Operating Systemsmedium

What are file descriptors and how does I/O buffering work?

Tags

#file-descriptors#io#buffering

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Answer

A file descriptor is a numeric handle to an open file or socket. Buffering batches I/O in memory to reduce syscalls and improve throughput, at the cost of delayed writes.

Advanced answer

Deep dive

Descriptors are OS-managed references:

Each process has a descriptor table.
Descriptors refer to kernel objects (files, sockets, pipes).
Buffering reduces syscall overhead and enables readahead.
fsync/flush forces data to disk when durability matters.

Examples

Buffered write flow:

write() -> user buffer -> kernel buffer -> disk later
fsync() -> force disk commit

Common pitfalls

Forgetting to close descriptors (leaks).
Assuming buffered writes are durable.
Mixing buffered and unbuffered I/O incorrectly.

Interview follow-ups

What happens on a partial write?
How do pipes use file descriptors?
When do you need fsync or O_DIRECT?

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