[PATCH] mention disabling GCC built-ins for customization

[Martin Sebor]

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
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