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Date:   Sun, 25 Jun 2017 21:53:02 +0200
From:   "Enrico Weigelt, metux IT consult" <>
To:     Joe Perches <>,
Subject: Re: [PATCH] lib: vsprintf: add printf format conversion %M for errno

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. 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() ?


Enrico, Sohn von Wilfried, a.d.F. Weigelt,
metux IT consulting

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