 +416,26 @@ definitions instead of typedefs in headers and function prototypes; this avoids problems with duplicated typedefs and reduces the need to include headers from other headers. +Bitfields +--------- + +C bitfields can be a cause of non-portability issues, especially under windows +where `MSVC has a different way to lay them out than GCC +<https://gcc.gnu.org/onlinedocs/gcc/x86-Type-Attributes.html>`_, or where +endianness matters. + +For this reason, we disallow usage of bitfields in packed structures and in any +structures which are supposed to exactly match a specific layout in guest +memory. Some existing code may use it, and we carefully ensured the layout was +the one expected. + +We also suggest avoiding bitfields even in structures where the exact +layout does not matter, unless you can show that they provide a significant +usability benefit. + +We encourage the usage of ``include/hw/registerfields.h`` as a safe replacement +for bitfields. + Reserved namespaces in C and POSIX ---------------------------------- |