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SubjectRe: do_div considered harmful

On Mon, 4 Aug 2003 Andries.Brouwer@cwi.nl wrote:
>
> Now that I compare, he wrote
> nativeMb = do_div(nativeMb, 1000000);
> to divide nativeMb by 1000000.
>
> So, it seems natural to expect that do_div() gives the quotient.
> But it gives the remainder.
> (Strange, Erik showed correct output.)

Actually, the above is "undefined behaviour", since it has _two_
assignments in the same thing. Exactly because "do_div()" modifies both
the first argument _and_ returns a value. So depending on the
implementation of do_div() (whether there are any sequence points etc) and
on random compiler behaviour (if there are no sequence points in do_div()
internally), in the example above "nativeMb" migth be totally undefined
after the above.

And yes, as a special case, it might be the divisor.

I agree that "do_div()" has strange semantics and is very likely misnamed,
but they are kind of forced upon us by the fact that C functions cannot
return two values. Renaming do_div() at this point is just going to make
it harder to write kernel- portable source, so I suspect we're better off
just commenting it, and making people aware of how do_div() works.

Not very many people should use "do_div()" directly (and a quick grep
shows that not very many people do). It's generally a mistake to do so, I
suspect. The thing was originally written explicitly for "printk()" and
nothing else.

Linus

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