Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!husc6!wjh12!gst
From: gst@wjh12.harvard.edu (Gary S. Trujillo)
Newsgroups: comp.unix.questions
Subject: Re: meaning of "count=" for dd?
Summary: There ain't no easy answer!
Message-ID: <420@wjh12.harvard.edu>
Date: 24 Oct 89 17:47:04 GMT
References: <419@wjh12.harvard.edu>
Reply-To: gst@wjh12.UUCP (Gary S. Trujillo)
Organization: Harvard University, Cambridge MA
Lines: 28

In article <419@wjh12.harvard.edu> gst@wjh12.UUCP (Gary S. Trujillo) writes:
>My empirical tests of dd leave me puzzled as to the real meaning of the
>"count=" argument.  The manual page says "copy only n input records."
>Trouble is, it doesn't say what sort of "record" it's talking about.
>I would have thought that it would be the number of characters in a
>block, as defined by one of the blocksize (e.g., "bs") arguments.

Thanks to everyone who sent email replies - including the AT&T person who
has actually worked on the dd code(!).  The upshot of the comments was that
my problem may come from the fact (it is a fact) that I'm reading from a
pipe in my application, and that a "record" (called "block" in other
versions of the man page, apparently) corresponds to one physical read.
Thus, the amount read may vary, depending on a number of factors.

There was a suggestion that I try 512-byte blocks, since that's normally
the size of a program buffer ("but it's not foolproof").  My tests indicate
that suggestion seems to work.

The AT&T guy also says that results vary between System V and BSD UNIX
implementations, having to do with different implementations of the file
system, I think: "Network files will usually break into 2K blocks, no
matter what you ask for."

Thanks again, and happy "dd-ing!" :-)

-- 
	Gary Trujillo
	(gst@wjh12.harvard.edu)