Re: [PATCH 0/4] drop some "int x = x" hacks to silence gcc warnings

From: Jeff King <peff@peff.net>
Date: Mar 21, 2013, 11:55 UTC
Message-ID: <20130321115545.GB21319@sigill.intra.peff.net>
In-Reply-To: <514AF2E1.7020409@viscovery.net>

On Thu, Mar 21, 2013 at 12:45:37PM +0100, Johannes Sixt wrote:

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> Am 3/21/2013 12:03, schrieb Jeff King:
> > I was fooling around with clang and noticed that it complains about the
> > "int x = x" construct under -Wall. That is IMHO a deficiency in clang,
> > since the idiom has a well-defined use in silencing -Wuninitialized
> > warnings.
> 
> IMO, that's a myth. The construct invokes undefined behavior at least
> since C99, and the compilers are right to complain about it.

While undefined behavior does leave the compiler free to do anything, including nasal demons, it would be a very poor implementation that did anything except leave random bytes in the value. And it also means that gcc is free to take it as a hint to silence the warning; given that clang tries to be compatible with gcc, I'd think it would want to do the same. But I may be wrong that the behavior from gcc is intentional or common (see below).

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> But you might just say that standards are not worth the paper they are
> printed on, and you may possibly be right for practical reasons. But I
> still consider it a myth that "int x = x" is an idiom. I'm in the C
> business since more than 25 years, and the first time I saw the "idiom"
> was in git code. Is there any evidence that the construct is used
> elsewhere? Have I been in the wrong corner of the C world for such a long
> time?

Git code was my introduction to it, too, and I was led to believe it was idiomatic, so I can't speak further on that. I think it was Junio who introduced me to it, so maybe he can shed more light on the history.

-Peff