Subj : Re: Can C++ local static objects be made thread safe?
To   : comp.programming.threads
From : Marcin 'Qrczak' Kowalczyk
Date : Thu Feb 17 2005 12:57 pm

gniccolai@yahoo.com (Giancarlo Niccolai) writes:

> That's not following the pthread_once semantic: all the threads get
> there even if once stuff is still NOT done.

This is not true at least on Linux/NPTL: other threads calling
pthread_once do wait for the first thread to finish.

The documentation is not clear enough to judge.

I believe this is never true, because otherwise pthread_once would be
useless, and the examples of the intended use of pthread_once in SUSv3
would be broken.

Is there an implementation where pthread_once works like you are
describing?

Here is an ugly test case (ugly because it relies on timing instead
of synchronization, but this is enough to detect the behavior of
pthread_once on the system):

#include <pthread.h>
#include <unistd.h>
#include <stdio.h>

static pthread_once_t once = PTHREAD_ONCE_INIT;

static void init(void) {
   printf("Started initialization\n");
   sleep(2);
   printf("Finished initialization\n");
}

static void *body(void *arg) {
   printf("Thread %d started\n", *(int *)arg);
   pthread_once(&once, init);
   printf("Thread %d continues\n", *(int *)arg);
}

int main(void) {
   pthread_t t1, t2;
   int arg1 = 1, arg2 = 2;
   pthread_create(&t1, NULL, body, &arg1);
   sleep(1);
   pthread_create(&t2, NULL, body, &arg2);
   sleep(5);
   return 0;
}

Result:
Thread 1 started
Started initialization
Thread 2 started
Finished initialization
Thread 1 continues
Thread 2 continues

Note that "Thread 2 continues" is not done directly after "Thread 2
started", but thread 2 waits for thread 1 to finish initialization.

> However, this semantic (do once AND lock the section, or lock the
> section and DO only once), may break a correctly built static
> initialization (i.e. a constructor called to initialize the static
> object) for the reason I already told.

They will not break it, as I've shown.

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

.