On Fri, Jun 24, 2016 at 03:07:44PM -0400, Jeff King wrote:
> > "dd bs=1 count=4096" is hopefully more portable.
>
> Hmm. I always wonder whether dd is actually very portable, but we do use
> it already, at least.
>
> Perhaps the perl monstrosity in t9300 could be replaced with that, too.
Hrm. So I wrote a patch for t9300 for this. But I wanted to flip the
order to:
dd bs=4096 count=1
because otherwise, dd will call read() 4096 times, for 1 byte each.
But it's not safe to do that on a pipe. For example:
{
echo 1
sleep 1
echo 2
} | dd bs=4 count=1
will copy only 2 bytes. So it's racily wrong, depending on how the
writer feeds the data to write().
The 1-byte reads do work (assuming blocking descriptors and that dd
restarts a read after a signal, which mine seems to). But yuck.
The difference in time between the two is measurable on my system, but
it's only a few milliseconds (for 4096 bytes). So maybe it's not worth
worrying about (though as a general technique, it does make me worry
that it's easy to get wrong in a way that will fail racily).
-Peff
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