linux-kernel - Re: [PATCH] lib/vsprintf: add %pT[C012] format specifier

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Date:	Sat, 28 Dec 2013 19:57:50 +0100
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Jiri Kosina <jkosina@...e.cz>, Joe Perches <joe@...ches.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	"David S. Miller" <davem@...emloft.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] lib/vsprintf: add %pT[C012] format specifier

On Sat, Dec 28, 2013 at 4:43 AM, Tetsuo Handa
<penguin-kernel@...ove.sakura.ne.jp> wrote:
> Andrew Morton wrote:
>> which is painful, so we also provide the new vsprintf token as a
>> convenience:
>>
>>       pr_warn("%|: hair on fire\n");
>>
>> but I don't know what we can use in place of %|.
>
> We are using %pEXTENSION where EXTENSION is [A-Za-z0-9]* because compiler does
> not need to understand what EXTENSION does; compiler needs to care about what
> character follows the % character and check type of corresponding argument if
> __printf() attribute is given.

Indeed,

> If we introduce a character which compiler does not know that follows the %
> character, compiler would be confused when checking type of corresponding
> argument.
>
>> I wonder if there's some way in which we can invent a vsprintf token
>> which means "insert corrent->comm here" and which doesn't require that
>> the caller pass in the additional argument?
>
> Therefore, if we want to omit passing corresponding argument, we should not
> introduce new character which compiler does not know that follows the %
> character.
>
> Also, % is the only character which everybody knows that it is reserved for the
> beginning of format specifier and %% is the only characters which everybody
> knows that it is reserved for literal % character.
>
> Therefore, what we could do for printing current thread's attributes would be
> either reserve a new character and add EXTENSION like
>
>   pr_warn("$comm$: hair on fire\n");
>   pr_warn("Process $pid$: hair on fire\n");
>
> or add EXTENSION after the %% characters like
>
>   pr_warn("%%comm%%: hair on fire\n");
>   pr_warn("Process %%pid%%: hair on fire\n");

ESC sequences? So far printk() doesn't parse them (a bit unfortunate, as I
always liked the idea of printing error messages in red, warnings in yellow,
etc.).

Is any of the "\x" (backslash + character) unused and thus available?

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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