Pointers and Offsets and Shared Memory Concerns

[Mark Lawrence]

I'm implementing a shared data area which has a companion data
dictionary (also in shared memory) to describe the contents of the
shared data area.

The shared data area is implemented as one big struct with lots of
elements.  The data dictionary is defined as an array of structs, each
describing one of the elements in the shared data area.  I use #define'd
symbols as indices into the array of structs to find the description of
an element in the shared data area.  A part of the description is the
address of the element of interest.

The dictionary and the shared data area will reside in shared memory
(System V style).  I maintain a global pointer to the beginning of the
shared data area and the shared dictionary area.  

Since the shared data area could potentially get mapped in at different 
virtual memory places among user processes, I had intended to store a
relative offset from the beginning of the shared data area for an element
into the dictionary structure for that element and simply add on the base
address for the shared data area (from the user processes perspective)
at time of access to yield the 'real' virtual memory address of the
element of interest (gad, that was long).  Of course, the converse is
required at startup time to cook up the offsets in the first place, e.g.
 shdata * sdp;
 offset = &(sdp->ElementOfInterest) - sdp;
/* then stuff the offset into the relevant element of the data dictionary */

Using gcc 1.35 on SunOS 4.0.3, I find that such attempts to + and - 
pointers and addresses yields compiler complaints about invalid operands 
to binary operators.  I get this whether I cast the pointers to void * or 
char * or simply leave them typed as is.

I understand why it probably doesn't make sense (in portable way) to
simple 'add' and 'substact' pointers to come up with offsets and
recalculate, but I'm sure there is an approved way.  Can anybody give me
pointers to an operator or 'standard' macro that achieve's what I
desire?
-- 
   mark at DRD.Com                (918) 743-3013              Jer. 9:23,24
   {uunet,rutgers}!drd!mark