Subj : Re: Can C++ local static objects be made thread safe?
To   : comp.programming.threads
From : Marcin 'Qrczak' Kowalczyk
Date : Fri Feb 04 2005 02:37 am

Giancarlo Niccolai <searchforme@ingoogle.com> writes:

>> Because it would change various instances of code written with no
>> threads in mind from unsafe to safe to use with threads.
>
> That is utterly impossible.

It is possible. Consider this function:

char32_t lookup_unicode_char_by_name(const string &name) {
   static map<string, char32_t> database =
      read_unicode_database_from_file();
   map<string, char32_t>::iterator it = database.find(name);
   return it == database.end() ? it->second : (char32_t)-1;
}

If C++ uses non-thread-aware checking for first-time use, it may break
in multithreaded environment when several threads try to lookup a name
at the same time, even though it may often work by chance. If C++ uses
an equivalent of pthread_once, it just works.

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

.