perror() changes orientation of non-oriented stderr when it is redirected

[Igor Liferenko]

Hi all,

I would like to recompile glibc locally with this patch:

    https://sourceware.org/bugzilla/attachment.cgi?id=9580

(this patch does not bring any incompatibility with existing programs)

This patch fixes a small bug, which can be seen by compiling the
following program and then running:

    $ ./a.out 2>/dev/null

The output is incorrect:

    initial fwide: 0
    fwide: -1

After applying the patch, the output becomes correct:

    initial fwide: 0
    fwide: 0

There are two issues with this patch, and I need help to figure them out.

1) what to do if "__write()" fails?
2) may "if (_IO_ferror_unlocked(fp))" test be safely deleted?

See also https://sourceware.org/bugzilla/show_bug.cgi?id=20677

Regards,
Igor

=================================

#include <stdio.h>
#include <wchar.h>
#include <errno.h>
int main(void)
{
  printf("initial fwide: %d\n", fwide(stderr, 0));
  errno = EINVAL;
  perror(""); /* note, that prior to this call stderr was not oriented */
  printf("fwide: %d\n", fwide(stderr, 0));
  return 0;
}