RE: Instantiating arrays with Inline

[bbcannon]

Thanks Neil,

I do know the size of the array before hand since I
create it in perl first.  
I am a rookie when it comes to C, so thanks a million.

In the Big Picture, I am just trying to pass a perl array
to C because I can then interface to Matlab's Math and Graphics libraries.  

Is there a more efficient/easy way to send an array
or array_ref to Inline C?

Thanks...Brady

-----Original Message-----
From: Neil Watkiss [mailto:[EMAIL PROTECTED]]
Sent: Monday, June 10, 2002 10:20 AM
To: bbcannon
Cc: '[EMAIL PROTECTED]'
Subject: Re: Instantiating arrays with Inline


bbcannon [10/06/02 09:30 -0600]:
> I am receiving a "parse" error when I try to declare an array of doubles.
> However, I don't see a problem with my syntax.
> 
> Is something special required to create arrays with Inline ?

The short answer:

Your syntax is valid Perl, almost valid C++, but not valid C. To create
(static) arrays within C, you must know the size of the array at
compile time.

The long answer:

Let me paraphrase the important section of your code:

    void
    some_function(SV *x) {
        AV *arr;
        int maxIndex;
        int numElements;
        
        arr = SvRV(x);
        maxIndex = av_len(arr);
        numElements = maxIndex + 1;

        double data[numElements];
    }

That's not legal C for two reasons:

   1. You can only declare new variables at the beginning of a new scope,
      before the first non-declaration.

      /* This is legal: */
      {
         int hello;
         printf("foo!\n");
      }

      /* This is not */
      {
         printf("foo!\n");
         int hello;
      }

      Move the "double data" declaration up to the top of the scope, above
      the first non-declaration statement. That leads us to...

   2. You can only declare fixed-size static arrays if the size is known at
      compile-time. In your case, where the size depends on the length of
the
      AV*, you will have to use New() or Newz() to allocate a temporary
array
      yourself:

      {
         int maxIndex, numElements;
         AV *arr;
         double *data;

         ...
         
         Newz(0, data, numElements, double);
         SAVEFREEPV(data); /* Safefree() 'data' on scope exit */
         
         ...
      }

Notice the SAVEFREEPV(), which automatically calls Safefree() for you when
you leave the Perl XSUB, no matter how you leave it. Pretty neat, IMHO.

Hope that helps!

Later,
Neil