[Flac-dev] const issue in FLAC__lpc_compute_residual_from_qlp_coefficients (libFLAC/lpc.c:233)

[Denis Chatelain]

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
>
>
>
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