[Bug libstdc++/52169] New: the ifstream readsome() method does not signal any bit on eof.

[Bug libstdc++/52169] New: the ifstream readsome() method does not signal any bit on eof.

[viriketo at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52169

             Bug #: 52169
           Summary: the ifstream readsome() method does not signal any bit
                    on eof.
    Classification: Unclassified
           Product: gcc
           Version: 4.6.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: viriketo@gmail.com


Created attachment 26608
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=26608
Run it with "./c2 c2.cpp" and it will loop forever

The ifstream readsome() method does not signal any bit on eof, and I think it
should.

Therefore, these loops loop forever:
    while(f && !f.eof())
    {
        char b[5000];
        size_t read = f.readsome(b, sizeof b);
        cerr << "Read: " << read << " bytes" << endl;
        ostr.write(b, read);
    }

According to http://www.cplusplus.com/reference/iostream/istream/readsome/ :

Errors are signaled by modifying the internal state flags:

eofbit    The get pointer is at the end of the stream buffer's internal input
array when the function is called, meaning that there are no positions to be
read in the internal buffer (which may or not be the end of the input
sequence). This happens when rdbuf()->in_avail() would return -1 before the
first character is extracted.

failbit    The stream was at the end of the source of characters before the
function was called.

badbit    An error other than the above happened.

[Bug libstdc++/52169] the ifstream readsome() method does not signal any bit on eof.

[tom at kera dot name]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52169

Tomalak Geret'kal <tom at kera dot name> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |tom at kera dot name

--- Comment #1 from Tomalak Geret'kal <tom at kera dot name> 2012-02-08 10:39:54 UTC ---
cplusplus.com is (a) not authoritative, (b) full of mistakes, and (c) otherwise
just awful.

Instead, we'll quote the standard(s):

[C++11: 27.7.2.3]:
      streamsize readsome(char_type* s, streamsize n);
32/ Effects: Behaves as an unformatted input function (as described in
27.7.2.3, paragraph 1). After constructing a sentry object, if !good() calls
setstate(failbit) which may throw an exception, and return. Otherwise extracts
characters and stores them into successive locations of an array whose first
element is designated by s. If rdbuf()->in_avail() == -1, calls
setstate(eofbit) (which may throw ios_base::failure (27.5.5.4)), and extracts
no characters;
— If rdbuf()->in_avail() == 0, extracts no characters
— If rdbuf()->in_avail() > 0, extracts min(rdbuf()->in_avail(),n)).
33/ Returns: The number of characters extracted.

[C++03: 27.6.1.3]:
      streamsize readsome(char_type* s, streamsize n);
30/ Effects: Behaves as an unformatted input function (as described in
27.6.1.3, paragraph 1). After constructing a sentry object, if !good() calls
setstate(failbit) which may throw an exception, and return. Otherwise extracts
characters and stores them into successive locations of an array whose first
element is designated by s. If rdbuf()->in_avail() == -1, calls
setstate(eofbit) (which may throw ios_base::failure (27.4.4.3)), and extracts
no characters;
— If rdbuf()->in_avail() == 0, extracts no characters
— If rdbuf()->in_avail() > 0, extracts min(rdbuf()->in_avail(),n)).
31/ Returns: The number of characters extracted.

[Bug libstdc++/52169] the ifstream readsome() method does not signal any bit on eof.

[tom at kera dot name]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52169

--- Comment #2 from Tomalak Geret'kal <tom at kera dot name> 2012-02-08 10:45:17 UTC ---
Are you sure it's not just that in_avail is 0? Why should it be -1 here?

i.e. doesn't readsome become a noop when there's nothing to read?

[Bug libstdc++/52169] the ifstream readsome() method does not signal any bit on eof.

[viriketo at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52169

--- Comment #3 from Lluís Batlle i Rossell <viriketo at gmail dot com> 2012-02-08 10:53:20 UTC ---
It looks also related to http://gcc.gnu.org/bugzilla/show_bug.cgi?id=8258 . The
post from Tomalak and that made all clear.

Sorry for the noise!

[Bug libstdc++/52169] the ifstream readsome() method does not signal any bit on eof.

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52169

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-02-08 10:56:18 UTC ---
(In reply to comment #1)
> cplusplus.com is (a) not authoritative, (b) full of mistakes, and (c) otherwise
> just awful.

This, x 1000

(In reply to comment #2)
> i.e. doesn't readsome become a noop when there's nothing to read?

Yes, which makes it very useful for non-blocking reads from pipes or sockets.

The testcase is simply wrong, as you can see by checking in_avail:

    cerr << "avail: " << f.rdbuf()->in_avail() << endl;

Since in_avail() doesn't return -1, readsome should not set eofbit.

There are better ways to read a whole file anyway, e.g. ostr << f.rdbuf()