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Re: SUSv3's "memory location" and threads Ian Lance Taylor Thu Aug 30 00:03:20 2007

"Adam Olsen" <[EMAIL PROTECTED]> writes:

> For example, if I had "struct { char foo; char bar[3]; }", where my
> first thread had a pointer to foo and was modifying it, while my
> second thread had a pointer to bar and was modifying it, would that
> meet the requirements?  My understanding is that a C compiler can (and
> in many cases, will) use larger writes so long as they appear the same
> for a single-threaded program; this obviously breaks threading though.

Yes, that can happen.

> Is there a specific variable size or alignment at which I can be sure
> (portably, etc) that there will be no larger writes, and that I can
> use locks correctly?

Well, volatile sig_atomic_t (sig_atomic_t is defined in <signal.h>) is
guaranteed to be safe in this scenario.  The C and C++ standards do
not provide any other guarantees, as they do not define thread
support.

That aside, if you use locks correctly, I believe you should be safe
in practice if you stick to variables which match the register size of
the processor, or variables which are not in structs.

Ian