Ah, ok-- I forgot my pointer arithmetic rules.

Once I remembered how it works, I still wasn't understanding how you could be doing pointer arithmetic with byte granularity when t_word is wider than 1 byte.  But then I see from template_find_field you are multiplying the onset by sizeof(t_word).

So now, further down the rabbit hole of knowledge, I'm wondering why you multiply by sizeof(t_word) at all.  If you didn't, couldn't the (char *) cast go away?

-Jonathan

On Monday, August 17, 2015 12:24 PM, Miller Puckette <msp@ucsd.edu> wrote:

I don't thing the width of (char *) enters into it (it's a pointer, 8 chars in
64 bit addr space)  - the direct contrast to be made is (char) vs (t_word).

Not sure if that answers the question though...

cheers
M

On Mon, Aug 17, 2015 at 03:52:27PM +0000, Jonathan Wilkes via Pd-list wrote:
> Thanks, I think I'm getting it.
> So is char* guaranteed to be the same width as sizeof(t_word)?  If so, are you just using it as a shorthand?
>
> Thanks,
> Jonathan
>
>
> 
>
>
>      On Monday, August 17, 2015 11:31 AM, Miller Puckette <msp@ucsd.edu> wrote:
>   
>
>  Here's an example...
>
> #include <stdio.h>
>
> float foo[2];
>
> main()
> {
>     printf("foo = %p\n", foo);
>     printf("incremented as float: %p\n", foo+1);
>     printf("incremented as (char *): %p\n", ((char *)foo)+1);
> }
>
> --->
>
> foo = 0x601038
> incremented as float: 0x60103c
> incremented as (char *): 0x601039
>
> Adding an integer to a pointer "increments" it - the effect depends on the type
> of pointer.  Another way to think of it is that foo[1], say, is semantically
> identical to *(foo+1).
>
> cheers
> Miller
>
> On Mon, Aug 17, 2015 at 03:10:35PM +0000, Jonathan Wilkes via Pd-list wrote:
> > But we're dealing with an array of t_words, so onset*sizeof(t_word) is what we want anyway, no?
> > -Jonathan
> >
> >
> >      On Monday, August 17, 2015 10:55 AM, Claude Heiland-Allen <claude@mathr.co.uk> wrote:
> >   
> >
> >  On 17/08/15 15:36, Jonathan Wilkes via Pd-list wrote:
> > > Hi list,Wondering if someone will give me a free lesson in C programming.
> > > In g_traversal.c, there's some code to retrieve a float from a t_word* vec.  It looks like this:
> > > *(t_float *)(((char *)vec) + onset));
> > > Why does vec need to be cast to char*?  t_word has to be as big as the largest member of the union, and the largest member has to be the same size as char*, right?  (Otherwise we'd have big problems...)
> >
> > aiui pointer arithmetic is in increments of sizeof(pointee)
> > if onset is measured in bytes (I don't know if it is in this case, but
> > it looks likely), then you need to have a pointer to bytes for the
> > addition to be meaningful.  vec is already a pointer, but adding onset
> > to a t_word* would offset the address by onset*sizeof(t_word) bytes
> >
> >
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> >
> > 
>
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>
> 

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