From: Hongli Lai <hongli@...99.net>
Date: 2008-03-05T20:08:06+09:00
Subject: Re: embedding Ruby 1.9.0 inside pthread

Suraj N. Kurapati wrote:
> Thanks for the tip.  I revised the example accordingly:

This is better, but now your program has a racing condition. Suppose the 
thread finished before your pthread_cond_wait() is called. Now you're 
waiting for a condition that will never be signalled.

The correct way to wait for a condition is to use the following pattern:

pthread_mutex_t lock;
pthread_cond_t cond;

void function_1() {
    pthread_mutex_lock(&lock);
    while (!"some expression which must evaluate to true") {
        pthread_cond_wait(&lock, &cond);
    }
    pthread_mutex_unlock(&lock);
}

void function_2() {
    pthread_mutex_lock(&lock);
    (...do some work, which makes "some expression which must evaluate 
to true" true...)
    pthread_cond_signal(&cond);   // use broadcast() if there can be 
multiple threads waiting on this condition
    pthread_mutex_unlock(&lock);
}