[boost] Re: "Thread-safe", etc. definitions [was: Re: Threads Review]

From: Alexander Terekhov (terekhov_at_[hidden])
Date: 2002-10-21 04:57:47

Joe Gottman wrote:
> 
>  David Abrahams wrote:
>  [...]
>  > * "Thread-safe": this is a term which is well-defined for programs. It
>  >   is /not/ well-defined for classes or functions, though the
>  >   documentation uses it that way repeatedly. ....
> 
> The SGI STL implementation gives a good definition of the minimal acceptable
> level of thread safety in a class that may be used in multi-threaded
> programs (see http://www.sgi.com/tech/stl/thread_safety.html ).  To

Well, the POSIX version is here [missing pthread_once() and *erroneously*
mentioned pthread_cond_signal()/ _broadcast() aside]:

http://www.opengroup.org/onlinepubs/007904975/basedefs/xbd_chap04.html#tag_04_10
(General Concepts, Memory Synchronization)

> paraphrase, it says that  simultaneous accesses to distinct objects are
> safe, and simultaneous read accesses to shared objects are safe.  If a class
> obeys these two rules, then it should be possible to make a shared object of
> that class thread safe by putting a mutex, critical section, etc. arround
> any write accesses to that object.

Unfortunately, this is NOT true because on one hand, POSIX doesn't address 
the problem of word-tearing with respect to "adjacent" objects in neither 
C nor C++, and, on the other hand, neither C nor C++ acknowledge the 
existence of threads/shared memory access with respect to threads/processes. 
You might want to take a look at this thread:

http://groups.google.com/groups?threadm=3D36E35C.D8C511AE%40web.de
(comp.std.c, Subject: "memory location")

regards,
alexander.