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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documentation, particularly for programming, you should not rely on the information found here, as it will be woefully out of date.

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─────C/C++ Language─────────────────────────────────────────────────────────
 
     An error-checking routine (_nullcheck) 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 _nullcheck library routine works only for near pointers and is
     therefore not useful for the compact, large, and huge memory
     models. Its source code (CHKSUM.ASM) is included as part of the
     startup code that is provided with the compiler.
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