From:	Jin <GoldCode AT yahoo DOT com>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Problem hooking DOS int 21h
Date:	Mon, 20 Jul 1998 14:06:31 -0700
Lines:	96
Message-ID:	<35B3B157.1D3F3DE@yahoo.com>
References:	<35B36E78 DOT 2BC87A6C AT yahoo DOT com> <Oa3$xAAt9GA DOT 171 AT upnetnews05>
NNTP-Posting-Host:	jinhehui.stanford.edu
Mime-Version:	1.0
To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp

Thanks Shelby Cain.

In the following program, `test.c',  I do nothing than intercepting DOS
interrupt and then passing it back to DOS without any change.  But I
cannot get it work under Window95.  It seems whenever you chain into DOS
in DJGPP,  functions that executing other program,   like spawn*. exec*
and system(), cease to work.  But other functions work fine.

/***START test.c ***/
#include <dpmi.h>
#include <go32.h>
#include <crt0.h>
#include <stdlib.h>

int _crt0_startup_flags = _CRT0_FLAG_LOCK_MEMORY;

_go32_dpmi_seginfo  old_int21_vector;
_go32_dpmi_registers  my_int21_regs;
_go32_dpmi_seginfo  my_int21_info;

/*
 * My int 21h handler
 */
static
void call_dos(_go32_dpmi_registers *r)
{
 r->x.cs = old_int21_vector.rm_segment;
 r->x.ip = old_int21_vector.rm_offset;
 _go32_dpmi_simulate_fcall_iret(r);
}

int grab_dos() {
    _go32_dpmi_get_real_mode_interrupt_vector(0x21, &old_int21_vector);
    my_int21_info.pm_offset = (int) call_dos;

if(_go32_dpmi_allocate_real_mode_callback_iret(&my_int21_info,&my_int21_regs)

        || _go32_dpmi_set_real_mode_interrupt_vector(0x21,
&my_int21_info) )
    {
        return 1;
    }
    return 0;
}

void release_dos(void)
{
 _go32_dpmi_set_real_mode_interrupt_vector(0x21, &old_int21_vector);
 _go32_dpmi_free_real_mode_callback(&my_int21_info);
}

int main()
{
 if( grab_dos() == 0 ) {
  system("dir");
  release_dos();
 }
 return 0;
}

/*** END test.c ***/


Shelby Cain wrote:

> You have to make sure that you only intercept the particular function
> (AH=??) that you are interested in... all other functions must be
> passed to
> the original Int 21h handler.
>
> Jin wrote in message <35B36E78 DOT 2BC87A6C AT yahoo DOT com>...
> >Hi:
> >
> >I need to hook DOS interrupt before running other program, so I can
> >monitor
> >the program's console output without redirection (with redirection,
> the
> >output
> >will not be displayed on the console).  The problem is, after hooking
>
> >DOS,
> >the functions spawn*, exec* and system no longer work.
> >
> >Is there anyone tell me how to make the following codes work?
> >
> >Thanks in advance.
> >
> >
> >JIN
> >
> >
> >
> >

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