[Plplot-general] Question about C 2D arrays for Ada binding

[Jerry <lan...@qw...>]

I've had a nearly-complete thin binding of PLplot to Ada for a few  
weeks now, and am working on a thick binding. However, even at the  
thin binding level, I've encountered a problem with passing 2D Ada  
arrays to C. I refer the interested to this thread at comp.lang.ada  
(as hosted by Google):

http://groups.google.com/group/comp.lang.ada/browse_thread/thread/ 
6b3a3c920575b35a/193ba675e86cb69a?

Being a relative Ada newbie and just learning enough C in the process  
to get by, I thought the gurus on the list would have the answer.  
However, it hasn't been forthcoming and even they seem befuzzled =  
befuddled + puzzled.

I don't expect any Ada per se help from this post, and this is really  
a C question, but since there might be a vested interest from the  
PLplot community to have functioning Ada bindings, maybe you will  
humor this question:

How does C map 2D arrays such as PLFLT **z to memory (e.g. plmesh)?  
Is this an array of pointers that then each point to the first  
element of a row (column??) of z, or is this simply a pointer to a  
pointer to z[0, 0] which merely needs to be twice de-referenced?

Let me try to be even more confusing. Say z is M x N. Is the address  
that is pointed to by PLFLT **z the first byte of a blob of memory  
that is M*N*sizeof(int), with M and N used in parsing up this blob  
into addressable array elements? Or are there actually M pointers to  
M memory locations, each of which represents N array elements as a  
contiguous blob of memory amounting to N*sizeof(int) bytes? If the  
latter is true, then it seems that z would require M*N*sizeof(int) 
+M*sizeof(pointer) bytes, and somehow this seems weird. If the latter  
is true, then could one access array element i,j by an offset M + M*i  
+ j or something like that?

Note that the standard C interfaces provided by Ada deal perfectly  
and easily with 1D arrays.

Thanks for any comments.
Jerry