From:	"Shelby Cain" <NOSPAMvhorrit AT email DOT msn DOT com>
References:	<35B36E78 DOT 2BC87A6C AT yahoo DOT com>
Subject:	Re: Problem hooking DOS int 21h
Date:	Mon, 20 Jul 1998 12:01:27 -0500
Lines:	87
Message-ID:	<Oa3$xAAt9GA.171@upnetnews05>
Newsgroups:	comp.os.msdos.djgpp
NNTP-Posting-Host:	[207.43.206.174]
To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp

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
>
>
>/***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;
>
>/*
> * This function is called only within our 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;
>}
>
>
>

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