[PD] free C lesson

Jonathan Wilkes jancsika at yahoo.com
Mon Aug 17 20:50:45 CEST 2015


Thanks, Miller.  Seems every time I have a question about C, even forming the question requires me to relearn pointer arithmetic.
-Jonathan
 


     On Monday, August 17, 2015 2:42 PM, Miller Puckette <msp at ucsd.edu> wrote:
   

 I can't remember why I thought that was the right thing to do... maybe I
was trying to keep the possibility open to pack the data more efficiently
(sticking two floats nto the size of one 'word').  But that seems like a
lot of awkwardness for not much payoff.  Maybe I had some other reason :)

M

On Mon, Aug 17, 2015 at 06:31:26PM +0000, Jonathan Wilkes via Pd-list wrote:
> Hi Spencer,The onset is in bytes, hence the cast to (char *).  I'm just wondering why we're measuring things in bytes since an onset must always lie on a t_word boundary.
> Thanks,Jonathan
>  
> 
> 
>      On Monday, August 17, 2015 2:29 PM, Jonathan Wilkes <jancsika at yahoo.com> wrote:
>    
> 
>  I guess what I'm saying is that sc_vec is a loaf of bread, and the slices are t_word wide.  So why are we doing the bookeeping (i.e., jumping to a particular location) in units smaller than one slice?
> -Jonathan
> 
> 
>      On Monday, August 17, 2015 1:54 PM, Forrest Curo <treegestalt at gmail.com> wrote:
>    
> 
>  *char is guaranteed to be the same size as a 'char', 8 bits -- but if you're treating t_word as an array of char's, you can get into t_word and process it in 8 bit chunks.
> 
> On Mon, Aug 17, 2015 at 9:53 AM, Jonathan Wilkes via Pd-list <pd-list at lists.iem.at> wrote:
> 
> 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 at 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 at 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 at 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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