Re: Need help mapping a C struct to Ada

["David Thompson" <david.thompson1@worldnet.att.net>]

(null) <cts@kampong.aedinc.net> wrote :
> I'm trying to call some C functions from my Ada program.  The C function
> prototype and data types are basically
...
>   typedef struct {
>   int      num_elements;
>      /*typedef to struct blah*/ the_elements[0];
>   } element_table;
...
> My question is basically about the element_table structure.  In C
> this means basically that element_table.the_elements doesn't have
> a fixed size.  It is up to the programmer to figure out how much
> memory is needed for the desired number of elements and manual
> allocate the memory.
>
Only in GNU C, the language "not entirely unlike" C
compiled by the gcc C compiler by default, not in
standard C and not in (most?) other C compilers.
The 1989/90 standard requires the array bound to be
(fixed and) strictly positive.  Since C does not require
bounds checking, and implementations rarely do it,
it *almost* always works to declare T foo [1],
malloc(sizeof(S)+ (N-1)*sizeof(T)), and use 0..N-1,
although this is ugly(er).  Since this actually works
on reasonable implementations, the new 1999
standard makes it official but with a new, distinct
syntax, called "flexible array member":
  typedef struct { ... T foo []; /* no bound at all */ } S;
  ptr = malloc(sizeof(S) + N * sizeof(T));

--
- David.Thompson 1 now at worldnet.att.net