fgetwc returns various errors when it shouldn't

[Roland Illig]

$ cat wcw.c
#define _GNU_SOURCE

#include <errno.h>
#include <locale.h>
#include <stdio.h>
#include <stdlib.h>
#include <wchar.h>

int
main(void)
{
        wint_t c;

        if (!setlocale(LC_CTYPE, "")) {
                perror("setlocale");
                exit(EXIT_FAILURE);
        }

        while ((errno = 0, c = fgetwc(stdin)) != WEOF) {
                printf("U+%04x has width %d\n", (unsigned)c, wcwidth(c));
        }

        if (errno != 0) {
                fprintf(stderr, "errno = %d\n", errno);
                perror("fgetwc");
                exit(EXIT_FAILURE);
        }
        return 0;
}

$ cc -Wall -W wcw.c -o wcw
$ ./wcw < empty
$ ./wcw < /dev/null
errno = 25
fgetwc: Inappropriate ioctl for device
$ cat /dev/null | ./wcw
errno = 29
fgetwc: Illegal seek

On the other hand, on Solaris 10, I get the following behavior, which lets me distinguish EOF from errors:

$ ./wcw < empty
$ ./wcw < /de
$ ./wcw < /dev/null
$ cat empty | ./wcw