[ILUG] Test-and-set in C/C++

[Brian Foster]

  | Date: Fri, 01 Oct 2004 10:56:01 +0100
  | From: Vincent Cunniffe <vincent at cunniffe.net>
  | 
  | I've been unable to find any way of doing a proper test-and-set
  | [ atomic instruction ] in C/C++.  anyone know a way of doing this?

 yes.
 you cannot.
 not in _portable_ C anyways.
 there is no way, none, nothing, nil, nana, of writing
 any form of an atomic read-modify-write in de jour
 (ISO) standard conformant portable C (or C++, AFAIK).

 this always has to be done with machine-specific code,
 usually assembler, such as the hint/example someone
 posted previously --- which was specific to the x86
 (I think) _and_ GNU gcc(1).  that sort of specificity
 is very common.

 some people think/claim that the following (or similar)
 is always atomic:

    volatile int x;        /* type as appropriate */
     ...
    if (x++ == 42) {
         /* false claim:  read 42 and wrote 43 (correct),
          *    and must have done so atomically (WRONG!).
          */
      ...
    }

 that may be atomic on some machines (I don't know and
 certainly wouldn't bet on it!), but it _definitely_ is
 _not_ on other machines.   and it is Ok for it not to be,
 since <volatile> does not mandate any read-modify-write
 access (nor un-cached access, the other myth); only that
 that accesses are non-speculative, in-order, and precise.

cheers!
	-blf-
-- 
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