[Bug c++/21286] New: GNU extension stdio_filebuf problems when reading from pipe

["ralfixx at gmx dot de" <gcc-bugzilla at gcc dot gnu dot org>]

Attach stdin (or any other FILE*) to an iostream via the rdbuf() created from a
FILE* by using the GNU extension stdio_filebuf() (see attached C++-File).

Input is just some big file with +11MB:

% wc -c big_file
11601584 big_file

This reads incorrect number of bytes when stdin is a pipe and using an block
size which is not some power of 2. The number of bytes read differs from run to run.
% cat big_file | ./t400 8687
total bytes read 9347072, blocks 1076
% cat big_file | ./t400 8687
total bytes read 978944, blocks 113
% cat big_file | ./t400 8687
total bytes read 8679424, blocks 1000
% cat big_file | ./t400 8687
total bytes read 3424256, blocks 395
% cat big_file | ./t400 8687
total bytes read 11601584, blocks 1336
% cat big_file | ./t400 2048
total bytes read 11601584, blocks 5665
% cat big_file | ./t400 2048
total bytes read 11601584, blocks 5665

No errors when stdin is redirected directly to the file.  

% ./t400 8676 < big_file
total bytes read 11601584, blocks 1338
% ./t400 8676 < big_file
total bytes read 11601584, blocks 1338
% ./t400 8676 < big_file
total bytes read 11601584, blocks 1338
% ./t400 8676 < big_file
total bytes read 11601584, blocks 1338

Also no problems when reading in 2048 or 4096 blocks:

% cat big_file | ./t400 2048
total bytes read 11601584, blocks 5665
% cat big_file | ./t400 2048
total bytes read 11601584, blocks 5665
% cat big_file | ./t400 2048
total bytes read 11601584, blocks 5665

% cat big_file | ./t400 4096
total bytes read 11601584, blocks 2833
% cat big_file | ./t400 4096
total bytes read 11601584, blocks 2833
% cat big_file | ./t400 4096
total bytes read 11601584, blocks 2833

Same problem on HP-UX 10.20 with gcc-4.0.0 (and 3.4.3 if it matters).

The same program runs w/o problems in gcc-3.4.1 and gcc-3.3.3 on the same platforms.

I actually had a shell loop trying block sizes from 1 to 10000 to find the
problematic ones, so if 8687 does not reproduce the bug, try the follwoing shell
loop:

i=8000
while test $i -lt 9000 ; do
  echo -e "$i: \\c"
  cat big_file | ./t400 $i
  i=`echo $i+1 | bc`
done


R'

-- 
           Summary: GNU extension stdio_filebuf problems when reading from
                    pipe
           Product: gcc
           Version: 4.0.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: ralfixx at gmx dot de
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21286