Re: odd array subscript is above array bounds error

[Jack Howarth]

On Thu, Mar 05, 2009 at 03:56:18AM +0000, Dave Korn wrote:
> Jack Howarth wrote:
> 
> > Ortho.c: In function 'OrthoFree':
> > Ortho.c:1973: warning: array subscript is above array bounds
> 
> > #define CMD_QUEUE_MASK 0x3
> 
> >    CQueue *cmdQueue[CMD_QUEUE_MASK+1];
> 
> >     int a;
> > 
> >     for(a=0;a<=CMD_QUEUE_MASK;a++)
> >       ;
> >     I->cmdQueue[a] = ((void *)0);
> > }
> > 
> > which produces the warning...
> > 
> > test.c: In function ‘OrthoFree’:
> > test.c:24: warning: array subscript is above array bounds
> > 
> > ...for the last assignment of I->cmdQueue[a] = ((void *)0).
> > It would seem to me that 'a' should be CMD_QUEUE_MASK+1
> > on exiting the for statement so that the assignment should
> > be valid. 
> 
>   Am I tired or are you?  This looks like a typical off-by-one in the code to
> me.  CMD_QUEUE_MASK is 0x3, meaning that the array is cmdQueue[4], consisting
> of entries numbered 0 ... 3.  The loop exits when is no longer <= 3, i.e. a ==
> 4.  That's not a valid index between 0 and 3.
> 
>   OTOH it is 4am over here and I could be blind.  I'll just go over here now
> and zzzzzzzzzzzzzzzzzzzzzzzZZZZZZSNRK
> 
>     cheers,
>       DaveK

Dave,
    My original guess was that as well, however when I tried a simplier test
case of...

#define CMD_QUEUE_MASK 0x3
void test()
{
    int cmdQueue[CMD_QUEUE_MASK+1];

    int a;

    for(a=0;a<=CMD_QUEUE_MASK;a++)
      ;
    cmdQueue[a] = 0;
}

compiled with -O3 -Wall, the "array subscript is above array bounds"
warning wasn't triggered for that case. Odd.
                   Jack