### Question 5.20

What does a run-time ``null pointer assignment'' error mean? How do I track it down?

This message, which typically occurs with MS-DOS compilers (see, therefore, section 19) means that you've written, via a null (perhaps because uninitialized) pointer, to location 0. (See also question 16.8.)

A debugger may let you set a data breakpoint or watchpoint or something on location 0. Alternatively, you could write a bit of code to stash away a copy of 20 or so bytes from location 0, and periodically check that the memory at location 0 hasn't changed.

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