[PATCH] manual: Document the standardized scanf flag, "m". [BZ #16376]

[Joseph Myers]

On Fri, 9 Feb 2018, Zack Weinberg wrote:

> > +As a GNU extension predating @samp{m}, @samp{a} is also available, but
> > +its use is considered deprecated.
> 
> let's be a little more specific here:
> 
> +As a GNU extension, the modifier @samp{a} has the same effect as @samp{m}.
> +This extension predates POSIX.1-2008 and is now deprecated.  Other C libraries
> +may interpret e.g.@: @samp{%as} as the @samp{%a} format for reading
> +floating-point numbers, followed by a literal @samp{s}.

Which glibc does in the absence of _GNU_SOURCE, since __USE_XOPEN2K is 
defined by default.  The redirection to __isoc99_scanf etc. is done if:

#if defined __USE_ISOC99 && !defined __USE_GNU \
    && (!defined __LDBL_COMPAT || !defined __REDIRECT) \
    && (defined __STRICT_ANSI__ || defined __USE_XOPEN2K)

-- 
Joseph S. Myers
joseph@codesourcery.com