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

[<hba...@ma...>]

On Jul 28, 2006, at 6:15 PM, Jerry wrote:
>
> 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?

PLFLT **z is an array of pointers each of which point to the first  
element of a row/column.

> 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

It does allow you to use M non-contiguous blocks of memory of size N  
* sizeof(int), rather than one large contiguous block of size  
M*N*sizeof(int). This might have been handy back in the day when  
computers had small fragmented memories :).

-Hazen