2024-10-19 11:00:03 +01:00
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---
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tags: [operating-systems]
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created: Wednesday, July 17, 2024
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---
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# Operating system API
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A user can interact with the operating system either via the GUI or the
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[shell](Intro_to_the_Unix_shell.md). (Both ultimately access the OS via the
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shell since the UI is an abstraction layer of the shell.)
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When _programs_ interact with the operating system they use the **operating
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system's API** directly. The GUI and shell also invoke the OS API but the access
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is mediated.
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Every OS has an API. This specification details how a program should interact
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with the OS, exposing functions, properties and data types.
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Whether using the GUI or a terminal emulator, the shell translates the user's
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commands into API calls. The API the invokes internal operating system code to
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perform the action.
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2024-10-20 19:50:20 +01:00
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2024-10-19 11:00:03 +01:00
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## Example: opening a file
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A user can use a file explorer and the mouse to open a file. Or they can open
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the file by invoking it from the terminal.
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A running application doesn't have to do this. If it's remit includes opening
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files, it can do so by calling the OS API.
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On a Unix-like system the applicaton would use the `open` API function to open
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the file:
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```
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open("foo.txt", 0_WRONLY|0_CREAT)
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```
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This command makes the action write-only and creates the file because it does
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not yet exist. When the user opens a file in their File Explorer, the
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application translates their graphical actions into this API call.
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## Different OSs have different APIs
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Unix-like and Linux operating systems use variations on the **Portable Operating
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System Interface** (POSIX) standard. This defines the OS API and also the rules
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for the shell's behaviour and its included utilities.
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Variations on this include _Cocoa_, macOS's API and Android's _Android Platform_
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API.
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Windows has its own API called _The Windows API_. The original version was
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16-bit called _Win16_, then when it moved to 32-bits it became _Win32_. When it
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moved to an x86 [instruction_set_architecture](Instruction_set_architectures.md)
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it became _Win64_. Starting with Windows 10, it introduced the _Universal
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Windows Platform_ which aims to make app development consistent accross
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different devices that run Windows.
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