[PATCH] mention disabling GCC built-ins for customization

[Martin Sebor]

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