What is a “file descriptor”, really?

People say “in UNIX, everything is a file”. And that we interact with the operating system through “file descriptors”.

I don’t find this very enlightening. All it means is that the word “file” is overloaded to mean “some sort of resource”. Here are things that a file descriptor can represent:

A handle to an honest-to-goodness file
One end of a TCP connection
A listening TCP port
A listening UDP port
A reference to a kernel event queue
...

Depending on how you got your “file descriptor”, the operations you can do on it are different. There is no universal API for interacting with a “file descriptor”. For example, if you call kqueue() to get a new kernel queue, you should not perform a write operation on it. Just because it’s a “file descriptor”, it doesn’t mean you can write to it like that. It’s a kernel queue, which is completely different from a file.

“File descriptor” should be renamed “resource descriptor”.

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