Re: NULL pointer arithmetic issues

[Johnny Billquist <bqt%softjar.se@localhost>]

On 2020-02-25 12:33, Tom Ivar Helbekkmo wrote:
> Johnny Billquist <bqt%softjar.se@localhost> writes:
>
> > But yes, on the PDP11 [having nothing mapped at address 0] was/is not
> > the case. Memory space is too precious to allow some of it to be
> > wasted for this...
>
> Yup - and I assume the "hack" Kamil alludes to is the practice of
> actually starting the data segment for split I/D programs at address 1
> instead of 0, to make sure that no actual pointer is 0, thus allowing
> the straightforward comparison of a pointer with 0 to see if it's set.

Well, the d-dpace don't start at 1, and also, the PDP-11 isn't that fond 
of odd addresses. :-)
Actually, you could not even start a page at address 1 if you wanted.

> (I believe they also initialized address 0 to 0, to stop indirect
> references through it from reaching random data.  I guess Franz may have
> depended on this in some way, e.g. expecting to be able to test *p
> directly, instead of first p and then *p.  Do enough of this, and you've
> soon bummed a significant amount of valuable code space...)

It used to, but not for some time now.

Here is the current "state":

/*
 * Paragraph below retained for historical purposes.
 *
 * The following zero has a number of purposes - it serves as a null 
terminated
 * string for uninitialized string pointers on separate I&D machines for
 * instance.  But we never would have put it here for that reason; programs
 * which use uninitialized pointer *should* die.  The real reason it's 
here is
 * so you can declare "char blah[] = "foobar" at the start of a C program
 * and not have printf generate "(null)" when you try to print it because
 * blah is at address zero on separate I&D machines ...  sick, sick, 
sick ...
 *
 * In porting bits and pieces of the 4.4-Lite C library the global program
 * name location '___progname' was needed.  Rather than take up another two
 * bytes of D space the 0th location was used.   The '(null)' string was
 * removed from doprnt.s so now when programs use uninitialized pointers
 * they will be rewarded with argv[0].  This is no sicker than before and
 * may cause bad programs to die sooner.
*/
        .data
        .globl  ___progname, _strrchr

___progname: 0


  Johnny

--
Johnny Billquist                  || "I'm on a bus
                                  ||  on a psychedelic trip
email: bqt%softjar.se@localhost             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol