Volatile and CreateThread

Question

I just asked a question involving volatile: volatile array c++

However my question spawned a discussion on what volatile does.

Some claim that w

Answer 1

Compiling on linux, g++ 4.1.2, I put in the equivalent of your example:

#include 

bool done = false;

void* thread_func(void*r) {
  while(!done) {};
  return NULL;
}

void* write_thread_func(void*r) {
  done = true;
  return NULL;
}


int main() {
  pthread_t t1,t2;
  pthread_create(&t1, NULL, thread_func, NULL);
  pthread_create(&t2, NULL, write_thread_func, NULL);
  pthread_join(t1, NULL);
  pthread_join(t2, NULL);
}

When compiled with -O3, the compiler cached the value, so it checked once and then enters an infinite loop if it wasn't done the first time.

However, then I changed the program to the following:

#include 

bool done = false;
pthread_mutex_t mu = PTHREAD_MUTEX_INITIALIZER;

void* thread_func(void*r) {
  pthread_mutex_lock(&mu);
  while(!done) {
    pthread_mutex_unlock(&mu);
    pthread_mutex_lock(&mu);
  };
  pthread_mutex_unlock(&mu);
  return NULL;
}

void* write_thread_func(void*r) {

  pthread_mutex_lock(&mu);
  done = true;
  pthread_mutex_unlock(&mu);
  return NULL;
}


int main() {
  pthread_t t1,t2;
  pthread_create(&t1, NULL, thread_func, NULL);
  pthread_create(&t2, NULL, write_thread_func, NULL);
  pthread_join(t1, NULL);
  pthread_join(t2, NULL);
}

While this is still a spin (it just repeatedly locks/unlocks the mutex), the compiler changed the call to always check the value of done after the return from pthread_mutex_unlock, causing it to work properly.

Further tests show that calling any external function appears to cause it to re-examine the variable.