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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2024-07-17 08:00:04 +01:00
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A user can interact with the operating system either via the GUI or the shell.
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(The UI is in fact an abstraction for interfacing with the shell.) When
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_programs_ interact with the operating system they use the operating system's
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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. However a running
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application doesn't have to do this. Instead it can call 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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