Subj : Re: Can C++ local static objects be made thread safe?
To   : comp.programming.threads
From : Marcin 'Qrczak' Kowalczyk
Date : Fri Feb 04 2005 09:00 am

Giancarlo Niccolai <searchforme@ingoogle.com> writes:

> To put that thing in a compiler, you have to mathematically demonstrate
> that:
>
> 1) ALL ever possible ST code can be made threadsafe by just adding a mutex
> at its initialization code.
> 2) ALL ever possible MT code won't be broken by your "help".

No, 1 is not necessary for this change to be useful. And I believe 2 is true.

> Suppose that read_unicode_database_from_file() requires ANOTHER static
> initializer, or is replaced in the MT code with something different. In
> example, suppose that read_unicode_database_from_file() requires a locked
> variable to be prepared by the thread that calls
> lookup_unicode_char_by_name(). Then, chances are that
> read_unicode_database_from_file() will deadlock.

If there was a cyclic dependency between local static objects, then the
library would have been already broken in a single-threaded environment.

I claim that it doesn't break code which is correct in ST.

> You can EASILY avoid this situation by calling lookup_unicode_char_by_name()
> before any thread is started; you won't need any synchronization
> thereafter. 

This defeats the advantage of lazy initialization.

-- 
   __("<         Marcin Kowalczyk
   \__/       qrczak@knm.org.pl
    ^^     http://qrnik.knm.org.pl/~qrczak/

.