Re: sed bug on seekable stdin

[Paul Jarc]

Eric Blake <address@hidden> wrote:
> I disagree.  According to POSIX, "If the most recent operation, other than
> ftell(), on a given stream is fflush(), the file offset in the underlying
> open file description shall be adjusted to reflect the location specified
> by fseek()", with no restriction that the stream must not be an input-only
> stream.

But there is still no specification of what fflush() does on an input
stream.  It's undefined by omission.  It follows that fflush() +
fseek() can only be used with output files, even though this is not
made explicit on the fseek() page.

> In other words, the only POSIX-sanctioned way to set the offset of
> the underlying fd of a buffered, seekable input stream is to use
> fflush then fseek on that stream.

It's not clear to me that there's any way to do it; this may be a
defect in POSIX.  The best I can come up with is fileno() + lseek(),
but then you still have to figure out what offset to jump to.


paul