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Xref: unogate vmsnet.internals:1419 vmsnet.misc:1011 comp.lang.c:21899
From: <bruce@ais.com>
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Subject: RE: C Language question
Message-ID: <1992Oct9.110102.5773@ais.com>
Date: 9 Oct 92 11:01:02 GMT
References: <8543225@MVB.SAIC.COM>
Organization: Applied Information Systems, Chapel Hill, NC
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In article <8543225@MVB.SAIC.COM>, "Andy, Systems Manager"
 <UDAA055@ELM.CC.KCL.AC.UK> writes:
> Nigel Arnot writes:
>
>>Question: in C, if one declares char a[I][J], is there anything in the
>>language standard that guarantees that &a[0][0] is the first of a set of
>>I*J contiguous chars, and that a[i+1][0] is guaranteed to be the same
>>storage location as a[i][J] ).
>>
>
> I suppose that it is conceivable for a compiler to use a mechanism for
> indexing that allows the individual elements of an array to be scattered
> in virtual memory. Assuming this is not the case then I think the
> following explanation holds.
>
> A single `dimensioned' array or vector, such as a[i], is contiguous in
> memory in any language. (Obviously, otherwise indexing couldn't work).

This isn't quite true.  Most languages don't have a construct for
non-connected storage, but it is by no means impossible.  PL/I for
example has a mechanism for dealing with non-connected (non-contiguous)
storage:

        non_connected:  procedure;
        declare array (100, 100) fixed binary;
        declare sub entry ((*) fixed binary);

                call sub (array (*, 1));
                end non_connected;

Since elements are stored in row-major order in PL/I (as in just
about every major language except Fortran), this fragment passes to
the `sub' procedure a `slice' of the `array' which is formed from the
first column of the array - or in other words, from every 101st element
of the `array' as it is stored in memory.  The usual implementation is
for the compiler to write a descriptor that describes the required `step'
size for the array, and to pass that descriptor to the sub-procedure
rather than the address of the array itself.  Because this is less
efficient than indexing items in connected storage (because of the
implied multiply that the optimizer can't reduce in strength since
it doesn't know the multiplier at compile-time, unlike connected
storage where it does know the element size and therefore the step
size between elements), most implementations will pass the address
of the array rather than the descriptor if the formal argument is
declared to be `connected' or to have an explicit array size (the
(*) in the declaration for `sub' says the array can have any size).

Anyway this is rather off the track.  C doesn't have a concept of
`connected' versus `unconnected' storage of array items, and has
the explicit definitions that the address of the array is equal to
the address of array[0] and that successive elements of the array
are stored in sequential addresses.  Since a multi-dimensional
array in C is just an array of arrays, this implies that the entire
object must be in connected storage.  I don't think that you could
build a conforming compiler that didn't treat arrays the way Nigel
describes.

If there is really a lot of interest in this topic maybe it should be
moved to comp.lang.c or possibly vmsnet.misc rather than remain in
vmsnet.internals, which is a VMS internals newsgroup ...

Bruce C. Wright