BZ #1198: stream behaviour on encountering an EOF

[Siddhesh Poyarekar <siddhesh at redhat dot com>]

Hi,

The aforementioned bz describes an incompatibility of glibc with posix
when it comes to behaviour of fread, fgets, etc. The POSIX standard
implies that if an end-of-file is encountered in a stream, subsequent
calls on that stream will continue to return EOF until the stream state
is changed with fseek or similar. The behaviour we have however is
different and the reason for this seems to be to maintain compatibility
with SysV, as mentioned in fileops.c:

...
int
_IO_new_file_underflow (fp)
     _IO_FILE *fp;
{
  _IO_ssize_t count;
#if 0
  /* SysV does not make this test; take it out for compatibility */
  if (fp->_flags & _IO_EOF_SEEN)
    return (EOF);
#endif

...

glibc behaviour is to do a sys_read if there's nothing on the buffer.
It is inconsequential for actual files that have not changed, since the
read would return a 0 again, but it results in blocking on stdin and I
imagine, on pipes as well.

Should we really be fixing this at all? A lot of applications could be
depending on this behaviour and fixing this to comply with POSIX would
imply breaking all those programs.

The fix itself is quite simple - uncomment the above and place some
more similar checks in other places where a read doesn't go through the
underflow mechanism.

Regards,
Siddhesh