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Text File | 1991-09-11 | 5KB | 265 lines

  PRODUCT  :  PARADOX                                NUMBER  :  542
  VERSION  :  ALL
       OS  :  DOS
     DATE  :  September 11, 1991                       PAGE  :  1/4

    TITLE  :  STACK SPACE/PAL ALLOCATION OF STACKS




  This is an explanation of why the error "Run out of stack space"
  is displayed by Paradox.

  Stack space problems are the result of scripts or procedures
  calling each other.  The Paradox stack is used to keep track of
  the script (or procedure) active at any time and which scripts
  have been played.

  The stack works on the premise that the last item placed on the
  stack must be cleared before accessing any other item, sometimes
  referred to as LIFO (Last In, First Out).  This means that each
  new procedure you call is placed on the stack, pushing down the
  previous procedure.  In order to develop an understanding of the
  discussion, let's work with some examples.

  If script FIRST calls script SECOND, the stack would look like
  this:

       FIRST
       SECOND

  Then, SECOND calls script FIRST again and now the stack becomes:

       FIRST
       SECOND
       FIRST

  This can continue until all the stack space is used up. The way
  to prevent this recursive system from filling up all the stack
  space is to use the RETURN command detailed on page 371 of the
  PAL User's Guide.  Using RETURN on the stack problem yields the
  following allocation of stack space:

       FIRST
       SECOND

  If script SECOND uses RETURN, instead of issuing PLAY FIRST,
  SECOND will be "popped off" the stack, and control returns to
  script FIRST.  Now the stack looks like this:

       FIRST















  PRODUCT  :  PARADOX                                NUMBER  :  542
  VERSION  :  ALL
       OS  :  DOS
     DATE  :  September 11, 1991                       PAGE  :  2/4

    TITLE  :  STACK SPACE/PAL ALLOCATION OF STACKS




  All of this information holds true whether you are using scripts
  or procedures.  The use of the WHERE? (press [Ctrl-W]) command is
  very useful in locating where recursion happens.  Page 126 of the
  PAL User's Guide explains the usage of WHERE?.

  The following pages contain scripts demonstrating the incorrect
  method and correct method for stack usage.

















































  PRODUCT  :  PARADOX                                NUMBER  :  542
  VERSION  :  ALL
       OS  :  DOS
     DATE  :  September 11, 1991                       PAGE  :  3/4

    TITLE  :  STACK SPACE/PAL ALLOCATION OF STACKS




  release vars all
  ;This script demonstrates a STACK OVERFLOW error.
  ;
  ;--------------What NOT to DO-----------------
  Proc main()                            ;Proc starts
  Showmenu                               ;a menu
  "Recursive" : "play this several times (15 or 20 to get debug)",
  "Quit"      : "Quit"
      to z                               ;to a variable
      Switch
          case z = "Recursive":          ;if they select this choice
             jcdowrong()                 ;call jcdowrong procedure
          case z = "Quit":
             message "Adios"             ;if they select quit
             sleep 750                   ;give a message
      Endswitch
  EndProc

  Proc jcdowrong()                       ;the bad procedure
     if not isassigned(z1)
     then z1 = 1
     endif
      Message "looped " + strval(z1)+ " times"
      z1=z1+1
      sleep 200
      main()                             ;this is the offending line
  EndProc

  Main()                                 ;the actual script



























  PRODUCT  :  PARADOX                                NUMBER  :  542
  VERSION  :  ALL
       OS  :  DOS
     DATE  :  September 11, 1991                       PAGE  :  4/4

    TITLE  :  STACK SPACE/PAL ALLOCATION OF STACKS




  ; Rather than call the original procedure, let the procedure
  ; return naturally or use a "return" command
  ;----------------What to DO------------------

  Proc goodmain()                        ;Proc starts
  While(true)                            ;use the loop to eliminate
  having to
                                         ;to recursive calls
  Showmenu                               ;a menu
  "NON-Recursive" : "play this several times(15 or 20 to get debug)",
                                         ;the choices
  "Quit"          : "Quit"
      to z                               ;to a variable
      Switch
          case z = "NON-Recursive":      ;if they select this choice
             jcdoright()                 ;call jcdoright procedure
          case z = "Quit":
             message "Adios"             ;if they select quit
             sleep 750                   ;give a message
             quitloop
      Endswitch
  endwhile
  EndProc


  Proc jcdoright()
     if not isassigned(z2)
     then z2 = 1
     endif
      Message "looped " + strval(z2)+ " times"
      z2=z2+1
      sleep 200
  EndProc

  goodmain()