> An errno+ferror approach works except for two cases:
> 
>   (1) MS fgets() does not consider reading from a write-only stream
>       an error. While annoying, I think this is standard conforming.
> 
>   (2) OpenSolaris sets errno unreliably after getc.
> 
> 
> I do not know how to get around those without using OS specifics.

Ok, then perhaps it's better to have some internal {readable, writable}
flags based on the original mode string.