Re: [PATCH v4 1/5] t9300: factor out portable "head -c" replacement

From: Jeff King <peff@peff.net>
Date: Jul 1, 2016, 18:01 UTC
Message-ID: <20160701180142.GC16235@sigill.intra.peff.net>
In-Reply-To: <xmqqd1mxz30m.fsf@gitster.mtv.corp.google.com>

On Fri, Jul 01, 2016 at 10:23:05AM -0700, Junio C Hamano wrote:

Show 12 quoted lines

> Jeff King <peff@peff.net> writes:
> 
> > In shell scripts it is sometimes useful to be able to read
> > exactly N bytes from a pipe. Doing this portably turns out
> > to be surprisingly difficult.
> 
> I'd rotate the above by three words ;-).
> 
>     It is sometimes useful to be able to read
>     exactly N bytes from a pipe. Doing this portably turns out
>     to be surprisingly difficult
>     in shell scripts.

Yeah, I'd very much agree with that (feel free to mark it up as you apply).

Show 8 quoted lines

> >   - "dd bs=1 count=$n" fixes the partial read problem (all
> >     reads are 1-byte, so there can be no partial response).
> >     It does make a lot of write() calls, but for our tests
> >     that's unlikely to matter.
> 
> It makes me wonder if it helps to use different ibs and obs if many
> writes bother you, but because this patch moves us away from dd,
> that is a moot point.

Actually, I just mis-spoke (mis-wrote?). I meant to say that it makes a lot of read() calls. (It probably also makes a lot of write() calls, but that is beside the point).

-Peff