58 lines
2.2 KiB
Markdown
58 lines
2.2 KiB
Markdown
---
|
|
tags: [operating-systems]
|
|
created: Wednesday, July 17, 2024
|
|
---
|
|
|
|
# Operating system API
|
|
|
|
A user can interact with the operating system either via the GUI or the
|
|
[shell](Intro_to_the_Unix_shell.md). (Both ultimately access the OS via the
|
|
shell since the UI is an abstraction layer of the shell.)
|
|
|
|
When _programs_ interact with the operating system they use the **operating
|
|
system's API** directly. The GUI and shell also invoke the OS API but the access
|
|
is mediated.
|
|
|
|
Every OS has an API. This specification details how a program should interact
|
|
with the OS, exposing functions, properties and data types.
|
|
|
|
Whether using the GUI or a terminal emulator, the shell translates the user's
|
|
commands into API calls. The API the invokes internal operating system code to
|
|
perform the action.
|
|
|
|
|
|
|
|
## Example: opening a file
|
|
|
|
A user can use a file explorer and the mouse to open a file. Or they can open
|
|
the file by invoking it from the terminal.
|
|
|
|
A running application doesn't have to do this. If it's remit includes opening
|
|
files, it can do so by calling the OS API.
|
|
|
|
On a Unix-like system the applicaton would use the `open` API function to open
|
|
the file:
|
|
|
|
```
|
|
open("foo.txt", 0_WRONLY|0_CREAT)
|
|
```
|
|
|
|
This command makes the action write-only and creates the file because it does
|
|
not yet exist. When the user opens a file in their File Explorer, the
|
|
application translates their graphical actions into this API call.
|
|
|
|
## Different OSs have different APIs
|
|
|
|
Unix-like and Linux operating systems use variations on the **Portable Operating
|
|
System Interface** (POSIX) standard. This defines the OS API and also the rules
|
|
for the shell's behaviour and its included utilities.
|
|
|
|
Variations on this include _Cocoa_, macOS's API and Android's _Android Platform_
|
|
API.
|
|
|
|
Windows has its own API called _The Windows API_. The original version was
|
|
16-bit called _Win16_, then when it moved to 32-bits it became _Win32_. When it
|
|
moved to an x86 [instruction_set_architecture](Instruction_set_architectures.md)
|
|
it became _Win64_. Starting with Windows 10, it introduced the _Universal
|
|
Windows Platform_ which aims to make app development consistent accross
|
|
different devices that run Windows.
|