[PATCH] mention disabling GCC built-ins for customization

[Andreas Schwab]

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.

Andreas.

-- 
Andreas Schwab, SUSE Labs, schwab@suse.de
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