Re: libstdc++/9439: filebuf::sputbackc ignores beginning-of-file

Re: libstdc++/9439: filebuf::sputbackc ignores beginning-of-file

[Paolo Carlini]

The following reply was made to PR libstdc++/9439; it has been noted by GNATS.

From: Paolo Carlini <pcarlini@unitus.it>
To: =?ISO-8859-1?Q?P=E9tur_Run=F3lfsson?= <peturr02@ru.is>
Cc: paolo@gcc.gnu.org,  bkoz@redhat.com,  gcc-bugs@gcc.gnu.org, 
 nobody@gcc.gnu.org,  gcc-gnats@gcc.gnu.org
Subject: Re: libstdc++/9439: filebuf::sputbackc ignores beginning-of-file
Date: Mon, 03 Feb 2003 15:21:09 +0100

 P=E9tur Run=F3lfsson wrote:
 
 >Hi,
 > =20
 >
 >>    Hi. I don't think there is a bug here: I cannot find in the
 >>    standard a specific prescription for the behaviour you
 >>    expect (in particular in 27.5.2.4.4) Can you?
 >>   =20
 >>
 >
 >Yes. 27.8.1.4 [lib.filebuf.virtuals] p5 describes how pbackfail may
 >put back the character c. The only cases that can apply here are the
 >ones starting with
 >
 [snip]
 
 Ok. I agree with your reading.
 
 Therefore it seems that seekoff should be changed to return error in=20
 this case. Then pbackfail would pay attention to the negative returned=20
 value. The latter changes will also fix libstdc++/9425.
 
 >>    Indeed, sputbackc calls, as expected, pbackfail, which in=20
 >>turns calls seekoff (fstream.tcc, line 218) (pay attention
 >>    to the preceding comment which means that this specific=20
 >>situation was considered and _not_ supposed to lead to an
 >>    obvious failure).
 >>   =20
 >>
 >I read "at the beginning of the buffer" as simply meaning that
 >gptr() =3D=3D eback(), not as "at the beginning of the file".
 >
 Yes, you are right.
 
 Paolo.
 
 http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=3Dview%20audit-trail&database=3D=
 gcc&pr=3D9439
 

Re: libstdc++/9439: filebuf::sputbackc ignores beginning-of-file

[paolo]

Synopsis: filebuf::sputbackc ignores beginning-of-file

State-Changed-From-To: feedback->closed
State-Changed-By: paolo
State-Changed-When: Wed Feb  5 19:23:48 2003
State-Changed-Why:
    Fixed for 3.3 and 3.4.

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=9439

RE: libstdc++/9439: filebuf::sputbackc ignores beginning-of-file

[Pétur Runólfsson]

The following reply was made to PR libstdc++/9439; it has been noted by GNATS.

From: =?iso-8859-1?Q?P=E9tur_Run=F3lfsson?= <peturr02@ru.is>
To: <paolo@gcc.gnu.org>,
	<bkoz@redhat.com>,
	<gcc-bugs@gcc.gnu.org>,
	<nobody@gcc.gnu.org>,
	<gcc-gnats@gcc.gnu.org>
Cc:  
Subject: RE: libstdc++/9439: filebuf::sputbackc ignores beginning-of-file
Date: Mon, 3 Feb 2003 13:56:12 -0000

 Hi,
 >     Hi. I don't think there is a bug here: I cannot find in the
 >     standard a specific prescription for the behaviour you
 >     expect (in particular in 27.5.2.4.4) Can you?
 
