Re: [Mingw-users] mingw-get with mintty; (was: old MSYS design gone)

[Andy K. <and...@gm...>]

On 22 February 2011 17:03, Keith Marshall wrote:
> On 22 February 2011 15:14, Andy Koppe wrote:
>> On 22 February 2011 15:01, Keith Marshall wrote:
>>> On 22 February 2011 13:54, Albrecht Schlosser wrote:
>>>> Instead of using flush, it is also possible to set stdout and stderr
>>>> unbuffered unconditionally at program initialization:
>>>>
>>>>   setvbuf (stdout,NULL,_IONBF,0);
>>>>   setvbuf (stderr,NULL,_IONBF,0);
>>
>> _IOLBF ought to do the job too, except that according to MSDN "For
>> some systems, this provides line buffering. However, for Win32, the
>> behavior is the same as _IOFBF - Full Buffering." :(
>
> So, yet another example of "broken windows" behaviour. :-)
>
> AFAICT, the ISO C Standard insists that stderr shall *not* be fully
> buffered, when opened.  Ever.  (That is, irrespective of whether it is
> connected to a terminal or otherwise).  Thus, it might be unbuffered, or
> it might be line buffered, but it should never be fully buffered.  Okay,
> Windows apparently does not have any concept of line buffered (IOLBF)
> output, so that means, for standards conformance[*] stderr should never
> be anything other than unbuffered; yet it appears that it becomes fully
> buffered when connected to a pipe.

You're right. I wrote the little test below to check. It prints a row
of asterisks for each of the six combinations of stdin/stderr streams
and default/full/no buffering, with a small delay between each
asterisk. The asterisks should appear one-by-one when unbuffered and
all-at-once when buffered.

It shows that enabling and disabling buffering with setvbuf works
correctly both in the console and with a pty, i.e. a pipe. It also
confirms that on a pipe, both stdout and stderr are buffered by
deault. Finally, with a console, both stdout and stderr are unbuffered
by default.

Andy


#include <stdio.h>
#include <stdlib.h>

void test(FILE *f, char *s) {
  int i;
  fputs(s, f);
  fflush(f);
  for (i = 0; i < 60; i++) {
    fputc('*', f);
    usleep(10000);
  }
  fputc('\n', f);
  fflush(f);
}

int main(void) {
  test(stdout, "stdout default: ");
  test(stderr, "stderr default: ");
  setvbuf(stdout, 0, _IONBF, BUFSIZ);
  test(stdout, "stdout _IONBF:  ");
  setvbuf(stderr, 0, _IONBF, BUFSIZ);
  test(stderr, "stderr _IONBF:  ");
  setvbuf(stdout, 0, _IOFBF, BUFSIZ);
  test(stdout, "stdout _IOFBF:  ");
  setvbuf(stderr, 0, _IOFBF, BUFSIZ);
  test(stderr, "stderr _IOFBF:  ");
}