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Path: utzoo!watmath!clyde!rutgers!husc6!ut-sally!utah-cs!b-davis
From: b-davis@utah-cs.UUCP
Newsgroups: comp.sys.ibm.pc
Subject: Major bug in all(?) versions of MS-DOS.
Message-ID: <4274@utah-cs.UUCP>
Date: Fri, 6-Feb-87 17:48:53 EST
Article-I.D.: utah-cs.4274
Posted: Fri Feb  6 17:48:53 1987
Date-Received: Sun, 8-Feb-87 00:40:09 EST
Organization: University of Utah VCIS Group
Lines: 95
Keywords: bug

; Here is a probable bug (or feature) in MS-DOS.  Does anyone know a
; work around (without closing the first file)?  Would Gordon Letwin at
; Microsoft care to comment?  This bug appears on PC-DOS 2.0, PC-DOS 3.0,
; and MS-DOS 3.1.  (Unix has no problems with this algorigthm.)

; The test goes like this:
; 	Create a file with name 'xxx'.  Call this file FD1.
;	Write 80 bytes to FD1.
;	Open the file named 'xxx' a second time.  Call this file FD2.
;		Note that NO errors have happened yet.
;	Try to read 80 bytes from FD2.  No bytes are read.
;		Note that NO error is reported.
;	If in symdeb push to a new shell.  See that the file 'xxx' has
;		been created but has a size of 0.
;	Exit the program.  See that the file 'xxx' is now 80 bytes long.
; If FD1 is closed at POINT A (see source) then FD2 will perform the read.
; If FD1 is closed at POINT B (see source) then the read of FD2 still fails
;	even though the disk has been updated before the read happens.

xtest	segment	para public 'prog'
	assume	cs:xtest,ds:xtest

	org	100h
start:	jmp	main

; int fd1, fd2;
; char buffer[80];
; int a, b;
fd1	dw	0
fd2	dw	0
buffer	db	80 dup (0)
a	dw	0
b	dw	0
xxx	db	"xxx", 0

main:
	; modify memory so symdeb can push a shell
	mov	bx, 4096
	mov	ah, 4ah
	int	21h
	; create a file
	; fd1 = open("xxx", O_TRUNC | O_CREAT | O_RDWR | O_BINARY, 0600);
	mov	dx, offset xxx
	mov	cx, 0
	mov	ah, 3ch
	int	21h
	mov	fd1, ax
	; write out 80 bytes
	; a = write(fd1, buffer, sizeof(buffer));
	mov	bx, fd1
	mov	dx, offset buffer
	mov	cx, size buffer
	mov	ah, 40h
	int	21h
	mov	a, ax
; POINT A
; If these three lines are included then all works well.
;	mov	bx, fd1
;	mov	ah, 3eh
;	int	21h
	; open the file a second time
	; fd2 = open("xxx", O_RDWR | O_BINARY, 0);
	mov	dx, offset xxx
	mov	al, 2
	mov	ah, 3dh
	int	21h
	mov	fd2, ax
; POINT B
; If these three lines are included then the read still fails.
;	mov	bx, fd1
;	mov	ah, 3eh
;	int	21h
	; read the data from the second file
	; b = read(fd2, buffer, sizeof(buffer));
	mov	bx, fd2
	mov	dx, offset buffer
	mov	cx, size buffer
	mov	ah, 3fh
	int	21h
	mov	b, ax
	; Here a = 80 and b = 0.  Shouldn't b = 80 also?
	; return to system
	mov	al, 0
	mov	ah, 4ch
	int	21h
	ret	
xtest	ends
	end	start
;--------------------------- End of Test -----------------------


-- 
Brad Davis	{ihnp4, decvax, seismo}!utah-cs!b-davis	
		b-davis@utah-cs.ARPA
One drunk driver can ruin your whole day.