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The pages on this site contain documentation for very old MS-DOS software, purely for historical purposes. If you're looking for up-to-date documentation, particularly for programming, you should not rely on the information found here, as it will be woefully out of date.

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     An error-checking routine is automatically invoked after your
     program has terminated to determine whether the contents of the
     NULL segment have changed. If they have, the routine displays the
     following error message:
 
          run-time error R6001
          - null pointer assignment
 
     This error does not cause your program to terminate. The error
     message is displayed following normal termination of the program.
 
     The NULL segment is a location in low memory that is normally not
     used. If the contents of the NULL segment change during program
     execution, it means the program has written to this area, usually
     by an inadvertent assignment through a null pointer.
 
     The null-pointer error message reflects a potentially serious
     program error. Although the program may appear to operate
     correctly, it is likely to cause problems and may fail to run in
     a different operating environment.
 
     The library routine that performs the null-pointer check is named
     _nullcheck. You can suppress the null-pointer check by defining
     your own routine named _nullcheck that does nothing. Place the
     _nullcheck definition in the file containing the main function.
     Use the /NOE option to avoid multiple symbol definitions. The call
     to _nullcheck will be satisfied by your definition of _nullcheck,
     and the library version will not be loaded.
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