fflush, EBADF, POSIX

[Bruno Haible]

Hi Eric,

According to POSIX:2008
<http://pubs.opengroup.org/onlinepubs/9699919799/functions/fflush.html>,
fflush() must fail with EBADF if the file descriptor is invalid.

This test program
==================================== foo.c ====================================
#include <errno.h>
#include <stdio.h>
#include <unistd.h>

int
main ()
{
  FILE *f;

  f = fopen ("test-fflush.txt", "w");
  if (f == NULL)
    return 1;
#if 0 /* fflush should also fail without this statement. */
  fputc ('x', f);
#endif
  if (close (fileno (f)) < 0)
    return 2;
  errno = 0;
  if (!(fflush (f) == EOF && errno == EBADF))
    {
      fprintf (stderr, "fflush did not fail, as expected by POSIX\n");
      return 3;
    }
  fclose (f);
  return 0;
}
===============================================================================

shows that this requirement is not fulfilled, not even on glibc platforms:

$ gcc -Wall foo.c
$ ./a.out
fflush did not fail, as expected by POSIX

But it is actually stupid to require that even if the buffer is empty, that is,
even if fflush(f) is normally a no-op, the implementation should make system
calls to test whether the file descriptor is valid or not.

Can you bring this before the Austin group, please?

I would say, a good fix would be to change the "shall fail" requirement for 
EBADF
to a "may fail" requirement.

Bruno
-- 
In memoriam Bernhard Bästlein <http://en.wikipedia.org/wiki/Bernhard_Bästlein>