* Bug in gets function?
@ 2019-02-12 2:55 Keyhan Vakil
2019-02-12 3:48 ` Rich Felker
0 siblings, 1 reply; 9+ messages in thread
From: Keyhan Vakil @ 2019-02-12 2:55 UTC (permalink / raw)
To: musl
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.
Thanks,
Keyhan
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: Bug in gets function?
2019-02-12 2:55 Bug in gets function? Keyhan Vakil
@ 2019-02-12 3:48 ` Rich Felker
2019-02-12 3:51 ` Rich Felker
0 siblings, 1 reply; 9+ messages in thread
From: Rich Felker @ 2019-02-12 3:48 UTC (permalink / raw)
To: musl
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.
Rich
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: Bug in gets function?
2019-02-12 3:48 ` Rich Felker
@ 2019-02-12 3:51 ` Rich Felker
2019-02-12 14:41 ` James Larrowe
0 siblings, 1 reply; 9+ messages in thread
From: Rich Felker @ 2019-02-12 3:51 UTC (permalink / raw)
To: musl
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
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: Bug in gets function?
2019-02-12 3:51 ` Rich Felker
@ 2019-02-12 14:41 ` James Larrowe
2019-02-12 14:55 ` Ponnuvel Palaniyappan
0 siblings, 1 reply; 9+ messages in thread
From: James Larrowe @ 2019-02-12 14:41 UTC (permalink / raw)
To: musl
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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
>
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^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: Bug in gets function?
2019-02-12 14:41 ` James Larrowe
@ 2019-02-12 14:55 ` Ponnuvel Palaniyappan
2019-02-12 16:30 ` Rich Felker
0 siblings, 1 reply; 9+ messages in thread
From: Ponnuvel Palaniyappan @ 2019-02-12 14:55 UTC (permalink / raw)
To: musl
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> 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.
Regards,
Ponnuvel
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
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^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: Bug in gets function?
2019-02-12 14:55 ` Ponnuvel Palaniyappan
@ 2019-02-12 16:30 ` Rich Felker
2019-02-13 21:39 ` Alexey Izbyshev
0 siblings, 1 reply; 9+ messages in thread
From: Rich Felker @ 2019-02-12 16:30 UTC (permalink / raw)
To: musl
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
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: Bug in gets function?
2019-02-12 16:30 ` Rich Felker
@ 2019-02-13 21:39 ` Alexey Izbyshev
2019-02-13 22:13 ` Rich Felker
0 siblings, 1 reply; 9+ messages in thread
From: Alexey Izbyshev @ 2019-02-13 21:39 UTC (permalink / raw)
To: musl; +Cc: Rich Felker, Rich Felker
On 2019-02-12 19:30, Rich Felker wrote:
> 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.
>
gets() must also return NULL if EOF is reached and no bytes were read.
Alexey
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: Bug in gets function?
2019-02-13 21:39 ` Alexey Izbyshev
@ 2019-02-13 22:13 ` Rich Felker
2019-02-13 23:19 ` Alexey Izbyshev
0 siblings, 1 reply; 9+ messages in thread
From: Rich Felker @ 2019-02-13 22:13 UTC (permalink / raw)
To: musl
On Thu, Feb 14, 2019 at 12:39:07AM +0300, Alexey Izbyshev wrote:
> On 2019-02-12 19:30, Rich Felker wrote:
> >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.
> >
> gets() must also return NULL if EOF is reached and no bytes were read.
So if (c != '\n' && (!feof(stdin) || !i)) ?
Rich
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: Bug in gets function?
2019-02-13 22:13 ` Rich Felker
@ 2019-02-13 23:19 ` Alexey Izbyshev
0 siblings, 0 replies; 9+ messages in thread
From: Alexey Izbyshev @ 2019-02-13 23:19 UTC (permalink / raw)
To: musl; +Cc: Rich Felker
On 2019-02-14 01:13, Rich Felker wrote:
> On Thu, Feb 14, 2019 at 12:39:07AM +0300, Alexey Izbyshev wrote:
>> On 2019-02-12 19:30, Rich Felker wrote:
>> >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.
>> >
>> gets() must also return NULL if EOF is reached and no bytes were read.
>
> So if (c != '\n' && (!feof(stdin) || !i)) ?
>
Yes, looks good.
Alexey
^ permalink raw reply [flat|nested] 9+ messages in thread
end of thread, other threads:[~2019-02-13 23:19 UTC | newest]
Thread overview: 9+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2019-02-12 2:55 Bug in gets function? Keyhan Vakil
2019-02-12 3:48 ` Rich Felker
2019-02-12 3:51 ` Rich Felker
2019-02-12 14:41 ` James Larrowe
2019-02-12 14:55 ` Ponnuvel Palaniyappan
2019-02-12 16:30 ` Rich Felker
2019-02-13 21:39 ` Alexey Izbyshev
2019-02-13 22:13 ` Rich Felker
2019-02-13 23:19 ` Alexey Izbyshev
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