 Yes. 27.8.1.4 [lib.filebuf.virtuals] p5 describes how pbackfail may
 put back the character c. The only cases that can apply here are the
 ones starting with
 
   If traits::eq_int_type( c ,traits::eof()) returns false and if the
   function makes a putback position available
 
 The meaning of "a putback position available" is defined in
 27.5.1 [lib.streambuf.reqts] p3
 
   If xnext is not a null pointer and xbeg < xnext for an input
   sequence, then a putback position is available
 
 and 27.5.1 [lib.streambuf.reqts] p2 states that xbeg points to the
 beginning of an array that
 
   represents, [...], a (sub)sequence of characters from the sequence
 
 Finally, "the sequence" is defined in 27.8.1.1 [lib.filebuf] p1
 
   The class basic_filebuf<charT,traits> associates both the input
   sequence and the output sequence with a file.
 
 >     Indeed, sputbackc calls, as expected, pbackfail, which in=20
 > turns calls seekoff (fstream.tcc, line 218) (pay attention
 >     to the preceding comment which means that this specific=20
 > situation was considered and _not_ supposed to lead to an
 >     obvious failure).
 
 I read "at the beginning of the buffer" as simply meaning that
 gptr() =3D=3D eback(), not as "at the beginning of the file".
 
 > The latter call then does _not_ fail and
 >     a put back buffer is created by _M_pback_create(), hosting
 >     the put back char, _exactly as happens_ when __testpb &&
 >     !__testeof && !__testeq, above.=20
 >     I agree that a few widespread implementations behave
 >     differently in this case, but we are, I maintain, in the
 >     realm of implementation defined behaviour.
 >    =20
 >     Paolo.
 >=20
 > http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=3Dview%20audit-trail&
 database=3Dgcc&pr=3D9439
 
 Petur

Re: libstdc++/9439: filebuf::sputbackc ignores beginning-of-file

[paolo]

Synopsis: filebuf::sputbackc ignores beginning-of-file

Responsible-Changed-From-To: unassigned->paolo
Responsible-Changed-By: paolo
Responsible-Changed-When: Sun Feb  2 17:38:23 2003
Responsible-Changed-Why:
    Analyzed a bit.
State-Changed-From-To: open->feedback
State-Changed-By: paolo
State-Changed-When: Sun Feb  2 17:38:23 2003
State-Changed-Why:
    Hi. I don't think there is a bug here: I cannot find in the
    standard a specific prescription for the behaviour you
    expect (in particular in 27.5.2.4.4) Can you?
    Indeed, sputbackc calls, as expected, pbackfail, which in turns calls seekoff (fstream.tcc, line 218) (pay attention
    to the preceding comment which means that this specific situation was considered and _not_ supposed to lead to an
    obvious failure). The latter call then does _not_ fail and
    a put back buffer is created by _M_pback_create(), hosting
    the put back char, _exactly as happens_ when __testpb &&
    !__testeof && !__testeq, above. 
    I agree that a few widespread implementations behave
    differently in this case, but we are, I maintain, in the
    realm of implementation defined behaviour.
    
    Paolo.

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=9439

libstdc++/9439: filebuf::sputbackc ignores beginning-of-file

[peturr02]

>Number:         9439
>Category:       libstdc++
>Synopsis:       filebuf::sputbackc ignores beginning-of-file
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Jan 25 12:06:00 UTC 2003
>Closed-Date:
>Last-Modified:
>Originator:     peturr02@ru.is
>Release:        gcc-3.2.1
>Organization:
>Environment:
Red Hat Linux 8.0
>Description:
filebuf::sputbackc succeeds even if the filebuf is already at the beginning of the file. I don't think this makes much sense, sputbackc should fail at the beginning.
>How-To-Repeat:
See attachment.
>Fix:

>Release-Note:
>Audit-Trail:
>Unformatted:
----gnatsweb-attachment----
Content-Type: text/plain; name="pbackfailbug2.cc"
Content-Disposition: inline; filename="pbackfailbug2.cc"

#include <fstream>
#include <cstdio>
using namespace std;

#undef NDEBUG
#include <cassert>

int main()
{
	filebuf fbuf;
	fbuf.open("pbackfailbug2.cc", ios_base::in);
	int r = fbuf.sputbackc('a');
	assert(r == EOF);

	return 0;
}