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Re: [PATCH] mention disabling GCC built-ins for customization
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.
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).
Martin