Information on atomics in the gnu extensions: is a dereference atomic?

Andrew Haley
Tue Apr 29 13:42:00 GMT 2014

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On 04/29/2014 02:11 PM, Leo Ferres wrote:

> Suppose the following C code.
> int t = __sync_fetch_and_add(&s->r, 1);
> s is a struct with member r, which is an volatile int.
> Is this an atomic operation, even if you include the indirection? In
> other words, can a thread come between the dereferencing operation and
> the atomic increment of another thread?

Yes.  Or rather, which dereference?  There are two.

> If so, is there a solution besides whileing with a compare and swap?
> What about stdatomic.h in C11, is there a way to do what I want there?

You need an atomic operation which covers a CAS and a fetch.  Unless you
want to use transactional memory you're going to have to use a lock.


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