Re: [PATCH] handle whitespace before %% in scanf

[Rich Felker <dalias@libc.org>]

On Mon, Jul 10, 2017 at 09:20:39PM -0400, Rich Felker wrote:
> On Sun, Jul 09, 2017 at 11:00:18PM +0200, Bartosz Brachaczek wrote:
> > this is mandated by C and POSIX standards and is in accordance with
> > glibc behavior.
> > ---
> >  src/stdio/vfscanf.c  | 10 +++++++---
> >  src/stdio/vfwscanf.c |  8 ++++++--
> >  2 files changed, 13 insertions(+), 5 deletions(-)
> > 
> > diff --git a/src/stdio/vfscanf.c b/src/stdio/vfscanf.c
> > index d4d2454b..9e030fc4 100644
> > --- a/src/stdio/vfscanf.c
> > +++ b/src/stdio/vfscanf.c
> > @@ -89,15 +89,19 @@ int vfscanf(FILE *restrict f, const char *restrict fmt, va_list ap)
> >  			continue;
> >  		}
> >  		if (*p != '%' || p[1] == '%') {
> > -			p += *p=='%';
> >  			shlim(f, 0);
> > -			c = shgetc(f);
> > +			if (*p == '%') {
> > +				p++;
> > +				while (isspace((c=shgetc(f))));
> > +			} else {
> > +				c = shgetc(f);
> > +			}
> >  			if (c!=*p) {
> >  				shunget(f);
> >  				if (c<0) goto input_fail;
> >  				goto match_fail;
> >  			}
> > -			pos++;
> > +			pos += shcnt(f);
> >  			continue;
> >  		}
> 
> Assuming your interpretation is correct, I have no objection to going
> forward with the change, but I don't think this is the right way to do
> it. The only reason %% was handled in the code that handles literal
> characters is because I assumed it behaves like one, but if it
> doesn't, it should just be handled as a format specifier that consumes
> space where it can use the existing code that does that, rather than
> complicting the code for literals and adding a duplicate of the
> space-skipping code to it.

I tried going forward with the idea I proposed, but it looks like it's
actually more invasive: in addition to adding the final case to
actually handle '%', it adds a new case where a conversion specifier
does not consume a variadic input, and a new case where width is
forced to 1 and modifier flags and explicit widths are rejected.

As such I think your patch as originally submitted is probably the
best approach. Sorry for the delay in reviewing and accepting it.

Rich