Path: utzoo!mnetor!uunet!seismo!sundc!pitstop!sun!decwrl!labrea!agate!ucbvax!LBL.GOV!forrest
From: forrest@LBL.GOV
Newsgroups: comp.os.vms
Subject: Bug in VAX C Run-Time Library
Message-ID: <880126085340.21e04d69@Csa2.LBL.Gov>
Date: 26 Jan 88 16:53:40 GMT
Sender: usenet@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 66

The following program shows a bug in the ftell/fseek
routines. I reread the documentation on these routines
to make sure what I'm doing is correct and it is.

What I'm doing is reading records from a file. If I use
ftell to tell me where a certain record is located and then
use fseek to go back to the record, I find that the next
record differs depending on if the input file is a
stream or a variable record file. Note that I am using
ftell/fseek at a record boundary, as mentioned in the
documentation.

---------------------------------------------------
#include stdio
#include string

char in_line[128];

main()
{
	FILE *ifp;
	int start_pos;

	ifp = fopen("testdata","r");

	fgets(in_line, sizeof in_line, ifp);	/* should read line 1 */
	fgets(in_line, sizeof in_line, ifp);	/* should read line 2 */

	start_pos = ftell(ifp);		/* current pos is start of line 3 */

	fgets(in_line, sizeof in_line, ifp);	/* should read line 3 */
	fseek(ifp, start_pos, 0);		/* go back to line 3 */
	fgets(in_line, sizeof in_line, ifp);	/* read unknown line */
	printf("Read - %s",in_line);
	fclose (ifp);
}
------------------------------------------------------

Assume the following input file:

-------------------------------------------------------
This is line 11111111111111
This is line 222222222222222
This is line 333333333333333
This is line 444444444444444
This is line 555555555555555

-----------------------------

When the input file is a stream record file the program says that
it read the third line. But, when the input is a variable record file
the programs says it read the second line.

I thought that one of the advantages of RMS is that all access methods
have the same results even if the record type varies. Even given the
requirement that fseek/ftell only work at record boundaries, this
program shows that this isn't the case. Furthermore, it means that
porting programs from Unix or MS-DOS is made even more difficult.

Comments?

Jon Forrest
FORREST@LBL.GOV		(internet)
FORREST@LBL		(bitnet)

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