Re: [PATCH 04/11] sio: fix passing negative size_t to sio_add_to_iov

[Vladimir Davydov <vdavydov.dev@gmail.com>]

On Fri, Nov 30, 2018 at 06:39:36PM +0300, Vladislav Shpilevoy wrote:
> sio_add_to_iov moves struct iov position on a
> specified offset, positive or negative. But its offset
> argument has size_t type, which is unsigned. Make it
> be ssize_t.
> 
> This worked before thanks to how negative numbers are
> stored. For example, consider
> 
> uint8_t value = 100;
> uint8_t offset = -5;
> 
> Value is stored as  0110 0100.
> Offset is stored as 1111 1011. (Yes, 1011, not 1010).
> 
> Sum of the values above is 0001 0101 1111 - first quad
> overflows and is truncated, so the result is
> 0101 1111 = 95 - correct.
> ---
>  src/sio.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/src/sio.h b/src/sio.h
> index ab0a243cd..ff383aa36 100644
> --- a/src/sio.h
> +++ b/src/sio.h
> @@ -84,7 +84,7 @@ sio_move_iov(struct iovec *iov, size_t nwr, size_t *iov_len)
>   * to adjust to a partial write.
>   */
>  static inline void
> -sio_add_to_iov(struct iovec *iov, size_t size)
> +sio_add_to_iov(struct iovec *iov, ssize_t size)
>  {
>  	iov->iov_len += size;

'iov_len' has type size_t so 'size' will be converted to size_t before
the operation, in other words this patch has, in fact, no effect.

Anyway, it's OK to apply unary minus to an unsigned variable: no matter
how integer types are stored, whether the machine uses two's-complement
or not, it should work so that (-x + x) equals 0.

That being said, I don't think we need this patch.