Question

I am writing an network application.
and have some problem regarding thread race condition.

“cd” is a socket descriptor.
one of my thread retrieves socket descriptor
and send some data through the socket.

lets say map_sd returns 5.
however another thread might close the socket 5 and
reassign another. which will destroy the logic of program.

                // wait until there is valid descriptor mapping
                while( !(cd = map_sd( sd )) ){
                    sleep(1);
                }                   

                // forward PAYLOAD header
                if( send(cd, &payload, sizeof(PAYLOAD), MSG_NOSIGNAL) < 0 ){    
                    printf("send fail 813\n");
                }

what I want is to make the code above “atomic”
how can I do this when I am using pthread library in linux??

thank you in advance.

Answer 1

You need a condition variable and a mutex:

pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;

In the thread passing cd:

pthread_mutex_lock(&lock);
/* here you pass cd through a data structure or whatever */
pthread_cond_signal(&cond);
pthread_mutex_unlock(&lock);

In the thread waiting for cd:

pthread_mutex_lock(&lock);
if (pthread_cond_wait(&cond, &lock) != 0) {
    /* handle error */
}
/* here you can acquire cd */
pthread_mutex_unlock(&lock);

That should be it – you use a condition variable and a lock for exclusive access and notifying the other thread that a resource is now available. pthread_cond_wait() releases the lock to wait and will reacquire it once it has been notified by the other thread with pthread_cond_signal().

Edit: formatting.