Re: %n in fscanf

["Michael Kerrisk (man-pages)" <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

Joern,

On 03/08/2010 12:38 PM, Joern Heissler wrote:
> The fscanf manpage contains this:
> 
> n      Nothing  is  expected;  instead, the number of characters consumed thus far from the input is 
> stored through the next pointer, which must be a pointer to int.  This is not a conversion, although it 
> can be suppressed with the * assignment-suppression character.  The  C  standard  says:
> "Execution  of  a  %n  directive does not increment the assignment count returned at the completion of 
> execution" but the Corrigendum seems to contradict this.  Probably it is wise not to make any 
> assumptions on the effect of %n conversions on the return value.
> 
> 
> I wondered about this and looked at c89, c99+tc3, c1x, k&r2, posix manpages; all say that %n does *not* 
> increase the counter. %*n causes undefined behaviour according to c99+tc3.
> I wasn't able to find proof that any Corrigendum says otherwise.
> Therefore I think it's safe to say that you can indeed make the assumption that %n does not affect the 
> return value.

Long after the fact, I've applied this patch.

Cheers,

Michael



> Suggested patch:
> --- scanf.3.orig        2010-03-08 12:21:59.000000000 +0100
> +++ scanf.3     2010-03-08 12:29:59.000000000 +0100
> @@ -506,18 +506,15 @@
>   .IR int .
>   This is
>   .I not
> -a conversion, although it can be suppressed with the
> -.B *
> -assignment-suppression character.
> -The C standard says: "Execution of a
> -.B %n
> -directive does not increment
> -the assignment count returned at the completion of execution"
> -but the Corrigendum seems to contradict this.
> -Probably it is wise
> -not to make any assumptions on the effect of
> -.B %n
> -conversions on the return value.
> +a conversion and does
> +.I not
> +increase the count returned by the function.
> +The assignment can be suppressed with the
> +.B *
> +assignment-suppression character, but the effect on the
> +return value is undefined. Therefore
> +.B %*n
> +conversions should not be used.
>   .SH "RETURN VALUE"
>   These functions return the number of input items
>   successfully matched and assigned,
> 
> 
> Cheers,
> Jörn Heissler
> 


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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