[PD] t_scalar member sc_vec

[Miller Puckette]

Hi all -

I don't know if this is a reasonable thing to do in 2014 - it's a coding trope
I learned around 1981.  As far as I know C has no clean way to describe a
packed data structure with a header and then a variable number of identical
elements (like a soundfile with a header followed by samples).  In this case,
the object consists of a structure,

    t_gobj sc_gobj;
    t_symbol *sc_template;

followed in memory by an array of t_words.  One can't make a zero-size arary
in standard C (as far as I know) and so it's declared as having the artificial
size 1.  If there are n elements, the size of the structure is 

    sizeof(t_scalar) + (n-1) * sizeof(t_word)

subtracting 1 for the one that stood in for the array.

cheers
Miller

On Thu, Feb 20, 2014 at 06:32:48PM -0800, Jonathan Wilkes wrote:
> Can anyone explain what's going on with this in m_pd.h:
> 
> typedef struct _scalar      /* a graphical object holding data */
> {
>     t_gobj sc_gobj;         /* header for graphical object */
>     t_symbol *sc_template;  /* template name (LATER replace with pointer) */
>     t_word sc_vec[1];       /* indeterminate-length array of words */
> } t_scalar;
> 
> How is a static t_word array of size 1 an indeterminate-length array?  Is its placement as the last member of the struct required?
> 
> I see lots of mysterious casts in the internals of Pd's data structures.  What's the trick here, and is it documented anywhere on the interwebs, a C standard doc, etc.?
> 
> Thanks,
> Jonathan

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