Funny NULL pointers: (was: awk bug)

[Joel Clark]

In article <275 at ho7cad.ATT.COM> ekb at ho7cad.ATT.COM (Eric K. Bustad) writes:
>In article <3763 at megaron.arizona.edu>, rupley at arizona.edu (John Rupley) writes:
>> This is a bit off the thread of the awk bug, but if the 3B20 can't 
>> handle a NULL pointer in awk, how does it handle C code like:
> [DELETED]
>Any code that does this will happen to work on a VAX running UNIX, but
>will fail on many more machines than just AT&T's 3B20.  I seem to recall
>that on some machines you will get a memory access error if you dereference
>a NULL pointer! 

Another war story regarding dereferencing NULL pointers.  I once worked on a
compiler that used the 4 bytes previous to a pointer address to validate a
pointer by putting the pointer's value there.  If the pointer had a value of
1000 decimal then location 996 would contain the value 1000.  Checking code
then did something like:  cmpl -4(r4), r4.  The checking code handled the
memory faults from this.
This worked fine until we came to a Masscomp, where 'cmpl -4(0), 0' was
a valid instruction.  Seems Masscomp uses small negative addresses to access
the FPU.  Since we did not have an FPU address -4 contained 0.  Ha Ha.

Joel Clark
joel at intelisc.com
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