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# Thread
## **TCP Echo Server**
### **Multiple Processes (Ver. 2)**
* **child process**
* `要close(listenfd)`
* `doit 做完 close(connfd), main 要再 close(connfd)`
* <img src="https://i.imgur.com/Fa7w0GK.png" width = "400"/>

### **Multiple Thread (Ver. 3)**
* `call pthread_create instead of fork`
* **child thread**
* `不用close(listenfd), since creating new thread doesn't affect the ref_cnt for open descriptor`
* `doit 做完 close(connfd), main不用再close(connfd)`
* <img src="https://i.imgur.com/ZdMaqNu.png" width = "400"/><img src="https://i.imgur.com/ShtoAGi.png" width = "400"/>

## **sharing data among threads**
### **static vs. stack-dynamic**
* `process是copy, thread 共享 static`
* <img src="https://i.imgur.com/28uM2b9.png" width = "500"/>
### **data inconsistence**
* `原因：hardware instr is not atomic : `
`mov eax, a `
`add 1`
`mov a, eax`
* <img src="https://i.imgur.com/pv3RTIm.png" width = "500"/>
### **calling non-rentrant function**
* `避免re-entrant`
* `avoid static data`
* `synchronization primitives`
* `程式撰寫`
* avoid static data
* 每個 funct 有自己的 structure：
caller packs all the args and stores static var into a structure
* `provide thread specific data`

### **provide thread-specific data**

* **Key Structure**
* `For each process, kernal maintain a KEY structure (an array)`
* <img src="https://i.imgur.com/qIrgleE.png" width = "400" /></img>
* **Concept**
* `利用 mapping 取得 data`
* <img src="https://i.imgur.com/HsmbMCg.png" width = "400"/></img>
* **Steps**
* call ==pthread_key_create / pthread_once== 向 kernal 要一個未使用的 key idx 作為 static data 的 name
* thread call ==malloc== 取得 heap-dynamic storage，用以儲存static data
* thread call ==pthread_setspecific(key, ptr)== 將此 storage 位址對應到取得的Key index
* 在 re-ent funct call ==pthread_getspecific(key)== 取得 key idx 對應的 thread-specific data
<br>
* <img src="https://i.imgur.com/f6pYizC.png" width = "400"/></img>
* **Storage**
* `Different Threads Have Different Storages for the Same Name / Idx`
`One Name to Different Location`
* <img src="https://i.imgur.com/W37hS6b.png" width = "400"/></img>

* **KEY idx 存取**
* 為確保 thread-safe 之變數 ：利用 pthread_key_create 得到一個唯一的key index
* 不同 key idx 供同一 function 中不同 static data 使用
* 對同一 static data，由第一個執行此 function 的 thread 去請求key idx 即可。因為每次呼叫會傳回新的未使用的 idx。
* pthread_once 來達成此功能 (對同一 static data 只請求一次 key idx)

###### tags: `OS`