mention disabling GCC built-ins for customization

  On 06/14/2018 01:25 AM, Andreas Schwab wrote:
> On Jun 13 2018, Martin Sebor <msebor@gmail.com> wrote:
>
>> diff --git a/manual/stdio.texi b/manual/stdio.texi
>> index 38be236..d945955 100644
>> --- a/manual/stdio.texi
>> +++ b/manual/stdio.texi
>> @@ -2963,7 +2963,11 @@ The facilities of this section are declared in the header file
>>
>>  @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.
>
> The manual already says that redefining existing conversions causes
> problems:
>
>     You can redefine the standard output conversions, but this is probably
>     not a good idea because of the potential for confusion.  Library routines
>     written by other people could break if you do this.
>
> We should extend that with a stronger language, independent of any
> compiler behaviour.

That sounds fine to me.  Attached is an updated patch that also
adds text to this paragraph.  I added a Portability Note before
the text to highlight the portability impact, similarly to
the prior paragraph.

Martin
  

On Jun 14 2018, Martin Sebor <msebor@gmail.com> wrote:

> diff --git a/manual/stdio.texi b/manual/stdio.texi
> index 38be236..26a570f 100644
> --- a/manual/stdio.texi
> +++ b/manual/stdio.texi
> @@ -2963,7 +2963,11 @@ The facilities of this section are declared in the header file
>  
>  @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.

Is that really needed?  Didn't you say that GCC disables its
optimisations when an unknown format is encountered?

Andreas.
  

On 06/18/2018 01:41 AM, Andreas Schwab wrote:
> On Jun 14 2018, Martin Sebor <msebor@gmail.com> wrote:
>
>> diff --git a/manual/stdio.texi b/manual/stdio.texi
>> index 38be236..26a570f 100644
>> --- a/manual/stdio.texi
>> +++ b/manual/stdio.texi
>> @@ -2963,7 +2963,11 @@ The facilities of this section are declared in the header file
>>
>>  @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.
>
> Is that really needed?  Didn't you say that GCC disables its
> optimisations when an unknown format is encountered?

Saying the behavior is undefined is the conservative thing to do.
I would not be comfortable providing a stronger guarantee for GCC
but if it were thought necessary to say more, the place to go into
the details would the GCC manual where they could be kept in sync
with the implementation.  IMO, the Glibc manual (and the manual
of any other I/O library) should defer to the compiler manual,
and that's what this text does.

Martin
  

Ping: https://sourceware.org/ml/libc-alpha/2018-06/msg00428.html

If there are no further comments I'd like to commit this change
later this week.

On 06/14/2018 01:11 PM, Martin Sebor wrote:
> On 06/14/2018 01:25 AM, Andreas Schwab wrote:
>> On Jun 13 2018, Martin Sebor <msebor@gmail.com> wrote:
>>
>>> diff --git a/manual/stdio.texi b/manual/stdio.texi
>>> index 38be236..d945955 100644
>>> --- a/manual/stdio.texi
>>> +++ b/manual/stdio.texi
>>> @@ -2963,7 +2963,11 @@ The facilities of this section are declared in
>>> the header file
>>>
>>>  @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.
>>
>> The manual already says that redefining existing conversions causes
>> problems:
>>
>>     You can redefine the standard output conversions, but this is
>> probably
>>     not a good idea because of the potential for confusion.  Library
>> routines
>>     written by other people could break if you do this.
>>
>> We should extend that with a stronger language, independent of any
>> compiler behaviour.
>
> That sounds fine to me.  Attached is an updated patch that also
> adds text to this paragraph.  I added a Portability Note before
> the text to highlight the portability impact, similarly to
> the prior paragraph.
>
> Martin
  

I have committed the change:
   http://tinyurl.com/y7jbvfs4

Martin

On 06/27/2018 05:37 PM, Martin Sebor wrote:
> Ping: https://sourceware.org/ml/libc-alpha/2018-06/msg00428.html
>
> If there are no further comments I'd like to commit this change
> later this week.
>
> On 06/14/2018 01:11 PM, Martin Sebor wrote:
>> On 06/14/2018 01:25 AM, Andreas Schwab wrote:
>>> On Jun 13 2018, Martin Sebor <msebor@gmail.com> wrote:
>>>
>>>> diff --git a/manual/stdio.texi b/manual/stdio.texi
>>>> index 38be236..d945955 100644
>>>> --- a/manual/stdio.texi
>>>> +++ b/manual/stdio.texi
>>>> @@ -2963,7 +2963,11 @@ The facilities of this section are declared in
>>>> the header file
>>>>
>>>>  @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.
>>>
>>> The manual already says that redefining existing conversions causes
>>> problems:
>>>
>>>     You can redefine the standard output conversions, but this is
>>> probably
>>>     not a good idea because of the potential for confusion.  Library
>>> routines
>>>     written by other people could break if you do this.
>>>
>>> We should extend that with a stronger language, independent of any
>>> compiler behaviour.
>>
>> That sounds fine to me.  Attached is an updated patch that also
>> adds text to this paragraph.  I added a Portability Note before
>> the text to highlight the portability impact, similarly to
>> the prior paragraph.
>>
>> Martin
>