> It can tell the difference between the other thread getting
> the mutex first and it getting the mutex first. But it cannot tell the
> difference between an implementation that puts random sleeps before calls
> to 'pthread_mutex_lock' and an implementation that has the allegedly
> non-compliant behavior. That makes the behavior compliant under the
> 'as-if' rule.
>
> If you don't believe me, try to write a program that prints
> 'non-compliant' on a system that has the alleged non-compliance but is
> guaranteed not to do so on any compliant system. It cannot be done.
>
> In order to claim the alleged compliance, you would have to
> know that a thread waiting for a mutex did not get it. But there is no
> possible way you can know that another thread is waiting for the mutex
> (as opposed to being about to wait for it). So you can never detect the
> claimed non-compliance, so it's not non-compliance.
An excellent point, but the existence of pthread_mutex_trylock()
invalidates it.
To be very specific, the following will do the job:
volatile unsigned shared_variable;
pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
void
thread_function()
{
unsigned prev_value = shared_variable;
for (;;) {
unsigned cur_value, delta;
if (pthread_mutex_trylock(&lock) == 0) {
cur_value = ++shared_variable;
pthread_mutex_unlock(&lock);
delta = cur_value - prev_value;
} else {
/* Another thread is holding the lock. */
pthread_mutex_lock(&lock);
cur_value = ++shared_variable;
pthread_mutex_unlock(&lock);
delta = cur_value - prev_value;
if (delta == 1)
fatal("non-compliant");
}
/* Assuming we don't wrap */
if (delta == 0)
fatal("buggy as a roach motel");
}
}
You need to run more than one instance of the thread_function()
to have a chance of triggering the non-compliant message, of course.
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