From: Bruno Haible <bruno@clisp.org>
To: bug-gnulib@gnu.org
Cc: Paul Eggert <eggert@cs.ucla.edu>
Subject: Re: intptr_t vs. uintptr_t
Date: Sat, 14 Dec 2019 03:35:09 +0100 [thread overview]
Message-ID: <2169340.l2UmpkpDhn@omega> (raw)
In-Reply-To: <20db9cb5-da09-fe26-f7fc-884fc194daaa@cs.ucla.edu>
Paul Eggert wrote:
> https://www.gnu.org/software/emacs/manual/html_node/elisp/C-Integer-Types.html
Quoting it:
"Prefer intptr_t for internal representations of pointers"
I disagree with this advice. uintptr_t ought to be used for representing the
address of a pointer.
Why? Because when signed comparisons or pointer differences come into play,
- uintptr_t creates a boundary line at 0x00000000,
- intptr_t creates a boundary line at 0x80000000.
Now look at the virtual memory map of a process (e.g. by compiling vma-iter.c
with -DTEST).
On all OSes, there is a natural boundary line at 0x00000000 - simply because
there is the null-pointer catching area there.
On many OSes, memory allocations can lie around 0x80000000.
So, it is possible to have ptr1 = 0x7fffc000 and ptr2 = 0x80003000 point into
the same object (allocated through mmap or malloc). Then
- you DO want ptr1 < ptr2 to evaluate to true, not false,
- you DO want ptr2 - ptr1 to evaluate to 0x7000, not to a signed integer
overflow.
Bruno
next prev parent reply other threads:[~2019-12-14 2:35 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-03-22 23:06 [PATCH] grep: a kwset matcher not work in a grep matcher Norihiro Tanaka
2019-03-23 2:49 ` bug#34951: " Norihiro Tanaka
2019-03-23 2:58 ` Budi
2019-03-23 2:59 ` Budi
2019-03-23 12:39 ` Eric Blake
2019-03-29 10:58 ` arnold
2019-12-11 23:25 ` Paul Eggert
2019-12-12 7:23 ` arnold
2019-12-12 7:31 ` arnold
2019-12-12 7:47 ` arnold
2019-12-12 22:26 ` Paul Eggert
2019-12-13 8:09 ` arnold
2019-12-13 12:08 ` arnold
2019-12-13 17:53 ` Jim Meyering
2019-12-13 20:00 ` Paul Eggert
2019-12-14 2:35 ` Bruno Haible [this message]
2019-12-14 3:19 ` intptr_t vs. uintptr_t Paul Eggert
2019-12-14 9:14 ` Bruno Haible
2019-12-14 22:29 ` Paul Eggert
2019-12-15 0:35 ` Bruno Haible
2019-12-16 10:02 ` Paul Eggert
2019-12-15 8:14 ` bug#34951: [PATCH] grep: a kwset matcher not work in a grep matcher arnold
2019-12-16 9:56 ` Paul Eggert
2019-12-16 10:12 ` arnold
2019-12-20 3:18 ` Paul Eggert
2019-12-20 10:35 ` arnold
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