🚀 go-pugleaf

X-Original-Date: 11 Jan 1996 12:45:44 +0100

Tom Payne (thp@cs.ucr.edu) wrote:

|> 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.

Close.  The program itself won't read any partially written non-atomic
shared variables, because signal handlers are not allowed to write such
variables in programs with defined behavior.

Note that your statement, if allowed, could lead to anomalies if a
non-atomic read is interrupted, and the signal handler writes the
object.  The first part of the read sees the old value, and the second
part the new value.
-- 
James Kanze           (+33) 88 14 49 00          email: kanze@gabi-soft.fr
GABI Software, Sarl., 8 rue des Francs Bourgeois, 67000 Strasbourg, France
Conseils, itudes et rialisations en logiciel orienti objet --
              -- A la recherche d'une activiti dans une region francophone

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