Re: Bug in gets function?

[Rich Felker <dalias@libc.org>]

On Tue, Feb 12, 2019 at 02:55:19PM +0000, Ponnuvel Palaniyappan wrote:
> >   Is gets(s) equivalent to scanf("%[^\n]%*1[\n]",s)?
> 
> I think it has at least one minor issue: it doesn't null-terminate the
> buffer on empty input i.e., just a newline as input.

Indeed, I omitted what the logic for handling the return value of
scanf would be. But it also seems more complicated than we might like.
If input begins with a newline, it would also fail to consume the
newline without an additional call, and the additional call would make
the operation as a whole non-atomic with respect to the FILE lock,
which is what I was trying to avoid.

Here's an alternate proposal via direct implementation:

char *gets(char *s)
{
	size_t i=0;
	int c;
	FLOCK(stdin);
	while ((c=getc_unlocked(stdin)) != EOF && c != '\n') s[i++] = c;
	s[i] = 0;
	if (c != '\n' && !feof(stdin)) s = 0;
	FUNLOCK(stdin);
	return s;
}

Does this look ok? Of course it's slow compared to a fgets-like
operation on the buffer, but gets is not a usable interface and I
don't see any reason to care whether it's fast.

Rich

> On Tue, Feb 12, 2019 at 2:42 PM James Larrowe <larrowe.semaj11@gmail.com>
> wrote:
> 
> > I could probably try patching it. That C99 specification seems descriptive
> > enough.
> >
> > On Mon, Feb 11, 2019 at 10:51 PM Rich Felker <dalias@libc.org> wrote:
> >
> >> On Mon, Feb 11, 2019 at 10:48:38PM -0500, Rich Felker wrote:
> >> > On Mon, Feb 11, 2019 at 06:55:24PM -0800, Keyhan Vakil wrote:
> >> > > Hi. It seems that the gets function does not follow the C99 spec. In
> >> > > particular, if the input contains a null byte in the middle of the
> >> > > input, then the new-line character is not discarded.
> >> > >
> >> > > For reference, here's the relevant part in the C99 standard
> >> > > (7.19.7.7):
> >> > >
> >> > > > The gets function reads characters from the input stream pointed to
> >> > > > by stdin, into the array pointed to by s, until end-of-file is
> >> > > > encountered or a new-line character is read. Any new-line character
> >> > > > is discarded, and a null character is written immediately after the
> >> > > > last character read into the array.
> >> > >
> >> > > Here is an example:
> >> > >
> >> > >     #include <stdio.h>
> >> > >     char s[8];
> >> > >     int main() {
> >> > >         gets(s);
> >> > >         for (int i = 0; i < sizeof s; i++) {
> >> > >             printf("%02x ", s[i]);
> >> > >         }
> >> > >         printf("\n");
> >> > >         return 0;
> >> > >     }
> >> > >
> >> > > When compiled against gcc:
> >> > >
> >> > >     $ echo -e 'A\x00B' | ./a.out
> >> > >     41 00 42 00 00 00 00 00
> >> > >
> >> > > When compiled against musl:
> >> > >
> >> > >     $ echo -e 'A\x00B' | ./a.out
> >> > >     41 00 42 0a 00 00 00 00
> >> > >
> >> > > Note the terminating newline, which contradicts the spec.
> >> >
> >> > I think this bug report is correct; however the gets function is
> >> > awful, removed in C11, and should never be used. :-)
> >> >
> >> > I will see what can be done to fix it though.
> >>
> >> Is gets(s) equivalent to scanf("%[^\n]%*1[\n]",s)? If so that would be
> >> an appropriately hideous way to implement it that avoids the current
> >> bug? :-)
> >>
> >> Rich
> >>
> >
> 
> -- 
> Regards,
> Ponnuvel P