GNU bug report logs - #15924
[PATCH] dfa: avoid undefined behavior of "1 << 31"

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Message #8 received at 15924 <at> debbugs.gnu.org (full text, mbox):

From: Paul Eggert <eggert <at> cs.ucla.edu>
To: Jim Meyering <jim <at> meyering.net>, 15924 <at> debbugs.gnu.org
Subject: Re: bug#15924: [PATCH] dfa: avoid undefined behavior of "1 << 31"
Date: Mon, 18 Nov 2013 18:16:10 -0800

Jim Meyering wrote:
>  static int
>  tstbit (unsigned int b, charclass const c)
>  {
> -  return c[b / INTBITS] & 1 << b % INTBITS;
> +  return c[b / INTBITS] & 1U << b % INTBITS;
>  }

On a machine with 32-bit int and where b % INTBITS is 31,
the expression c[b / INTBITS] & 1U << b % INTBITS
is of type 'unsigned' and can have the value 2**31, and
this will overflow when tstbit converts that value as an int,
leading to implementation-defined behavior, which can include
raising a signal.

Better would be something like this:

static bool
tstbit (unsigned int b, charclass const c)
{
  return c[b / INTBITS] >> b % INTBITS & 1;
}

and it'd probably be better to encourage this style in
other places where the problem occurs, e.g., quotearg.

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