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# Semaphore
## **How to use**
### **Binary Semaphore**
* **用法**
* `sem_t s;`
`sem_init(&s, 0, 1);`
`sem_wait(&s);`
`// criticle section`
`sem_post(&s);`
* **trace**
* T1
* 初始 s=1，wait 使s=0，繼續執行critical
* T2
* wait 使s=-1 ( -x , x 為等候者數)，進入 sleep
* T1
* critical 做完 post 使 s=0，wake T2
* T2
* wait return 後執行 critical，完成後 call post 使 s=1
* **Producer/Consumer**
* <img src="https://i.imgur.com/5OUI7rV.png" width = "600"/></img>
* **Reader/Writer**
* `Allow 1 writer and multiple reader`
* <img src="https://i.imgur.com/HFpYobv.png" width = "250"/></img>

* <img src="https://i.imgur.com/d60RJAb.png" width = "400"/></img>

* **Dining-Philosophers**
* `Represent chopsticks by semaphore[idx]`
* `所有人拿左等右 --> hold and wait --> deadlock`
`wait(chopstick[i]);`
`wait(chopstick[i＋1]);`
* `解法`
* 最多 n-1 人吃
* pick only if both sides are available
* asymmetric solution : 奇選左 偶選右
* **Thread Throttling**
* `限制 concurrent thread 數`
* `初始 s = the max_num of threads`
* `5->4->3->2->1->0，第六個進來時卡住`
* sem_wait(&s)
...
sem_post(&s)
## **How to implement**
* <img src="https://i.imgur.com/ISilFUU.png" width = "550"/></img>

###### tags: `OS`