NetNews Usenet Archive 1992 #19/NN_1992_19.iso/spool/comp/lang/c/13054

Internet Message Format | 1992-08-31 | 2.0 KB

Xref: sparky comp.lang.c:13054 comp.os.msdos.programmer:8925
Path: sparky!uunet!decwrl!access.usask.ca!ccu.umanitoba.ca!ciit85.ciit.nrc.ca!brandonu.ca!dueck
Newsgroups: comp.lang.c,comp.os.msdos.programmer
Subject: Re: file positioning with buffered/unbuffered I/O
Message-ID: <[e-mail]>
From: [e-mail]
Date: 31 Aug 92 15:08:50 CST
References: <[e-mail]> <[e-mail]> <[e-mail]>
Organization: Brandon University, Brandon, Manitoba, Canada
Lines: 26

In article <[e-mail]>, [e-mail] (Christopher H Gregg) writes:
> Hi, I am writing an assembly language program and I am having some
> problems with it.  The program is memory resident and it traps int 1Ch.
> After it traps the interrupt, it checks if a certain amount of time
> has passed and then it deletes a certain file on a directory.  The
> problem is that when I try to use either DOS function 13h (delete file)
> or DOS function 41h (which I would prefer), the program crashes.
>
You have become stuck on one of the worst problems in DOS: its not
re-entrant. However, you can tell when its safe to call DOS by checking
the DOS-safe flag. This is a one-byte value located one byte past the
address returned in DS:SI by interrupt 21 function 5D06 and it is zero
when it is safe to call DOS. Unfortunately the flag lies: It is safe to call
DOS if DOS is busy with a call <= 0x0C, mostly standard input and output
calls. However, when DOS is in a keyboard-polling loop, it invokes
interrupt 28, the DOS-idle interrupt.

Your program should acquire its data via int 1C and write it out from a
routine called via int 28.

This solution will break if you have hot-keyed back into a TSR, thereby
activating via int 28. The TSR can't use int 21 subfunction <= 0C, and
int 28 can't be re-entered. Does anyone know the solution to this problem?

Gery Dueck
[e-mail]