Guaranteed yielding with pthread_cond_wait and pthread_cond_signal

Assuming I have a C program with 3 POSIX threads, sharing a global variable, mutex, and condition variable, two of which are executing the following psuedocode:

...process data...
pthread_mutex_lock( &mutex );
variable = data_ptr;
pthread_cond_signal( &cond );
pthread_mutex_unlock( &mutex );

And the third running:

while(1) {
    while( variable == NULL ) {
        pthread_mutex_wait( &cond, &mutex );
    }
    printf( "Data is %d", *variable );
}

Is it safe to assume that the third thread will see the data from each of the first two?

Put a different way, if a thread is wating on a mutex and a condition variable, is it safe to assume that it will be the next one to get the lock if it is signaled, rather than some other thread that may be waiting on the lock?