Path: utzoo!attcan!uunet!lll-winken!lll-lcc!ames!haven!vrdxhq!bms-at!stuart
From: stuart@bms-at.UUCP (Stuart Gathman)
Newsgroups: comp.sys.ibm.pc
Subject: File handles in TSR's: A tip and a question
Keywords: Aaaargh! MesS DOS
Message-ID: <146@bms-at.UUCP>
Date: 5 Jan 89 02:03:49 GMT
Organization: Business Management Systems, Inc., Fairfax, VA
Lines: 68

First: A tip for writing TSR's in Turbo C 1.5 & 2.0.  The setjmp() and
longjmp() functions save ss:sp.  This allows easy context switching.
Non-trivial interrupt handlers usually need to switch to an internal stack
to avoid overflowing a caller's stack of unknown size.  (And so that
pointers to stack variables work as expected for a small model TSR.)
A handler plus main for small model looks something like this:
--------------------------
#include <setjmp.h>
#include <dos.h>

#define	VECT	0x7f
#define	DSPARAS	0x1000		/* 64K data segment */

static jmp_buf u_state, state;

void interrupt handler()
  unsigned ...,ax;
{
  _AX = setjmp(u_state);		/* save user stack */
  if (_AX == 0) longjmp(state,ax);	/* switch to internal stack */
}

main()
{
  void interrupt (*saveint)();
  int op;
  op = setjmp(state);
  if (op == 0) {
    /* should check if already installed, etc. . . . */
    saveint = getvect(VECT);
    setvect(VECT,handler);
    keep(_DS - _psp + DSPARAS);		/* TSR */
  }

  if (op != UNINSTALL) {
    doit(op);				/* do stuff */
    longjmp(u_state);			/* return to user */
  }

  /* uninstall */
  setvect(VECT,saveint);		/* restore vector */
  freemem(*(unsigned far *)MK_FP(_psp,0x2c));	/* free environment */
  freemem(_psp);				/* free program */
  longjmp(u_state);			/* return to user */
}
---------------
Nice & high level, huh?  The handler doesn't need to be interrupt for
huge model (assuming the caller doesn't need to save AX..DX).  The
interrupt form ensures that _DS is loaded with the local data segment.
This could also be accomplished with inline assembler as could pushing
only those registers not saved by setjmp().

NOW - my question is this:  is there any way to keep files open in a TSR?
DOS likes to close them whenever the last process loaded before they were
opened exits.  (Got that?)  Dos does not simply close all files whenever
any program exits.  It carefully keeps track of which files were opened
by which psp (apparently).  Unfortunately, TSR's don't count.  Is there any
way to fool DOS into saving TSR files also?  The alternative of keeping
an internal file descriptor table with qualified path names and replacements
for open/read/write/lseek/close doesn't appeal to me.

Strangely enough, file desciptors 0,1,2,3,4 are not closed even for TSR's.

Please reply by E-mail (in addition to posting if desired).  I read this
group, but can't keep up with the thousands of articles.
-- 
Stuart D. Gathman	<stuart@bms-at.uucp>
			<..!{vrdxhq|daitc}!bms-at!stuart>