Date: Mon, 7 May 2007 23:25:18 +0300 From: Giorgos Keramidas <keramida@freebsd.org> To: Hans Petter Selasky <hselasky@c2i.net> Cc: freebsd-arch@freebsd.org Subject: Re: Missing LIST_PREV() ? Message-ID: <20070507202517.GA88340@kobe.laptop> In-Reply-To: <20070507202034.GA80846@kobe.laptop> References: <200705051617.34162.hselasky@c2i.net> <20070507202034.GA80846@kobe.laptop>
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On 2007-05-07 23:20, Giorgos Keramidas <keramida@freebsd.org> wrote: >On 2007-05-05 16:17, Hans Petter Selasky <hselasky@c2i.net> wrote: >> Hi, >> >> Why should LISTs only be forward traversable? The following piece of >> code make lists backward traversable: >> >> /sys/sys/queue.h: >> >> +#define LIST_PREV(head,elm,field) \ >> + (((elm) == LIST_FIRST(head)) ? ((__typeof(elm))0) : \ >> + ((__typeof(elm))(((uint8_t *)((elm)->field.le_prev)) - \ >> + ((uint8_t *)&LIST_NEXT((__typeof(elm))0,field))))) >> >> Any comments? > > 1. The use of (uint8_t *) casts is relatively ugly. > > 2. What does LIST_PREV give us that cannot be done with TAILQ_PREV() > already? Even more importantly, which I missed in my original look (3) The use of the gcc-specific __typeof() extension makes this unusable with other compilers. The entire <sys/queue.h> header is otherwise very portable and I already use it successfully on other systems too (i.e. Solaris with the Sun Studio 11 compilers). Introducing unportable constructs like __typeof() shouldn't be allowed, IMHO. - Giorgos
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