Slides: Semaphores and Condition Variables

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# Semaphores and Condition Variables
## CSC 213 – October 27, 2025

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# Agenda for Today
1. Department Events
2. Condition Variables
3. Semaphores
4. Wrap Up

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# Department Events

## CS Table: Topic TBD
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  Tuesday, October 28th at noon in JRC 224C
]

## CS Poster Session
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  Thursday, October 30th at 4:30pm in HSSC A1231  
  Fruit, desserts, and beverages provided.
]

*To receive 0.25% extra credit, send an email to let me know you attended no more than 48 hours after the event.*

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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
]

.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 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.*

---
# 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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class: section, gray
# Implementing Tick/Tock with Semaphores

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# Wrap Up

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# Reminders

## Lab
You can submit the worm lab for up to 80% credit until this Wednesday.

## Assignment
There is no assignment this week.

## Reading

- **Review lab before class**