Interaction

• We have seen how threads are created, scheduled, and executed
• The tools we have so far allow threads to run independently
• Sometimes threads have dependencies

Independent Execution

• Create threads
• Run independently
• Combine thread results

What if threads need to share data?

Memory

• A pointer to a memory location can be shared when creating a thread
• The thread can allocate memory and use stack allocated memory
• Global variables and data are accessible to all threads

What behaviors do we need to avoid if we have shared memory?

Thread Race Conditions

• Two threads may be operating concurrently in an unsyncronized manner
• This can lead to output that differs over different program runs
• This may lead to bugs or unexpected behavior

Example

if(seatsRemaining > 0){
  dispenseTicket();
  seatsRemaining = seatsRemaining - 1;
} else {
  displaySorrySoldOut();
}

Can the example sell more tickets than are available?

What if it is running in multiple threads?

examples/race/tickets.c

Core Issue

• Multiple threads operating on the same data structure with interleaving modifications

Solution

• Allow only one thread to access the data at a time
• Sometimes referred to as locking the data structure
• This creates mutual exclusion (mutex)

Thread Safe Operations

• Only reading from shared data structure
• Interleaving operations without data dependencies
• Interleaving atomic operations