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From: Hpda@hplabs.UUCP (HP Data Systems)
Newsgroups: net.bugs.4bsd
Subject: dd(1) trashes file if input and output same
Message-ID: <2766@hplabs.UUCP>
Date: Fri, 23-Mar-84 20:51:11 EST
Article-I.D.: hplabs.2766
Posted: Fri Mar 23 20:51:11 1984
Date-Received: Sun, 25-Mar-84 12:30:33 EST
Organization: Hewlett Packard Labs, Palo Alto CA
Lines: 53

Subject:	dd(1) destroys file if input and output are the same.
Index:		/usr/src/bin/dd.c
Submitter:	Eric Wertz ( ..!ucbvax!hpda!eric,
			     wertz@su-glacier.ARPA,
			     eric@cmu-ee-faraday.ARPA )

Description:
	If file arguments given via "if=" and "of=" are the same, the
	file gets trashed.
	Programs like cp(1) usually catch this.

Repeat-By:
	% cat foo
	This is a test file.
	% dd if=foo of=foo
	0+0 records in
	0+0 records out			(oh dear, this doesn't look good...)
	% cat foo
	%				(sure enough...)

Fix:
	Add <sys/types.h> and <sys/stat.h> to the list of include files.
	and insert code with change bars...

	if(ibf < 0) {
		perror(ifile);
		exit(0);
	}	
|	if (ifile && ofile) {
|		struct stat sbuf_in, sbuf_out;
|
|		/* Can I stat ifile ? */
|		if ( stat(ifile,&sbuf_in) == -1 ) {
|			perror(ifile);
|			exit(0);
|		}
|		/* If ofile exists, test to see if it's the same as ifile */
|		if ( stat(ofile,&sbuf_out) == 0 &&
|		     sbuf_in.st_dev == sbuf_out.st_dev &&
|		     sbuf_in.st_ino == sbuf_out.st_ino ) {
|			fprintf(stderr,"dd: can't copy file onto itself\n");
|			exit(0);
|		}
|	}
	if (ofile)
		obf = creat(ofile, 0666);

Comments:
	This bug occurs in System III, System V, System V.2, and
	4.[12] BSD.

	Of course, there's no way to prevent...
	% dd if=foo >foo			...from trashing a file.