Re: Threads: incompatibilities between C and C++?

Pete Becker <>
Thu, 24 May 2012 16:28:57 -0700 (PDT)
On 2012-05-24 09:11:58 +0000, Rani Sharoni said:

Thanks for the explanation. I see your point. std::create-thread is a
full memory barrier so for example there is no need for additional
barriers in order to access (from the new thread) memory that was
initialized before the create-thread call (i.e. no re-ordering is
allowed by the caller).

There are two things that are enforced by barriers: no re-ordering, and
cache coherence. For those who aren't up on this stuff, no re-ordering
means the compiler can't rearrange the code in ways that invalidate the
read/write rules. Cache coherence means that the hardware can't
rearrange reads and writes. The underlying hardware issue is that,
typically, with multiple cpus you have multiple caches; a write by one
cpu writes data to its cache; a read by another cpu may read from its
own cache or from main memory; if the contents of the first cache
haven't been written to main memory, the read won't see the result of
that write. Memory barriers (which underly the implementation of the
"synchronizes with" relationship) do the appropriate flushes. A release
operation flushes the cpu's cache, storing newly written values to main
memory; an acquire operation invalidates the cpu's cache, forcing data
to be read from main memory instead of the cache. The combination of
the two produces "synchronizes with": a write operation with release
semantics on one cpu synchronizes with a read operation with acquire
semantics on another cpu if the read sees the value that was written.
Again, that means that writes prior to the release operation are
visible to readers after the acquire operation.

Roundhouse Consulting, Ltd. ( Author of "The
Standard C++ Library Extensions: a Tutorial and Reference

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