PRODUCT  :  Borland C++                            NUMBER  :  643
  VERSION  :  All
       OS  :  PC DOS
     DATE  :  October 22, 1993                         PAGE  :  1/4

    TITLE  :  Using a Dynamically Allocated Stack in C





  /*[]------------------------------------------------------------[]*/
  /*|                                                              |*/
  /*|     'Using a LARGE temporary stack in a small program'       |*/
  /*|                                                              |*/
  /*|                                                              |*/
  /*|     Copyright (c) 1991 by Borland International              |*/
  /*|     All Rights Reserved.                                     |*/
  /*|                                                              |*/
  /*[]------------------------------------------------------------[]*/

  /************************************************************************
   NOTE:  If you want to use the stack-checking code in Turbo C++ or
          Borland C++ then you must be in the LARGE or HUGE memory model.
   **********************************************************************/

  /*
      The theory here is the stack checking option generates the
      following code at the beginning of every function that was compiled
      with it on:

      _function proc far
                push bp
                mov bp, sp
                cmp word ptr DGROUP:_stklen, sp
                ja short @1@74
                call far ptr F_OVERFLOW@
      @1@74:
                .    ; the rest of the function...
                .
                .


     This is basically checking SP against the stack length and if SP ever
     becomes greater than _stklen, (which can only happen if you use all
     the stack space and then wrap SP from seg:0000 to seg:FFFE) then the
     overflow function gets called.

     All we are doing is allocating a 66K buffer from farmalloc,
     normalizing the pointer to get a full segment, saving the old values
     for ss, sp, and _stklen, setting the new values, and calling the
     stack-hungry function.














  PRODUCT  :  Borland C++                            NUMBER  :  643
  VERSION  :  All
       OS  :  PC DOS
     DATE  :  October 22, 1993                         PAGE  :  2/4

    TITLE  :  Using a Dynamically Allocated Stack in C




     This can be very useful for qsort() programs that wish to sort
     very large tables, and don't need 64k of stack all the time.

     If you don't use stack checking then you can use this trick from any
     model.
  */


  #if !defined (__LARGE__)
    #error Must be in the LARGE model!
  #endif


  #include <dos.h>
  #include <stdio.h>
  #include <stdlib.h>
  #include <alloc.h>
  #include <conio.h>

  unsigned _stklen = 2048;     // stack = 2k

  unsigned int goober = 0;
  char *p;

  void recurse1 (void)
  {
    goober++;
    cprintf ("\rCalling recurse1() %u ", goober);
    recurse1 ();
  }

  void do_reg (void)
  {
    recurse1 ();
  }

  void do_alloc (void)
  {
  // NOTE: these all MUST be static (or global) because if they were local
  //       the compiler wouldn't be able to access them after we switch
  //       our stacks.

    static char far *p;













  PRODUCT  :  Borland C++                            NUMBER  :  643
  VERSION  :  All
       OS  :  PC DOS
     DATE  :  October 22, 1993                         PAGE  :  3/4

    TITLE  :  Using a Dynamically Allocated Stack in C




    static unsigned old_ss, old_sp, old_stklen;

    if ((p = farmalloc (66000L)) == NULL)
    {
      printf ("\nNo room to allocate stack buffer!\n");
      exit (1);
    }
    /*
     We now have a pointer xxxx:0004. This example will overwrite the
     bottom and top of the segment.  To prevent serious memory
     corruption, we must allocate a full 64K segment and normalize it
     ourselves.  Now if we corrupt memory at xxxx:0000 or xxxx:FFFF we are
     still within the buffer we allocated, and thus are safe.  This
     saves us from incurring the overhead of having a HUGE pointer for
     the address of our buffer.
    */

    (long)p += 0x00010000;   // increment segment
    p -= 4;                  // set offset to zero

    // p is now = (xxxx + 1):0000

    old_ss = _SS;            // save old values
    old_sp = _SP;
    old_stklen = _stklen;

    _stklen = 60000U;        // change the stack length (for checking)

    _SS = FP_SEG (p);        // set up our new stack segment

    _SP = FP_OFF (&(p[_stklen]));  // set the stack to the top

    goober = 0;              // reset our counter
    recurse1();              // call the recursive DEATH function

     // If we were to get to this point...

    _SS = old_ss;            // reset the values
    _SP = old_sp;
    _stklen = old_stklen;

    (long)p -= (0x00010000 - 4);
                   // set the pointer back to what it was to free it













  PRODUCT  :  Borland C++                            NUMBER  :  643
  VERSION  :  All
       OS  :  PC DOS
     DATE  :  October 22, 1993                         PAGE  :  4/4

    TITLE  :  Using a Dynamically Allocated Stack in C




    free ((void *) p);
  }

  int main (void)
  {
    cputs ("\n\rBIG Stack Example (c) 1991 by Borland International\r\n");
   // do_reg();    // call this function to compare with regular stack
    do_alloc ();   // call this one to allocate a BIG temporary stack
    return 0;
  }

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  subject to the terms of the No-Nonsense License Statement that
  you received with the Borland product to which this information
  pertains.