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Re: proposal: some pointer and arithmetic utilities

On Thu, 21 Mar 2013, Taylor R Campbell wrote:
/* typeof-free roundup2/rounddown2 */
#define roundup2(x, m)  (((x) + ((m) - 1)) & ~((m) - 1 + ((x) - (x))))
#define rounddown2(x,m) ((x) & ~((m) - 1 + ((x) - (x))))

/* multiple-evaluation-free roundup2/rounddown2 */
#define roundup2(x, m)  ((((x) - 1) | ((m) - 1)) + 1)
#define rounddown2(x,m) ((x) &~ ((typeof(x))((m) - 1)))

#define offsetin(s, f)  (((const char *)&(s).f) - ((const char *)&(s)))

#define container_of(p, t, f)                                           \
        ((void)sizeof((p) -                                             \
                &((t *)(((char *)(p)) - offsetof(t, f)))->f),            \
            ((t *)(((char *)(p)) - offsetof(t, f))))

Fixing roundup2 seems like a good idea, and I have no objection to adding rounddown2.

I don't really see the point of offsetin -- if you have a struct that ends with an array of 1 element, but you want to allocate enough space for the struct plus an array of >1 elements, then why not just do the arithmetic without a macro, and if you do need a macro, then why not give it a name that sounds more like "sizeof"?

I also don't see the point of container_of -- why not redesign the code so that the the inner struct has a pointer to the outer struct?

Please don't evaluate macro arguments more than once, except in macros whose names are ALL_CAPS.

Since it's not standard C, I think __typeof__ would be preferable to typeof. My tests show that it's available in gcc, clang, and pcc.

--apb (Alan Barrett)

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