🚀 go-pugleaf

In article <BvsL61.LEE@resntl.bhp.com.au> Bernard Orenstein <bernie@resntl.bhp.com.au> writes:
>in the past with C6.0, but now, under C7.0 is giving the
>error "null pointer assignment" when it finishes executing
>(and it does execute correctly) . I have used 'CodeView' to step through it,
>which shows that after the last "}" in my main() program, it
>goes off to execute some machine langiage instructions, a
>call to some routine or other, and it is THIS call which gives
>me the "null pointer assignement" error.

"null pointer assignment" is described in the "Comprehensive Index and
Error Messages" book that came with your C7.0 compiler.

Your main program is not the beginning and end of the actual executable
program.  In fact, main( ) is a subroutine to the C startup code.  That
code does things like grab the command-line arguments and open
stdin/stdout/stderr, then calls main( ), then upon return from main it
closes any open files and checks for null pointer assignments.

Pointers are not checked for validity when you actually use them; to do so
would greatly add to the size of programs.  Instead, Microsoft code
checks after your program has executed whether 0000:0000 (*) has a different
value from before it began.  If so, your program must have written to
it so Microsoft code tells you this.  The actual assignment using an
uninitialized pointer may have occurred at any point in the program.

Your program did not execute correctly; it _appeared_ to execute
correctly because the consequences of the failure were not obvious.
The manual suggests that the code may fail in more obvious ways on
different platforms.  I add that "different platforms" means not just
other (kinds of) machines but even the same machine if you have loaded
different TSRs, or if a different program is executed before the one
that gets "null pointer assignment".

(*) If you're in small or medium memory model, data pointers are 16 bits
and a null pointer would assign to location 0000 in your data segment,
not to 0000:0000.  I don't know the internals of how Microsoft code
checks or what.  But the general principles given above are correct 
based on my experience and reading the manuals.
-- 
Stan Brown, Oak Road Systems                      brown@Ncoast.ORG
Cleveland, Ohio, USA