2024-07-17 07:00:04 +01:00
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---
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title: Operating_system_API
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2024-07-17 08:00:04 +01:00
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tags: [operating-systems]
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2024-07-17 07:00:04 +01:00
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created: Wednesday, July 17, 2024
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---
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# Operating system API
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2024-07-20 11:00:04 +01:00
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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 shell
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since the UI is an abstraction layer of the shell. When _programs_ interact with
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the operating system they use the **operating system's API**.
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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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## 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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