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Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!ihnp4!oddjob!matt
From: matt@oddjob.UUCP (Matt Crawford)
Newsgroups: net.bugs.4bsd
Subject: dd bug -- clobbers output file
Message-ID: <679@oddjob.UUCP>
Date: Thu, 18-Apr-85 15:51:57 EST
Article-I.D.: oddjob.679
Posted: Thu Apr 18 15:51:57 1985
Date-Received: Fri, 19-Apr-85 01:22:43 EST
Reply-To: matt@oddjob.UUCP (Matt Crawford)
Organization: U. Chicago, Astronomy & Astrophysics
Lines: 35

I can hardly believe that this hasn't been discovered before, but
since I've seen no mention of it I'm sending it out.

Subject: dd clobbers output file
Index:	bin/dd.c 4.2BSD

Description:
	dd truncates its output file before writing.  This is clearly
	not a `feature' because it makes the seek=n option useless.
Repeat-By:
	Create a file "infile" longer than, say, 50 bytes.
	  dd if=infile of=outfile bs=50 count=1
	  dd if=infile of=outfile bs=50 skip=1 seek=1
	outfile and infile should now be identical but in fact the
	first 50 bytes of outfile will be zeros.
Fix:
	dd opens its output file by calling creat().  This truncates.
	The later lseek() fills the first record with zeros.

RCS file: RCS/dd.c,v
retrieving revision 1.1
retrieving revision 1.2
diff  -r1.1 -r1.2
3a4
> #include <sys/file.h>
258c259
< 		obf = creat(ofile, 0666);
---
> 		obf = open(ofile, O_WRONLY|O_CREAT, 0666);


   "It's true, your honor.  This node has no disk."
_____________________________________________________
Matt		University	crawford@anl-mcs.arpa
Crawford	of Chicago	ihnp4!oddjob!matt