Trap Int21h problem

I saw a lot of info about the problems of trapping int21h, but I
really don=B4t understand what happens. I need to watch some serial
operations from another program, wich uses fopen() function as
interface for the comm port. Here=B4s my code:

void interrupt (*old21)();


void interrupt int21(void)
  {
    char far* str;
    if (_AH =3D=3D 0x3d)
      {
        str =3D MK_FP(_DS, _DX);
        if (_fstricmp(str, "com1:") =3D=3D 0)
          reboot();  //Debugging purpouses
      }
    old21();
  }


void main(void)
  {
    union REGS regs;
    long far* vect;


    old21 =3D getvect(0x21);
    vect =3D MK_FP(0, 0x21 * 4); //using direct address cause setvect
could cause conflict??
    *vect =3D  int21 // Not sure about this


    reboot =3D MK_FP(0xffff, 0x0000);


    regs.x.ax =3D 0x3100;
    regs.x.dx =3D 0x1000;
    intdos(&regs, &regs);
  }


Im not sure if this pointers operations are correct but Im interested
about what happens inside the int21 function.
Tks in advance