Re: [PATCH net-next v2 1/2] Makefile.extrawarn: Add symbol for W=1 warnings for today

From: Masahiro Yamada <masahiroy@kernel.org>
To: Arnd Bergmann <arnd@arndb.de>
Cc: Andrew Lunn <andrew@lunn.ch>,
	Nick Desaulniers <ndesaulniers@google.com>,
	netdev <netdev@vger.kernel.org>,
	David Miller <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	Michal Marek <michal.lkml@markovi.net>,
	Rohit Maheshwari <rohitm@chelsio.com>,
	Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>
Subject: Re: [PATCH net-next v2 1/2] Makefile.extrawarn: Add symbol for W=1 warnings for today
Date: Mon, 12 Oct 2020 10:00:02 +0900	[thread overview]
Message-ID: <CAK7LNARRchbhDNUT3paTVpOJYKR-D_+HLzjG-wsOOM+LO5p3sA@mail.gmail.com> (raw)
In-Reply-To: <CAK8P3a0tA9VMMjgkFeCaM3dWLu8H0CFBTkE8zeUpwSR1o31z1w@mail.gmail.com>

On Fri, Oct 2, 2020 at 9:21 PM Arnd Bergmann <arnd@arndb.de> wrote:
>
> I do care about the tree as a whole, and I'm particularly interested in
> having -Wmissing-declarations/-Wmissing-prototypes enabled globally
> at some point in the future.
>
>         Arnd

BTW, if possible, please educate me about the difference
between -Wmissing-declarations and -Wmissing-prototypes.

The GCC manual says as follows:

-Wmissing-prototypes (C and Objective-C only)

Warn if a global function is defined without a previous prototype
declaration. This warning is issued even if the definition itself
provides a prototype. Use this option to detect global functions that
do not have a matching prototype declaration in a header file. This
option is not valid for C++ because all function declarations provide
prototypes and a non-matching declaration declares an overload rather
than conflict with an earlier declaration. Use -Wmissing-declarations
to detect missing declarations in C++.

-Wmissing-declarations

Warn if a global function is defined without a previous declaration.
Do so even if the definition itself provides a prototype. Use this
option to detect global functions that are not declared in header
files. In C, no warnings are issued for functions with previous
non-prototype declarations; use -Wmissing-prototypes to detect missing
prototypes. In C++, no warnings are issued for function templates, or
for inline functions, or for functions in anonymous namespaces.

The difference is still unclear to me...

For example, if I add -Wmissing-declarations, I get the following:

kernel/sched/core.c:2380:6: warning: no previous declaration for
‘sched_set_stop_task’ [-Wmissing-declarations]
 2380 | void sched_set_stop_task(int cpu, struct task_struct *stop)
      |      ^~~~~~~~~~~~~~~~~~~

But, if I add both -Wmissing-declarations and -Wmissing-prototypes,
-Wmissing-declarations is superseded by -Wmissing-prototypes.

kernel/sched/core.c:2380:6: warning: no previous prototype for
‘sched_set_stop_task’ [-Wmissing-prototypes]
 2380 | void sched_set_stop_task(int cpu, struct task_struct *stop)
      |      ^~~~~~~~~~~~~~~~~~~

Do we need to specify both in W=1 ?
If yes, what is the difference between them?

--
Best Regards
Masahiro Yamada