Re: Problem with line buffering and getc function on 1.7.33.

[Kaz Kylheku <920-082-4242 at kylheku dot com>]

On 11.03.2016 14:14, Marco Atzeri wrote:
> On 11/03/2016 22:57, Kaz Kylheku wrote:
> > On 11.03.2016 12:08, Yaakov Selkowitz wrote:
> > > On 2016-03-11 13:16, Kaz Kylheku wrote:
> > >
> > > > On a Cygwin installation version 1.7.33-2(0.280/5/3), I encountered
> > > > an odd issue.
> > >
> > > This is a 15-month old release which is no longer supported. Please
> > > update to 2.4.1.
> >
> > Do you mean, "Please update to 2.4.1, it is fixed there!"
> > or do you mean "You haven't done enough with that minimal
> > C repro case; please keep investigating against
> > the latest code?"
>
> #2 please verify if the issue is still present in the last code.
> We don't have the bandwith to check claims on older release.
> on 2.4.1 the output is:
>
> $ ./test_stream.exe
> received REQUEST
>  from client
> received
>  from server

That confirms that the issue is still there.

We can reproduce the problem with just file streams using
a much simpler program:

#include <stdio.h>

int main(void)
{
  FILE *out = fopen("file", "w+");
  setvbuf(out, (char *) NULL, _IOLBF, 0);
  getc(out);
  clearerr(out);
  fseek(out, 0, SEEK_SET);
  putc('a', out);
  putc('b', out);
  putc('c', out);
  putc('d', out);
  putc('e', out);
  putc('\n', out);
  fclose(out);
  return 0;
}

The contents of file end up being "\n": one empty
line, instead of "abcde\n":

  $ cat file

  $

The necessary ingredients seem to be: open the stream for update;
put into line buffered mode; do some input. In this program,
replacing putc with fputs doesn't make the problem go away;
the workaround isn't working:

  fputs("a", out);
  fputs("b", out);
  fputs("c", out);
  fputs("d", out);
  fputs("e", out);
  fputs("\n", out);

However, if we do it as one string, then the file does contain
the correct line:

  fputs("abcde\n", out);

This sequence also gets us correct file contents:

  fputs("ab", out);
  putc('c', out);
  putc('d', out);
  putc('e', out);
  putc('\n', out);

HOWEVER, these distracting effects are because GCC is
optimizing one-character fputs calls into _fputc calls.
Annoyingly, this happens even with gcc -O0.

The clearerr and fseek calls are red herrings; they seem to make
make no difference. I put in the fseek for formal ISO C compliance:
ISO C says that before performing an output operation on an update
stream whose last operation was input, we "shall" perform a
positioning operation. (No sanely implemented libc actually
breaks code that doesn't meet this silly requirement.)





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