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”.

What can computers do? What are the limits of mathematics? And just how busy can a busy beaver be? This year, I’m writing Busy Beavers, a unique interactive book on computability theory. You and I will take a practical and modern approach to answering these questions — or at least learning why some questions are unanswerable!

It’s only $19, and you can get 50% off if you find the discount code ... Not quite. Hackers use the console!

After months of secret toil, I and Andrew Carr released Everyday Data Science, a unique interactive online course! You’ll make the perfect glass of lemonade using Thompson sampling. You’ll lose weight with differential equations. And you might just qualify for the Olympics with a bit of statistics!

It’s $29, but you can get 50% off if you find the discount code ... Not quite. Hackers use the console!

More by Jim

Tagged . All content copyright James Fisher 2016. This post is not associated with my employer. Found an error? Edit this page.