PRODUCT  :  Borland C++                            NUMBER  :  726
  VERSION  :  2.0
       OS  :  DOS
     DATE  :  September 18, 1991                       PAGE  :  1/3

    TITLE  :  "Null Pointer Assignment" Errors Explained




  Null Pointer Assignment

  1. What is a Null Pointer Assignment error?

  The Null Pointer Assignment error is generated in programs that
  corrupt the bottom of the data segment in such a fashion as to
  indicate a high probability that an improperly-initialized
  pointer has been used.  The error can only be generated in the
  small and medium memory models.

  2. What causes a Null Pointer Assignment error?

  Borland places four zero bytes at the bottom of the data segment,
  followed by the Borland copyright notice.  In the small and
  medium memory models, a null pointer points to DS:0000.  Thus
  assigning a value to the memory referenced by this pointer will
  overwrite the first zero byte in the data segment.  At program
  termination, the four zeros and the copyright banner are checked.
  If either has been modified, then the Null Pointer Assignment
  error is generated.  Note that the pointer may not truly be null,
  but may be a wild pointer that references these key areas in the
  data segment.

  3. How can I debug a Null Pointer Assignment error?

  In either the Integrated Development Environment (except BCX) or
  in Turbo Debugger, set two watches on these key memory locations.
  These watches, and what they should display in the watch window,
  are:

      *(char*)0,4m    "Borland C++ - Copyright 1991 Borland Intl."
      (char *)0       00 00 00 00

  Of course, the copyright banner will vary depending on your
  version of the Borland C/C++ compiler.

  Caution:  The first of these watches--*(char *)0,4m--cannot be
  used in BCX or a general protection fault will be generated;
  however, the second watch--(char *)0--will perform properly.

  Step through your program and monitor theses values in the watch
  window.  At the point where one of them changes, you have just














  PRODUCT  :  Borland C++                            NUMBER  :  726
  VERSION  :  2.0
       OS  :  DOS
     DATE  :  September 18, 1991                       PAGE  :  2/3

    TITLE  :  "Null Pointer Assignment" Errors Explained




  executed a statement that uses a pointer that has not been
  properly initialized.

  The most common cause of this error is probably declaring a
  pointer and then using it before allocating memory for it.  For
  example, compile this program in the small memory model and
  execute it:

  #include <dos.h>
  #include <stdio.h>
  #include <string.h>

  int main(void)
  {
      char *ptr, *banner;
      banner = (char *) MK_FP(_DS, 4);
      printf("banner: %s\n", banner);
      strcpy(ptr, "Where will this text be copied?!?");
      printf("&ptr = %Fp\n", (void far*) &ptr[0]);
      printf("banner: %s\n", banner); return 0;
  }

  One of the best debugging techniques is to turn on all warning
  compiler messages.  If the above program is compiled with
  warnings turned off, no warning messages will be generated.
  However, if all warnings are turned on, both the strcpy() and
  printf() calls using the buffer variable will generate warnings.
  Be particularly suspicious of any warnings that a variable might
  be used before being initialized, or of a suspicious pointer
  assignment.  With the command-line compiler, just include the -w
  switch. In the IDE, select the appropriate menu option for
  compiler messages and work through all the menu windows, ensuring
  all items are checked.  In Turbo C++ and Borland C++, some of the
  option windows have sub-menus indicated by a 'more' selection box
  in the lower left corner of the menu box.

  4. Why is a Null Pointer Assignment error not generated in all
     models?

  In the compact, large and huge memory models, far pointers are
  used for data.  Therefore, a null pointer will reference
  0000:0000, or the base of system memory, and using it will not
  cause a corruption of the key values at the base of the data













  PRODUCT  :  Borland C++                            NUMBER  :  726
  VERSION  :  2.0
       OS  :  DOS
     DATE  :  September 18, 1991                       PAGE  :  3/3

    TITLE  :  "Null Pointer Assignment" Errors Explained




  segment.  Modifying the base of system memory usually causes a
  system crash, however.  Although it would be possible that a wild
  pointer would overwrite the key values, it would not indicate a
  null pointer.

  In the tiny memory model, DS = CS = SS.  Therefore, using a null
  pointer will overwrite the beginning of the code segment.

  5.  Can anything else generate a Null Pointer Assignment error?

  Using a wild pointer that happens to reference the base area of
  the data segment--thus causing a corruption of the zeros or the
  copyright banner--will generate this error.  Since data
  corruption or stack corruption could cause an otherwise-valid
  pointer to be corrupted and point to the base of the data
  segment, any memory corruption could result in this error being
  generated.  If the pointer used in the program statement which
  corrupts the key values appears to have been properly
  initialized, place a watch on that pointer.  Step through your
  program again and watch for its value (address) to change.