Slides: Semaphores and Condition Variables

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# Semaphores and Condition Variables
## CSC 213 – October 14, 2026

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# Agenda for Today
1. Department Events
1. Upcoming Work
1. Condition Variables
1. Semaphores
1. Wrap Up

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# Department Events

## CS Extra: Graduate School in CS
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   Thursday, October 15th at 4pm in Noyce 3821  
   Refreshments provided
]

*To receive a token, send an email to let me know you attended no more than 48 hours after the event.*

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# Upcoming Work

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# Lab: Worm
**Due:** Wednesday, October 14th by 11:59pm  
**Late Deadline:** Monday, October 26th by 11:59pm

## Questions
_No new questions since last class._

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# Condition Variables

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# Discussion: Condition Variable Basics
**Questions for think, pair, share:**

1. What does a condition variable do?

2. Why would we choose to use a condition variable with a mutex instead of just a mutex?

*Start by thinking of answers on your own. I will let you know when to start discussion.*

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# Discussion: Condition Variable Interface
**Questions for think, pair, share:**

For each of the condition variable functions below, explain what the function does and when you would use it.

1. `pthread_cond_wait(pthread_cond_t* cv, pthread_mutex_t* m)`

2. `pthread_cond_signal(pthread_cond_t* cv)`

3. `pthread_cond_broadcast(pthread_cond_t* cv)`

*Start by thinking of answers on your own. I will let you know when to start discussion.*

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# Exercise: A Simple Parallel Program

.left-col[
  ## Main Function
  - Create two threads, tick_thread and tock_thread

- While there is input from the user:
    - If the user typed "go", both threads are allowed to move forward
    - Otherwise, wait for more input.

- Join with tick_thread and tock_thread
]

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  ## Thread Function
  - Wait for the main thread to allow this thread to run.

- Repeat five times:
    - Print this thread's message (either "tick" or "tock")
    - Pause for one second
]

<br>
## How should we make the threads wait, and how do we resume them?
Discuss strategies with your group. We'll complete the implementation together.

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# Implementing Tick/Tock with Condition Variables

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# Semaphores

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# Discussion: Semaphore Basics
**Questions for think, pair, share:**

1. What does a semaphore do?

2. How is a semaphore different from a mutex or condition variable?

*Start by thinking of answers on your own. I will let you know when to start discussion.*

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# Discussion: Semaphore Interface
**Questions for think, pair, share:**

For each of the semaphore functions below, explain what the function does and when you would use it.

1. `sem_init(sem_t* s, int cross_process, int state)`

2. `sem_wait(sem_t* s)`

3. `sem_post(sem_t* s)`

*Start by thinking of answers on your own. I will let you know when to start discussion.*

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# Exercise: A Simple Parallel Program

.left-col[
  ## Main Function
  - Create two threads, tick_thread and tock_thread

- While there is input from the user:
    - If the user typed "go", both threads are allowed to move forward
    - Otherwise, wait for more input.

- Join with tick_thread and tock_thread
]

.right-col[
  ## Thread Function
  - Wait for the main thread to allow this thread to run.

- Repeat five times:
    - Print this thread's message (either "tick" or "tock")
    - Pause for one second
]

<br>
## How can we implement this same program with semaphores?
Discuss strategies with your group. We'll complete the implementation together.

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# Implementing Tick/Tock with Semaphores

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# Wrap Up

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# Reminders

## Upcoming Deadlines
- **Worm Lab:** due tonight, or Monday after break with a token

## Need help? Use your resources!
- Book an appointment during office hours
- Attend mentor office hours
- Review slides, readings, and other course resources
- Use tools like `gdb` and AddressSanitizer to catch and diagnose issues

## Reading

*No reading for the next class.*