[Xcb] [PATCH libxcb] Add comments about how _xcb_conn_ret_error() works

[Uli Schlachter]

On 18.01.2014 16:10, Ran Benita wrote:
> If xcb_connect() fails, it doesn't return NULL. Instead, it always
> returns an xcb_connection_t*, and the user should check for errors with
> the xcb_connection_has_error() function. What this function does is
> check if conn->has_error contains a non-zero error code, and returns it.
> 
> If an error did occur, xcb doesn't actually return a full
> xcb_connection_t though, it just returns (xcb_connection_t *)
> error_code. Since the 'has_error' field is the first, it is still
> possible to check conn->has_error.
> 
> That last trick was not immediately obvious to me, so add some guiding
> comments. This also ensures no one obliviously rearranges the struct.
> 
> Signed-off-by: Ran Benita <ran234 at gmail.com>

Reviewed-by: Uli Schlachter <psychon at znc.in>

> ---
>  src/xcb_conn.c | 3 +++
>  src/xcbint.h   | 1 +
>  2 files changed, 4 insertions(+)
> 
> diff --git a/src/xcb_conn.c b/src/xcb_conn.c
> index 46390e1..00c458f 100644
> --- a/src/xcb_conn.c
> +++ b/src/xcb_conn.c
> @@ -374,6 +374,9 @@ void _xcb_conn_shutdown(xcb_connection_t *c, int err)
>  /* Return connection error state.
>   * To make thread-safe, I need a seperate static
>   * variable for every possible error.
> + * has_error is the first field in xcb_connection_t, so just
> + * return a casted int here; checking has_error (and only
> + * has_error) will be safe.
>   */
>  xcb_connection_t *_xcb_conn_ret_error(int err)
>  {
> diff --git a/src/xcbint.h b/src/xcbint.h
> index b25f03b..67cf571 100644
> --- a/src/xcbint.h
> +++ b/src/xcbint.h
> @@ -192,6 +192,7 @@ void _xcb_ext_destroy(xcb_connection_t *c);
>  /* xcb_conn.c */
>  
>  struct xcb_connection_t {
> +    /* This must be the first field; see _xcb_conn_ret_error(). */
>      int has_error;
>  
>      /* constant data */
> 


-- 
"Why make things difficult, when it is possible to make them cryptic
and totally illogical, with just a little bit more effort?" -- A. P. J.