🚀 go-pugleaf

X-Original-Date: 6 Jan 1996 21:57:16 GMT
[moderator's note:  This posting was apparently lost or delayed somewhere in the
'net, and we am re-posting it.  Please bear with us if this is a duplicate.]
J. Kanze (kanze@gabi-soft.fr) wrote:
[stuff deleted]
: On point concerning volatile that I have not seen made is the fact that
: the C/C++ concept of volatile is orthogonal to atomicity of access.
: Declaring an object volatile is *NOT* sufficient with regards to
: asynchronous events; the object must still be of a type for which the
: compiler will generate atomic accesses.  
[stuff deleted]

Because of the signal-handlers-can't-read provision of the ANSI C
standard, atomicity is unnecessary in the coordination of asynchronous
activity for programs with defined behavior:

  *  signal handlers won't read any partially written non-atomic 
     shared variables, because they are not allowed to read variables 
     of static storage class in programs with defined behavior.

  *  the program itself won't read any partially written non-atomic 
     shared varible, because signal handlers don't return control 
     until they have completed their writes of volatile variables.

With pre-emptive multithreading, of course, the possibility of
pre-emption during a non-atomic write requires atomicity of some
coordination variables, but it's impossible to introduce
multithreading and stay within defined behavior per the ANSI 
standard.

Tom Payne (thp@cs.ucr.edu)

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