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[Chicken-users] set! atomic?

From: Bryan Vicknair
Subject: [Chicken-users] set! atomic?
Date: Tue, 4 Jun 2013 19:15:52 -0700
User-agent: Mutt/1.5.21 (2010-09-15)

SRFI-18 states:

  "Read and write operations on the store (such as reading and writing a
  variable, an element of a vector or a string) are not required to be atomic.
  It is an error for a thread to write a location in the store while some other
  thread reads or writes that same location."

Is it possible to eval a variable that is in an inconsistent state?  I
understand that if there are a series of set-car! on a list, then a reader may
see the list as it is in between any of the set-car! calls.  But is it possible
that an update to a very large object will ever be interrupted by one thread
such that other threads will see a broken version?

  (use srfi-1)
  (define foo '(1 2 3))
  (thread-start! (make-thread (lambda () (set! foo (iota 1e8)))))
  (print foo)

In the above code, will the primordial thread ever print a list that
isn't exactly (1 2 3) or (iota 1e8)?

The context of my question is that for a small web app I have a hash table that
all threads read freely, but they coordinate writes with a mutex.  Do I need to
also have a read lock?

Bryan Vicknair

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