File Descriptor

A handle to an open file, socket or pipe.

Backend Engineer · Mid-level Data Engineer · Mid-level

Also known as: file descriptor, fd, open file handle

A file descriptor is a small non-negative integer the operating system gives a process to refer to an open resource — a file, a socket, a pipe, a terminal. When you open a file, you get back a number; you pass that number to every subsequent read or write. It’s a handle: an opaque token for something the kernel is managing on your behalf.

open("/etc/hosts") → 3        # read/write using descriptor 3
read(3, buf, 100)
close(3)

Three descriptors are open from the start of every process:

0 — standard input
1 — standard output
2 — standard error

The kernel keeps a per-process table mapping each descriptor to the underlying open resource, so the same integer can mean different things in different processes.

The classic mistakes:

Leaking descriptors. Every open, socket or connection you don’t close consumes a descriptor permanently. Eventually you hit the limit and get “too many open files” — a failure that starts as mysterious and snowballs.
Ignoring the limit. Each process has a cap on open descriptors (see resource limits). Busy servers routinely need it raised; the default is often too low for high concurrency.
Reusing a closed number accidentally. Descriptor numbers are reused; writing to a stale number writes to whatever now owns it — a subtle, dangerous bug (a cousin of a dangling pointer).
Assuming the number is an index or an ID. It’s just a slot in the table. It isn’t stable across processes and carries no meaning beyond “handle”.
Forgetting that sockets are files too. A network connection consumes a descriptor, so connection-heavy code is really descriptor-heavy code.

Descriptors are how the Unix “everything is a file” idea stays concrete: reading a socket and reading a text file use the same interface. They underpin epoll/kqueue (which watch many descriptors for readiness) and connection pooling (which bounds how many are in use). Watch them with ls /proc/<pid>/fd and tune the limit when a service handles many connections.