[TUHS] history of virtual address space

[Alan Coopersmith via TUHS]

On 1/19/26 15:10, Luther Johnson via TUHS wrote:
> I once found an old version of RCS that expected *0 = 0, and I think many other 
> programs did as well. I'm not sure which versions of Unix did or still do this 
> any more, but I believe many had the first page in virtual address space for 
> user program processes, mapped to a read-only page of zeroes, to support this. 
> If I'm remembering that wrong, I'm interested as the other posters here are, in 
> the details of which did which sorts of things.
Deep in the SunOS SCCS archives, the initial checkin of the string(3) man page,
labeled as:

D 1.1   83/08/30 15:57:14 wnj   1 0     00103/00000/00000
date and time created 83/08/30 15:57:14 by wnj

contains this warning in the "BUGS" section:

        On  the  Sun  processor (and on some other machines), you can NOT use a
        zero pointer to indicate a null string.  A zero pointer is an error and
        results in an abort of the program.  If you wish  to  indicate  a  null
        string, you must have a pointer that points to an explicit null string.
        On  PDP-11's  and VAX'en, a source pointer of zero (0) can generally be
        used to indicate a null string.  Programmers using NULL to represent an
        empty string should be aware of this portability issue.

This warning was apparently not sufficient, since in 1994, Bill Shannon added
the "0 at 0.so.1" object to Solaris, which you could use with LD_PRELOAD to mmap
a page from /dev/zero to the page at address 0, as described in the ld.so.1(1)
man page:

        The  user  compatibility library /usr/lib/0 at 0.so.1 provides a mechanism
        that establishes a value of 0 at location 0.  Some  applications  exist
        that  erroneously assume a null character pointer should be treated the
        same as a pointer to a null string. A segmentation violation occurs  in
        these  applications  when a null character pointer is accessed. If this
        library is added to such an application at  runtime  using  LD_PRELOAD,
        the  library provides an environment that is sympathetic to this errant
        behavior. However, the user compatibility library is  intended  neither
        to enable the generation of such applications, nor to endorse this par-
        ticular programming practice.


        In  many cases, the presence of /usr/lib/0 at 0.so.1 is benign, and it can
        be preloaded into programs that do not require it. However,  there  are
        exceptions.  Some applications, such as the JVM (Java Virtual Machine),
        require that a segmentation violation be generated from a null  pointer
        access.   Applications   such   as   the   JVM   should   not   preload
        /usr/lib/0 at 0.so.

There are more lines of comments than code in the source for it:

https://github.com/illumos/illumos-gate/blob/master/usr/src/cmd/sgs/0%400/common/0%400.c

-- 
         -Alan Coopersmith-                 alan.coopersmith at oracle.com
          Oracle Solaris Engineering - https://blogs.oracle.com/solaris