Re: fflush after ungetc

[Bruno Haible]

Eric Blake wrote on Saturday:
> Newlib has two bugs - first, fflush is failing to discard ungetc data when
> changing the underlying fd offset.

Do you know the wording that the newest POSIX has about this?

The following test program, run on various platforms, gives unconclusive
results.

========================== foo.c =========================
#include <stdio.h>
int
main (int argc, char **argv)
{
  /* Check that fflush after a non-backup ungetc() call discards the ungetc
     buffer.  */
  int c;

  c = fgetc (stdin);
  printf ("c = '%c'\n", c);

  c = fgetc (stdin);
  printf ("c = '%c'\n", c);

  c = ungetc ('@', stdin);
  printf ("ungetc result = '%c'\n", c);

  fflush (stdin);

  c = fgetc (stdin);
  printf ("c = '%c'\n", c);

  c = fgetc (stdin);
  printf ("c = '%c'\n", c);

  return 0;
}
=============================================================

$ gcc foo.c
$ ./a.out < foo.c
$ cat foo.c | ./a.out

On glibc-2.3.6: Different results.
When reading from the regular file:
c = '#'
c = 'i'
ungetc result = '@'
c = 'n'
c = 'c'
When reading from the pipe:
c = '#'
c = 'i'
ungetc result = '@'
c = '@'
c = 'n'

On MacOS X: twice
c = '#'
c = 'i'
ungetc result = '@'
c = '@'
c = 'n'

On HP-UX 11:
When reading from the regular file:
c = '#'
c = 'i'
ungetc result = '@'
c = 'i'
c = 'n'
When reading from the pipe:
c = '#'
c = 'i'
ungetc result = '@'
c = <EOF>
c = <EOF>

If I understand the comments in lib/fflush.c right, the second half of my
test is unspecified behaviour, and the first half is specified and exposes
in MacOS X the same bug as on Cygwin?

Bruno