Re: [Vala] how to *not* automagically copy arrays passed to methods?

[Matto Marjanovic <maddog mir com>]

On 12/20/11 13:11, Matto Marjanovic wrote:
> On 12/20/11 11:49, Evan Nemerson wrote:
> > On Tue, 2011-12-20 at 00:39 -0800, Matto Marjanovic wrote:
> > > The subject line explains what I am trying to do. In plain old C,
> > > I would simply use plain old libgio:
> > >
> > > GInputStream* s = ...;
> > > guint8 buffer[4096];
> > > GError* error = NULL;
> > > ...
> > > g_input_stream_read(s, buffer + M, N, NULL,&error);
> > >
> > > How do I do this in Vala?
> ...
> > Use array slicing. Something like stream.read (buffer[M:M + N]);
>
> Ah-ha! Thank you.
>
> The docs make it sound like slicing always duplicates the (slice of the)
> array --- but I now see that doesn't happen when the slicing happens in
> the method call. (Before writing my email, I had tried assigning a slice
> to a temp local array variable, and that invoked a dup in the generate
> C code....)

Ok, another bonus question (see revised subject) --- and I think this
issue was the root of the misfire with my earlier experiments with slices:

How does one tell valac to *not* copy arrays that are passed to methods?

For example:

   public void foo(uint8[] x) {
     ...
     x[0] = 7;
     ...
   }

The generated C code for the method duplicates the array x before executing
the body of the method; hence the common case of a method which modifies a
buffer, returning with side-effects, doesn't work.

Who has *ever* wanted to pass an array by value?

Even worse, this behavior is inconsistent with wrapped methods --- e.g.,
InputStream.read() happens to work because vala doesn't wrap an array-copying
wrapper around the underlying C/GIO function.

Is there some magic keyword to make the copying stop?

-m