# OS-Application Interface

## System calls


* 定義：作為 user process 與 kernel 之間的溝通界面，**當執行中的 process 需要 OS 提供某種服務時，則 user process 會對 OS 發出 system call 執行的請求（伴隨 ==Trap== 先行）**，當 OS 收到請求（含 system call ID 及相關參數）後，則會執行一連串的 system call 完成服務請求，並將服務結果回傳給 user process

* system call 種類
* Process Control
> ex: ```fork()```, ```exit()```, ```wait()```
* File management
> ex: ```open()```, ```read()```, ```write()```, ```close()```
* Device management
> ```ioctl()```, ```read()```, ```write()```
* Information maintenance
> ```getpid()```, ```alarm()```, ```sleep()```
* Communication
Process 間的 communication 且只針對 **message passing** 方式提供相關 system calls
> ```pipe()```, ```shmget()```, ```chown()```
* 補充：UNIX system calls 多一種類別 → protection
> ```chmod()```, ```umask()```, ```chown()```

* system call 參數傳遞方式
1. Register
2. Memory
3. Stack

* ==\[法一\] 使 register 保存參數==，OS 再從 register 取得
* 優點：存取速度快，simple
* 缺點：不適用於大量參數的情況
* ==\[法二\] 使用 memory，以**一個 table（or Block）** 來儲存這些參數，同時用**一個 register** 紀錄此 table 的起始位址==，then pass it to OS
* 優點：適用於大量參數的情況
* 缺點：存取速度比 register 慢
* ==\[法三\] 使用 stack 保存參數，即 push 參數到 stack 裡面，OS 再從 stack 取得這些參數
* 優點：操作較為 simple
* 缺點：stack space 預留要大一些，避免 overflow


## API: Application Program Interface

* Three most common APIs:
* ==Win32 API== for Windows
* ==POSIX API== for POSIX-based systems（including virtually all versions of UNIX, Linux, and Mac OS X）
* POSIX 全名：**P**ortable **O**perating **S**ystem **I**nterface for Uni**x**
* ==Java API== for the Java virtual machine（JVM）

## System Calls & API

* System Calls
* The **OS interface** to a running program
* An explicit request to the **kernel** made via a **software interrupt**
* Generally available as **assembly-language** instructions
* API: Application Program Interface
* **Users mostly program against API instead of system call**
> 純粹是為了 programming 方便，還有方便 application 而設計的，跟作業系統無關
* Commonly implemented by language libraries (ex: C library)
* An API call could involve **zero or multiple system call**
* Both ```malloc()``` and ```free()``` use system call ```brk()```
* Math API functions, suck as ```abs()```, don't need to involve system call
> 一個 API 可能會對應到好幾個 system call（一對多），也有可能一對 0（單純做數學運算），不用 OS


* 圖片說明：
* API 下才是 system call
* API call 到 system call 才會間接的進到 OS 裡面叫 OS 完成

* Interface vs. Library
* User program:

    * Interface:

* Library:
Imp 1: ```int exp2(int x, int y){ for(int i=0; i<y; i++)x=x+2; return x; }```
Imp 2: ```int exp2(int x, int y){ x = x << y; return x; }```
Imp 3: ```int exp2(int x, int y){ return HW_EXP(x,y);}```

* API & System Call & OS Relationship


* user 的 application 會先去 call API 的 library，如果需要 OS 幫忙，API 就會再去 call system call，就會進到 OS 裡面（kernel mode），透過 interrupt 去 call service routine，然後再回到 API 的部份

### Why Use API?

* Simplicity
* API is designed for applications

* Portability
* API is an unified defined interface

* Efficiency
* Not all functions require OS services or involve kernel
> 加一層 API 是最有效的方式去 implement OS service，讓使用者不用這麼了解下層如何執行

## Review

* What are the two commmuniaction models provided by OS？
> shared memory、mesage passing

* What is the relationship between system calls, API and C library?

* Whay use API rather than system calls？

###### tags: `OS` `共筆`