Xref:	news2.mv.net comp.os.msdos.djgpp:722
From:	rho AT hrz DOT tu-chemnitz DOT de (Robert Hoehne)
Newsgroups:	comp.os.msdos.djgpp
Subject:	Q: Why does my handler of int 0x21 not work?
Date:	1 Feb 96 15:07:48 GMT
Organization:	University of Technology Chemnitz, FRG
Lines:	91
Message-ID:	<4eql1u$2lm@pyrrhus-f.hrz.tu-chemnitz.de>
NNTP-Posting-Host:	ram.hrz.tu-chemnitz.de
To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp

/*
I want to write a handler for int 0x21 and call then external programs.
Every thing in my program work well with this handler, but if I call
an external program (like gcc), computer crashs. Where is the Error or
the problem, which I have to solve? (I'm using DJGPP V2Beta4 GCC V2.7.0
and the patched file gormcb.c). Program was comiled with

  gcc -o test21 test21.cc


   Robert

/* start of file test21.cc */
#include <stdio.h>
#include <stdlib.h>
#include <dpmi.h>
#include <go32.h>
#include <unistd.h>
#include <process.h>
#include <sys/farptr.h>

_go32_dpmi_seginfo oldint21,newint21;
__dpmi_regs regs;
int is_set = 0;

#define call_oldint(info,r)\
  r->x.cs = info.rm_segment;\
  r->x.ip = info.rm_offset;\
  _go32_dpmi_simulate_fcall_iret(r)

void _new_int21(__dpmi_regs *r,int origstack,int _sp,int _ss)
{
  int _es;
  call_oldint(oldint21,r);
/*
   save flags on caller's stack, because the iret-wrapper restores 
   them from this location and not from *r
*/
#if 1
/* after analysing the wrapper I got this code to access the callers stack */
  _es = _farpeekw(_ss,_sp);
  _farpokew(_es,origstack+4,r->x.flags); 
#else /* this shoul'd also work
  _farpokew(_dos_ds,(_ss << 4) + _sp + 4,r->x.flags);
#endif
}

void exit_func(void)
{
  int i;
  if (!is_set) return;
  _go32_dpmi_set_real_mode_interrupt_vector(0x21,&oldint21);
  _go32_dpmi_free_real_mode_callback(&newint21);
  is_set = 0;
}

void InitHook21()
{
  newint21.pm_offset = (int)_new_int21;
  _go32_dpmi_lock_code(_new_int21,
                    (unsigned long)exit_func - (unsigned long)_new_int21);
  _go32_dpmi_lock_data(&oldint21,sizeof(oldint21));
  _go32_dpmi_get_real_mode_interrupt_vector(0x21,&oldint21);
  _go32_dpmi_allocate_real_mode_callback_iret(&newint21,&regs);
  atexit(exit_func);
  _go32_dpmi_set_real_mode_interrupt_vector(0x21,&newint21);
  is_set = 1;
}

void DoneHook21()
{
  exit_func();
}

int main(int argc,char *argv[])
{
  fprintf(stderr,"before calling external program\n");
  spawnlp(P_WAIT,"gcc.exe","gcc.exe","-v",0);
  fprintf(stderr,"after calling external program\n");
  InitHook21();
  fprintf(stderr,"before calling external program\n");
  spawnlp(P_WAIT,"gcc.exe","gcc.exe","-v",0);
/*
  At this point the computer hangs. => Ctrl-Alt-Del
*/
  fprintf(stderr,"after calling external program\n");
  DoneHook21();
}

/* end of file test21.cc */

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