flac dev - Sep 2004 - const issue in FLAC__lpc_compute_residual_from_qlp_coefficients (libFLAC/lpc.c:233)

On Tue, Jan 13, 2004 at 02:04:48PM -0800, Josh Coalson wrote:
> --- Denis Chatelain <listes@octopodus.com> wrote:
> > Hello,
> > 
> > 
> > I just tried to compile libFLAC (using Borland C++ Builder 6 on
> > Windows).
> > 
> > The compilers yells at me on line 233 of libFLAC/lpc.c
> > 
> > *(residual++) = *(data++) - (sum >> lp_quantization);
> > --> data is const and cannot be modified
> > 
> > Funny thing is, if data is declared:
> >    const FLAC__int32 *data
> > instead of
> >    const FLAC__int32 data[]
> > 
> > everything is ok.
> > 
> > Is this a bug in my compiler, or a personnal lack of understanding of
> > the difference in C between *data and data[] ?
> 
> hmmm, not sure what the standard says on this.  but it's
> OK to change it to
> 
> const FLAC__int32 *data
> 
> in order to get it to compile.
I haven't checked, and I don't recall whether it covers the case of
const
arrays, but this article has helped me in the past:

http://www.parashift.com/c++-faq-lite/const-correctness.html

-- 
 - mdz

Hello,

I think you solved the mystery.

You're right, the basic problem is not a const problem, but a * [] problem

As a test.c:
int main (void)
{
int data[3];
*(data++);
return 1;
}

gcc test.c  gives:
 -- wrong type argument to increment

But:
int main (void)
{
int *data;
*(data++);
return 1;
}

Compiles just fine with gcc.


I didn't take a look at the latest CVS snapshot, but this might be a 
good thing to fix it so that the gcc build doesn't break in the future 
and others spend time asking themselves what's so funny about this */[] 
things :)

I'll post a patch later if anyone is interested, this kind of problem 
arises in a few lines.


Denis


Christian Hudon wrote:
>
>>>> The compilers yells at me on line 233 of libFLAC/lpc.c
>>>>
>>>> *(residual++) = *(data++) - (sum >> lp_quantization);
>>>> --> data is const and cannot be modified
>>>>
>>>> Funny thing is, if data is declared:
>>>>   const FLAC__int32 *data
>>>> instead of
>>>>   const FLAC__int32 data[]
>>>>
>>>> everything is ok.
>>>>
>>>> Is this a bug in my compiler, or a personnal lack of
understanding of
>>>> the difference in C between *data and data[] ?
>>>>     
>>>
>
> I'm not a great language lawyer, but I don't think you can modify
the
> pointer when something is declared int foo[]. You can modify the data 
> pointed to, with foo[0] = 42, etc. but you can't do stuff like foo++ 
> or foo = (int*)something. The same error would show up with 
> FLAC__int32 * const data. If you want to change the pointer, you need 
> to declare it as a pointer with int *foo (or const int *foo, which 
> means you can modify the pointer, but not the data it points to).
>
> My 2 cents. I'll go back to lurking now. :-)
>
>  Christian
>
>
>
> -------------------------------------------------------
> This SF.net email is sponsored by: Perforce Software.
> Perforce is the Fast Software Configuration Management System offering
> advanced branching capabilities and atomic changes on 50+ platforms.
> Free Eval! http://www.perforce.com/perforce/loadprog.html
> _______________________________________________
> Flac-dev mailing list
> Flac-dev@lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/flac-dev

>>>The compilers yells at me on line 233 of libFLAC/lpc.c
>>>
>>>*(residual++) = *(data++) - (sum >> lp_quantization);
>>>--> data is const and cannot be modified
>>>
>>>Funny thing is, if data is declared:
>>>   const FLAC__int32 *data
>>>instead of
>>>   const FLAC__int32 data[]
>>>
>>>everything is ok.
>>>
>>>Is this a bug in my compiler, or a personnal lack of understanding
of
>>>the difference in C between *data and data[] ?
>>>      
>>>
I'm not a great language lawyer, but I don't think you can modify the 
pointer when something is declared int foo[]. You can modify the data 
pointed to, with foo[0] = 42, etc. but you can't do stuff like foo++ or 
foo = (int*)something. The same error would show up with FLAC__int32 * 
const data. If you want to change the pointer, you need to declare it as 
a pointer with int *foo (or const int *foo, which means you can modify 
the pointer, but not the data it points to).

My 2 cents. I'll go back to lurking now. :-)

  Christian