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Date:   Sun, 25 Jun 2017 13:10:49 -0700
From:   Joe Perches <>
To:     "Enrico Weigelt, metux IT consult" <>,
Subject: Re: [PATCH] lib: vsprintf: add printf format conversion %M for
 errno strings

On Sun, 2017-06-25 at 21:53 +0200, Enrico Weigelt, metux IT consult
> On 25.06.2017 19:27, Joe Perches wrote:
> > Every use of %M is going to cause gcc when using __printf to emit
> > a warning like:
> > 
> > unknown conversion type character ‘M’ in format [-Wformat=]
> Yeah, that's still an open problem. Actually, I still haven't found out,
> how it's done w/ all the other kernel-internal conversions.

Everything else uses "%p<foo>", <object reference>

> I was under
> the impression, there was some magic to tell the compiler which letters
> correspond to which types - unfortunately, didn't find anything like
> that. Is that really hardcoded in gcc ?
> > Beyond that, why is this useful?
> Use that instead of %d where errno values are printed/logged.
> > There can't possibly be any fast-path use.
> I'm using it eg. for driver development - always having to look up the
> numbers is quite ugly and time consuming.
> > Why not just create a function that does errno/string conversions?
> Already was about to do so. Shall I call it strerror() ?

I presume kstrerror

So use something like
	"%d: (%s)", errno, kstrerror(errno)

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