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Date:	Sat, 28 Dec 2013 11:25:09 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	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, 28 Dec 2013 19:57:50 +0100 Geert Uytterhoeven <geert@...ux-m68k.org> wrote:

> > 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?

I guess control characters would work.

#define PRINTK_COMM	"\001"
#define PRINTK_PID	"\002"
#define PRINTK_TASK_ID	"\003"	/* "comm:pid" */

	printk(PRINTK_TASK_ID ": hair on fire\n");

It's certainly compact.  I doubt if there's any existing code which
deliberately prints control chars?

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