Re: feature request -- atomic add/dec return the old value

On Mon, 11 Mar 2002, Ian Holsman wrote:

> hi.
> I'm not sure if this is the right spot to ask,
> but I'd like to see freebsd return a value on it's
> add/subtract atomic functions.
>
> is this hard to implement?
>
> is there a reason not to ?
>

  Because many platforms (including the x86) implement addition and
subtraction instructions which modify the value in-place (whether in memory
or in a register).  To get the behaviour you desire, you can build your own
routine using the existing atomic operations like so (not actually tested):

	static inline int
	atomic_readandadd_int(int *P, int V)
	{
		register int orig;
		register int new;

		do {
			orig = *P;
			new = orig + V;
		} while (!atomic_cmpset_int(P, orig, new);

		return (orig);
	}


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