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

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Date:	Sat, 28 Dec 2013 12:43:24 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	akpm@...ux-foundation.org
Cc:	jkosina@...e.cz, joe@...ches.com, viro@...iv.linux.org.uk,
	davem@...emloft.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] lib/vsprintf: add %pT[C012] format specifier

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.

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");

.
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