strange issue reading /dev/null

[Sean C. Farley]

On Thu, 7 Aug 2008, Gabor Kovesdan wrote:

> Hello,
>
> I'm wondering why fgetc() returns 0xff if called with /dev/null:
>
> #include <stdio.h>
> #include <stdlib.h>
>
> int
> main(void)
> {
>       int      c;
>       FILE    *f;
>
>       f = fopen("/dev/null", "r");
>
>       if (c != EOF)
>               printf("%c\n", fgetc(f));
> }
>
>> gcc foo.c
>> ./a.out
> ÿ
>
> This causes a bug in BSD grep as /dev/null is not distinguished from
> ordinary files in the code, thus I was expecting it just returned EOF,
> but in reality this is not the case. How such cases should be handled?

You are testing c which has not been set.  It works OK if you set c then
do the test:

+       c = fgetc(f);
         if (c != EOF)
-               printf("%c\n", fgetc(f));
+               printf("%c\n", c);

Sean
-- 
scf at FreeBSD.org