Re: [avr-gcc-list] Got strange compilation problem

[Lars Noschinski]

* David Bourgeois <address@hidden> [2006-02-16 11:17]:
> Replacing the 2 lines
>     length = (p->inIdx) - (p->outIdx);
>     length %= FIFO_SIZE;
> by
>     length = (((p->inIdx) - (p->outIdx)) % FIFO_SIZE);
>
> completely broke my code. I'm using avr-gcc 3.4.5 (latest winavr release)  
> under avrstudio, -Os.
[...]
> Here, the problem occurs when p->inIdx) - (p->outIdx) is <0, then the  modulus 
> is not done, the return value is 0xF9 for example.

"%" is a signed modulus, so -1 % 8 == -1. Try

   length = ((uint8_t)((p->inIdx) - (p->outIdx)) % FIFO_SIZE);

to force the difference to be a positive. Which types do inIdx and
outIdx have?