- (void)buyFunction {

// Keep buying tickets if there are more than one

while (sum > 0) {

// Print the current ticket number before purchase

NSLog(@" thread %@ ready to buy tickets :%d tickets ", [NSThread currentThread].name, sum);

// Simulate purchase operation, vote -1

sum--;

}

// Print the current number of tickets when they are sold out

NSLog(@" thread %@: tickets sold out :%d tickets left ", [NSThread currentThread].name, sum);

}

Now we open two threads to perform this operation at the same time, starting with 10 tickets (sum = 10), check the console output:

Thread 2 ready to buy tickets :10 tickets left

Thread 1 ready to buy tickets :10 tickets left

Thread 1 ready to buy tickets :9 tickets left

Thread 2 ready to buy tickets :8 tickets left

Thread 1 ready to buy tickets :7 tickets left

Thread 2 ready to buy tickets :6 tickets left

Thread 1 ready to buy tickets :5 tickets left

Thread 2 ready to buy tickets :4 tickets left

Thread 1 ready to buy tickets :3 tickets left

Thread 2 ready to buy tickets :2 tickets left

Thread 1 ready to buy ticket remaining :1 ticket

Thread 2: Tickets sold out Remaining :0 tickets

Thread 1: Tickets sold out Remaining :-1 ticket

As a result of our observations, we found some problems:

(1) The remaining tickets are the same in the two ticket buying processes

I bought the same ticket twice

(3) I was bought once after I had no tickets left

Solution：Process/thread synchronization is a major problem in data sharing of operating systems. It requires not only hardware and system-level implementation, but also programmers to avoid deadlocks during development. Synchronization is inseparable from deadlocks