From: Paul Eggert <eggert@cs.ucla.edu>
To: Alan Mackenzie <acm@muc.de>
Cc: emacs-devel@gnu.org
Subject: Re: Building: alloc.c:766:1: error: negative width in bit-field `_gl_verify_error_if_negative'
Date: Mon, 13 Feb 2012 08:57:36 -0800 [thread overview]
Message-ID: <4F394100.2030704@cs.ucla.edu> (raw)
In-Reply-To: <20120213130702.GA26954@acm.acm>
On 02/13/2012 05:07 AM, Alan Mackenzie wrote:
> How can the compiler transform "INT_MAX <= PTRDIFF_MAX" into a message
> about bit-field widths?
That message is generated for compilers that are not known to
support C11-style static assertions
<http://gcc.gnu.org/ml/gcc-patches/2010-05/msg01188.html>.
In GCC 4.6.0 and later, verify(X) simply expands to _Static_assert(X).
Other compilers (including older GCCs) are not known to have
_Static_assert, so verify(X) expands to:
struct { unsigned _gl_verify_error_if_negative: (X) ? 1 : -1; }
which causes a compile-time error if X is false (the desired
effect); as you've noticed, the diagnostic isn't as nice as
with GCC 4.6.0 and later.
This is implemented in lib/verify.h -- more commentary is there.
My guess is that PTRDIFF_MAX is set incorrectly in your implementation.
Emacs's 'configure' script attempts to detect that, and replace it
with a correct PTRDIFF_MAX, but perhaps we need to improve 'configure'
so that it detects the problem with your implementation.
Can you send the compressed "gcc -E ..." output of the offending
compilation, and the contents of 'config.log' and 'src/config.h' and
(if present) 'lib/stdint.h'? That should help debug the situation.
Also, if you could determine which standard header is defining
ptrdiff_t and PTRDIFF_MAX, and what they're defined to, that might
help.
next prev parent reply other threads:[~2012-02-13 16:57 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-02-12 11:02 Building: alloc.c:766:1: error: negative width in bit-field `_gl_verify_error_if_negative' Alan Mackenzie
2012-02-12 21:31 ` Paul Eggert
2012-02-13 13:07 ` Alan Mackenzie
2012-02-13 16:57 ` Paul Eggert [this message]
2012-02-13 20:34 ` Alan Mackenzie
2012-02-13 20:50 ` Paul Eggert
2012-02-13 22:41 ` Alan Mackenzie
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