From: Martin Sebor <msebor@gmail.com>
To: Florian Weimer <fw@deneb.enyo.de>
Cc: GNU C Library <libc-alpha@sourceware.org>
Subject: Re: [PATCH] mention disabling GCC built-ins for customization
Date: Wed, 13 Jun 2018 21:31:00 -0000 [thread overview]
Message-ID: <bb13b364-dd87-4d48-c87a-9456e44cd4b9@gmail.com> (raw)
In-Reply-To: <87vaamxuy1.fsf@mid.deneb.enyo.de>
On 06/13/2018 03:01 PM, Florian Weimer wrote:
> * Martin Sebor:
>
>> On 06/13/2018 02:35 PM, Florian Weimer wrote:
>>> * Martin Sebor:
>>>
>>>> @strong{Portability Note:} The ability to extend the syntax of
>>>> @code{printf} template strings is a GNU extension. ISO standard C has
>>>> -nothing similar.
>>>> +nothing similar. When using the GNU C compiler or any other compiler
>>>> +that interprets calls to standard I/O functions according to the rules
>>>> +of the language standard it is necessary to disable such handling by
>>>> +the appropriate compiler option. Otherwise the behavior of a program
>>>> +that relies on the extension is undefined.
>>>
>>> Aren't there ISO extensions to C which define additional format
>>> specifiers which GCC knows nothing about? So maybe it makes more
>>> sense to say that if the application uses format specifiers not known
>>> by GCC, behavior is undefined (unless the compiler option is used).
>>
>> The GCC optimization is disabled when the format string contains
>> invalid or unhandled specifiers/modifiers etc, so even those may
>> still be undefined in Glibc they aren't a problem for GCC.
>
> Good.
>
>> What would cause a problem for the GCC optimization is a change
>> to the behavior of one of the standard conversions, like %i, or
>> %s. One example would be changing the number of bytes output by
>> the conversion. Another example of a future GCC optimization
>> that would lead to undefined behavior is a hook that modified
>> the string argument to %s (when GCC starts to assume that
>> the argument is not clobbered by a sprintf call).
>
> So it's not so much about extending the syntax, but altering the
> behavior of existing syntax, right?
Yes, that's probably pretty close.
Just to be clear, it extends beyond changes to the printf behavior
of directives. A %s hook, for example, cannot rely on being called
for every %s conversion, even if it doesn't change its behavior.
(Say if all it did was count its occurrences.) This is because
GCC transforms printf("%s", s) to puts(s) and sprintf(d, "%s", s)
to stcrpy(d, s).
But adding a hook for a new/undefined conversion specification
that doesn't match an existing one in any way should not be
okay.
Martin
next prev parent reply other threads:[~2018-06-13 21:31 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-06-13 18:19 Martin Sebor
2018-06-13 20:35 ` Florian Weimer
2018-06-13 20:55 ` Martin Sebor
2018-06-13 21:01 ` Florian Weimer
2018-06-13 21:31 ` Martin Sebor [this message]
2018-06-14 7:25 ` Andreas Schwab
2018-06-14 19:11 ` Martin Sebor
2018-06-18 7:42 ` Andreas Schwab
2018-06-19 3:00 ` Martin Sebor
2018-06-27 23:37 ` PING " Martin Sebor
2018-06-29 17:01 ` Martin Sebor
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=bb13b364-dd87-4d48-c87a-9456e44cd4b9@gmail.com \
--to=msebor@gmail.com \
--cc=fw@deneb.enyo.de \
--cc=libc-alpha@sourceware.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).